CA2625142A1 - Identity-coded surface - Google Patents

Abstract

A region is defined in relation to a surface. Coded data is disposed within the region. The coded data includes identity data for identifying the region.

Description

IDENTITY-CODED SURFACE
FIELD OF INVENTION
The present invention relates generally to a method of defining a region in relation to a surface, and to a region so defined.
The invention has been cleveloped primarily to provide the basis for a surface-based interface which allows a user to input and interact with information via a network, and to obtain interactive printed matter on demand via high-speed networked color printers. Although the invention will largely be described herein with reference to this use, it will be appreciated that the invention is not limited to use in this field.
CO-PENDING APPLICATIONS
Various methods, systems and apparatus relating to the present invention are disclosed in the following co-pending applications filed by the applicant or assignee of the present invention simultaneously with the present application:
PCT/AU00/00518, PCT/AU00/00519, PCT/AUOO/00520, PCT/AU00/00521, PCT/AUOO/00523, PCT/AUOO/00524, PCT/AU00/00525, PCT/AUOO/00526, PCT/AU00/00527, PCT/AUOO/00528, PCT/AUOO/00529, PCT/AU00/00530, PCT/AU00/00531, PCT/AUOO/00532, PCT/AUOO/00533, PCT/AU00/00534, PCT/AU00/00535, PCT/AUOO/00536, PCT/AU00/00537, PCT/AUOO/00538, PCT/AUOO/00539, PCT/AUOO/00540, PCT/AU00/00541, PCT/AUOO/00542, PCT/AUOO/00543, PCT/AUOO/00544, PCT/AUOO/00545, PCT/AU00/00547, PCT/AUOO/00546, PCT/AU00/00554, PCT/AUOO/00556, PCT/AUOO/00557, PCT/AU00/00558, PCT/AU00/00559, PCT/AU00/00560, PCT/AU00/00561, PCT/AUOO/00562, PCT/AUOO/00563, PCT/AUOO/00564, PCT/AU00/00566, PCT/AUOO/00567, PCT/AUOO/00568, PCT/AUOO/00569, PCT/AU00/00570, PCT/AU00/00571, PCT/AUOO/00572, PCT/AUOO/00573, PCT/AUOO/00574, PCT/AUOO/00575, PCT/AUOO/00576, PCT/AUOO/00577, PCT/AUOO/00578, PCT/AUOO/00579, PCT/AU00/00581, PCT/AU00/00580, PCT/AUOO/00582, PCT/AUOO/00587, PCT/AUOO/00588, PCT/AU00/00589, PCT/AUOO/00583, PCT/AUOO/00593, PCT/AUOO/00590, PCT/AU00/00591, PCT/AUOO/00592, PCT/AUOO/00594, PCT/AU00/00595, PCT/AU00/00596, PCT/AUOO/00597, PCT/AU00/00598, PCT/AU00/00516, and PCT/AU00/00517.

BACKGROUND
Presently, the majority of forms for accepting input from a user are either printed on paper or displayed on a computer screen. With a paper-based form, the user writes infonnation on the form by hand, usually in spaces or boxes provided adjacent printed questions or instructions. The information is later keyed into a computer system for online storage processing and storage. With a screen-based form, the user enters information into the form via a computer keyboard.
Both of these approaches have disadvantages. The screen-based approach is convenient to the form recipient, but requires that the user have access to a computer system. By contrast, the paper-based approach only requires the user to apply pen to paper, but requires the form recipient to wait for manual delivery of the form and perform data entry of its content.
OBJECT
It is an object of the present invention to combine advantages of paper-based forms and immediate obline information capture.

SUMMARY OF INVENTION
In a first aspect of the invention, there is provided a region defined in relation to a surface, coded data being disposed within the region, wherein the coded data includes identity data for identifying the region.
Preferably, the coded data takes the form of at least one tag. In a preferred form, there are a plurality of tags that substantially fill the region.
Preferably, each of the tags within a region are identical to each other, but are distinct from tags in a plurality of other regions.
In one preferred embodiment, the tags are positioned stochastically within the region. In an alternative preferment, the tags are positioned in a regular array within the region.
In a particularly preferred form, the tags are printed onto a surface in the form of a piece of paper, and are configured to be read by a sensing device in the form of an optical sensing stylus. The tags are preferably printed using an ink that absorbs near infrared light but is substantially invisible to a human viewer under normal lighting conditions.
When a user brings a sensing end of'the stylus close to the surface, one or more of the tags are imaged, interpreted and decoded to provide an indication of the identity of the region from which the tag was imaged. This information can be used in a number of useful ways, including controlling a computer via interactive elements.
Further aspects of the invention will become apparent from reading the following detailed description of preferred and other embodiments of the invention.
BRIEF DESCRIPTION OF DRAWINGS
Preferred and other embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:
Figure I is a schematic of a the relationship between a sample printed netpage and its online page description;
Figure 2 is a schematic view of a interaction between a netpage pen, a netpage printer, a netpage page server, and a netpage application server;
Figure 3 illustrates a collection of netpage servers and printers interconnected via a network;
Figure 4 is a schematic view of a high-level structure of a printed netpage and its online page description;
Figure 5 is a plan view showing a structure of a netpage tag;
Figure 6 is a plan view showing a relationship between a set of the tags shown in Figure 5 and a field of view of a netpage sensing device in the form of a netpage pen;
Figure 7 is a flowchart of a tag image processing and decoding algorithm;
Figure 8 is a perspective view of a netpage pen and its associated tag-sensing field-of-view cone;
Figure 9 is a perspective exploded view of the netpage pen shown in Figure 8;
Figure 10 is a schematic block diagram of a pen controller for the netpage pen shown in Figures 8 and 9;
Figure 11 is a perspective view of a wall-mounted netpage printer;
Figure 12 is a section through the length of the netpage printer of Figure 11;
Figure 12a is an enlarged portion of Figure 12 showing a section of the duplexed print engines and glue wheel assembly;
Figure 13 is a detailed view of the ink cartridge, ink, air and glue paths, and print engines of the netpage printer of Figures 11 and 12;
Figure 14 is a schematic block diagram of a printer controller for the netpage printer shown in Figures 11 and 12;
Figure 15 is a schematic block diagram of duplexed print engine controllers and MemjetT'" printheads associated with the printer controller shown in Figure 14;
Figure 16 is a schematic block diagram of the print engine controller shown in Figures 14 and 15;
Figure 17 is a perspective view of a single MemjetTM printing element, as used in, for example, the netpage printer of Figures 10 to 12;

Figures 10 to 12;
Figure 18 is a perspective view of a small part of an array of Memjet'*' printing elemeats;
Fignre 19 is a series of perspective views iUustrating the operating cycle of the Memjet"' priti<ting eleatent shown in Figttre 13;
Figure 20 is a perspective view of a short segment of a pagewidth MemjetTM
printhead;
Figure 21 is a schematic view of a user class diagratn;
Figure 22 is a schemadc view of a printer class diagram;
Figure 23 is a schematic view of a pen class diagtam;
Figure 24 is a schematic view of an application class diagram;
Fgure 25 is a schematic view of a doctunent and page description class diagram;
Figure 26 is a schematic view of a docutnent and page ownership class diagtatn;
Figure 27 is a schematic view of a terminal element specialization class diagram;
Figure 28 is a schematic view of a stadc element specialization class diagnmt;.
Figure 29 is a schematic view of a hyperlink element class diagrant;
Figure 30 is a schematic view of a hyperlink element specialization class diagram;
Figure 31 is a schematic view of a hyperlinked group class diagram;
Figun: 32 is a schematic view of a form class diagram;
Fgnre 33 is a schematic view of a digital ink class diagram;
Figure 34 is a schematic view of a field element specialization class diagram;
Figure 35 is a schematic view of a checkbox field class diagram;
Figure 36 is a schematic view of a text field class diagram;
Figuie 37 is a schematic view of a signature field class diagram;
Figure 38 is a flowchart of an input processing algorithm;
Figure 38a is a detailed flowchart of one step of the flowchart of Figure 38;
Figure 39 is a schematic view of a page server comtnand element class diagram;
Figure 40 is a schematic view of a resource description class diagram;
Figure 41 is a schematic view of a favorites list class diagram;
Figure 42 is a schematic view of a history list class diagram;
Figure 43 is a schematic view of a subscription deiivery protocol;
Figure 44 is a schematic view of a hyperlink request class diagrani;
Figure 45 is a schematic view of a hyperlink activation protocol;
Fgure 46 is a schematic view of a form submission protocol;
Figore 47 is a schentatic view of a commission payment protocol;
Figure 48 is a schematic view of a set of radial wedges making up a symbol;
Figure 49 is a schematic view of a ring A and B symbol allocation scheme;
Figure 50 is a schematic view of a first ring C and D symbol allocation scheme;
Figure 51 is a schematic view of a second ring C and D symbol allocation scheme;
Figure 52 is a schematic view of a triangular tag packing;
Figure 53 is a perspective view of an icosahedron;
Figure 54 is a perspective view of an icosahedral geodesic with frequency 3;
Figure 55 is a schematic view of a niinimum tag spacing;
Figure 56 is a schematic view of a niinimum tag spacing which avoids overlap;
Figure 57 is a schematic view of a first tag insertion case;

Figure 58 is a schematic view of a second tag insertion case;
Figure 59 is a schematic view of a third tag insertion case;
Figure 60 is a schematic view of a fourth tag insertion case;
Figure 61 is a schematic view of a pen orientation relative to a surface;
'rJ Fgtm 62 is a schematic view of a pen pitch geometry;
Figure 63 is a schematic view of a pen roll geometry;
Figure 64 is a schematic view of a pen coordinate space showing physical and optical axes of a pen;
Figure 65 is a schematic view of a curved nib geoatetry;
Figure 66 is a schematic view of an interaction between sampling frequency and tag frequency;
Figute 67 is a table containing equations numbered 1 to 10;
Figure 68 is a table containing equations numbered I 1 to 20;
Figune 69 is a table containing equations numbered 21 to 26;
Figure 70 is a table containing equapons nutnbered 27 to 34;
Figure 71 is a tabie containing equations nuntbered 35 to 41;
Figure 72 is a table containing equations numbered 42 to 44;
Figure 73 is a table containing equations numbered 45 to 47;
Figure 74 is a table containing equadons numbered 48 to 51;
Figure 75 is a table containing equations numbered 52 to 54;
Figure 76 is a table containing equations numbered 55 to 57;
Figure 77 is a table containing equations numbered 58 to 59;
Figure 78 is a table containing equations numbered 60 to 63;
Figure 79 is a table containing equations numberod 64 to 74;
Figure 80 is a table containing equations numbered 75 to 86;
Figure 81 is a table containing equations numbered 87 to 99;
Figure 82 is a table containing equations numbered 100 to 111;
Figure 83 is a table containing equations numbered 112 to 120;
Figure 84 is a table containing equations numbered 121 to 129;
Figure 85 is a table containing a set of degenerate forms of equations 64 to 71;
Figurc 86 is a first part of a table containing conditions and special handling for zero pitch and zero roll; and Figure 87 is a the second patt of the table of Figure 86.
DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS
Note: MemjetTM is a trade mark of Silverbrook Research Fty Ltd, Australia.
In the prefetred embodiment, the invention is configured to work with the netpage networked computer systeni, a detailed overview of which follows. It will be appreciated that not every implementation will ttecessarily embody all or even most of the specific details and extensions discussed below in relation to the basic system However, the system is described in its most complete fonn to reduce the need for external teference when attempting to understand the context in which the prefernd embodiments and aspects of the ptesent invention operate.
In brief summary, the prefen-ed form of the netpage system employs a computer interface in the form of a mapped surface, that is, a physical surface which contains references to a map of the surface ntaintained in a computer system. The map references can be, queried by an appropriate sensing device.
Depending upon the specific implementation, the map references may be encoded visibly or invisibly, and defined in such a way that a local query on the mapped surface yields an unambiguous map reference both within the map and among different maps. The computer system can contain infonnation about features on the mapped surface, and such infonmuion can be retrieved based on map references supplied by a sensing device used with the nutpped surface. The infonnation thus retrieved can take the .5-form of actions which are initiated by the computer system on behalf of the operator in response to the operator's interaction with the surface features.
In its preferred form, the netpage system relies on the production of. and human interaction with, netpages.
These are pages of text, graphics and images printed on ordinary paper,+ut which work like interactive web pages.
Information is encoded on each page using ink which is substantially invisible to the unaided htnnan eye. The ink, however, and thereby the coded data, can be sensed by an optically imaging pen attd transnritted to the netpage system.
In the preferred form, active butt.ons and hypedinks on each page can be clicked with the pen to request information from the network or to signal preferences to a network server. In one embodintent, text written by hand on a netpage is autonmtically recognized and converted to computer text in the netpage system, allowing forms to be filled in.
In other emboditnents, signatures recorded on a netpage are autAmatically verified, allowing e-commence transactions to be securely authorized.
As illustrated in Figure 1, a printed netpage I can represent a interactive form which can be fiUed in by the user both physically, on the printed page, and "elecxronically", via communication between the pen and the netpage systent. The example shows a"Request~ form containing name and address fields and a submit button. The netpage consists of graphic data 2 printed using visible ink, and coded data 3 printed as a eollection of tags 4 using invisible ink.
The corresponding page description 5, stored on the netpage network, describes the individual elements of the netpage. In particular it describes the type and spatial extent (wtte) of each intetactive element (i.e. text field or button in the example), to allow the netpage system to correctly interpret input via the netpage. 71x submit button 6, for example, has a zone 7 which carresponds to the spatial extent of the corresponding graphic B.
As illustrated in Figure 2, the netpage pen 101, a preferred form of which is shown in Figures 8 and 9 and described in more detail below, works in conjunction with a netpage printer 601, an Internet-connected printing applianee for home, office or mobile use. The pen is wireless and communicates securely with the netpage printer via a short-range radio link 9.
The netpage printer 601, a prefetred form of which is shown in Fgurt:s 11 to 13 and described in tnore detsil below, is able to deliver, periodically or on demand, personalized newspapers, magazitu:s, catalogs, bmchures and other publications, all printed at high quality as interactive netpages. Unlike a personal cotnputer, the netpage printer is an appliance which can be, for exantple, wall-mounted adjacent to an area where the morning news is first consumed, such as in a user's kitchen, near a breakfast table, or near the household's point of departure for the day. It also comes in tabletop, desktop, portable and ntiniature versions.
Netpages printed at the'u point of consumption combine the ease-of-use of paper with the titnelittess and interactivity of an interactive med'ntm.
As shown in Figure 2, the netpage pen 101 interacts with the coded data on a printed netpage I and communicates, via a short-range radio link 9, the intetaction to a netpage printa. 71te printer 601 sends the interaction to the relevant netpage page server 10 for interpretation. In appropriate circumstances, the page server sends a corresponding ntessage to application computer software ntnning on a netpage application server 13. The application server may in tum send a response which is printed on the originating printer.
The netpage system is made considerably more convenientin the prefen-ed etnbodintett by being used in conjunction with high-speed microelectromechanical system (MEMS) based inkjet (MemjetTm) printers. In the preferred form of this technology, relatively high-speed and high-quality printing is made more affordable to consumers. In its preferred form. a netpage publication has the physical characteristics of a traditional newsmagazine, such as a set of letter-siu glossy pages printed in full color on both sides, bound together for easy navigation and comfortable handling.
The netpage printer exploits the growing availability of broadband Intemet access. Cable service is available to 95% of households in the United States, and cable ntodem service offering broadband lntetnet aocess is already available to 20% of these. The netpage printer can also operate with slower connections, but with longer delivety times snd lower image quality. Indeed, the netpage system can be enabled using existing consumer inkjet and laser ptinters, although the system will operate more slowly and will therefore be less acceptable from a consumer's point of view. In other embodintents, the netpage system is hosted on a private intranet. In sdll other embodintents, the netpage system is hosted on a single computer or computer-enabled device, such as a printer.
Netpage publication servers 14 on the netpage network are configured to deliver print-qualit7 publications to netpage printers. Periodical publications are delivered automatically to subscribing netpage printers via pointcasting and multicasdng Internet protocols. Personalized pubGcations are filtered and forntatted acsording to individual user profiles.
A, netpage printer can be configured to support any number of pens, and a pen can work with any number of netpage printers. In the preferred implementation, each netpage pen has a unique identifier. A household may have a collection of colored netpage pens, one assigned to each member of the family.
This allows each user to maintain a distinct profile with respect to a netpage publication server or applicarion setver.
A netpage pen can also be registered with a netpage regisiration server 11 and linked to one or more payntent card accounts. This allows e-commerce payments to be securely authorized using the netpage pen. The netpage registration server compares the signature captured by the netpage pen with a previously n:gistered signatupe, allowing it to authenticate the user's identity to an e-commerce server. Other biometrics can also be used to verify identity. A version of the netpage pen includes fingerprint scanning, verified in a similar way by the netpage registration server.
Although a netpage printer may deliver periodicals such as the moming newspaper without user intervention, it can be configured never to deliver unsolicited junk mail. In its preferred forai, it only delivers periodicals from subscribed or otherwise authorized sources. In this respea, the netpage printer is unlike a fax ntachine or e-mail account which is visible to any junk mailer who knows the telephone number or email addrcss.
1 NETPAGE SYSTEM ARCHtTECfIJRE
Each object model in the system is described using a Unified Modeling Language (UML) class diagram. A
class diagram consists of a set of object classes connected by relationships, and two kinds of relationships are of intenw here: associations and generalizations. An association represents some kind of relationship between objects, i.e. between 2'rJ instances of classes. A generalization relates actual classes, and can be understood in the following way: if a class is thought of as the set of all objects of that class, and class A is a generalization of class B. then B is simply a subset of A.
The UML does not directly support second-order modelling = i.e. classes of classes.
Each class is drawn as a rectangle labelled with the name of the class. It contains a list of the attributes of the class, separated from the name by a horizontal line, and a list of the operations of the class, separated from the attribute list by a horizontal line. In the class diagrams which follow, however, operations are never modelled.
An association is drawn as a line joining two classes, optionally labeiled at either end with the multiplicity of the association. The default multiplicity is one. An asterisk (*) indicates a multiplicity of "ntany", i.e. zero or more. Each association is optionally labelled with its natne, and is 'also optionally labelled at either end with the role of the corresponding class. An open diamond indicates an aggregation association ("is-part-of"), and is drawii at the aggregator end of the association line.
A generalization relationship ("is-a') is drawn as a solid line joining two classes, with an arrow (in the form of an open triangle) at the generalization end.
When a class diagram is broken up into multiple diagrams, any class which is duplicated is shown with a dashed outline in all but the main diagraro which defines it. It is shown with attributes only where it is defined.
1.1 NETPAGES
Netpages are the foundation on which a netpage network is built. They provide a paper-based user interface to published information and interactive services.
A netpage consists of a printed page (or other surface region) invisibly tftged with references to an online description of the page. The online page description is maintained persistently by a netpage page server. The page description describes the visible layout and content of the page, including text, graphics and images. It also describes the inpat elements on the page, including buttons, hypedinks, and.input fields. A
netpage allows marldngs made with a netpage pen on its surface to be sintnltaneously captured and processed by the netpage system.
Muldple netpages can share the same page description. However, to allow input through otherwise identical pages to be distinguished, each netpage is assigned a unique page identifier.
This page ID has sufficieot pnxision to distinguish between a very large number of netpages.
Each reference to the page description.is encoded in a printed tag. llte tag identif=ies the unique page on which it appears, and thereby ind'uectly identifies the page description. The tag also identifles its own posidon on the page. Characterisdcs of the tags are described in more detail below.
Tags are printed in in5ared-absorptive ink on any substrate which is infrared-reflective, such as ordinary paper. Near-infrared wavelengths are invisible to the human eye but are easily sensed by a solid-state image sensor with an appropriate filter.
A tag is sensed by an area intage sensor in the netpage pen, and the tag data is transnritted to the netpage system via the nearest netpage printer. The pen is wireless and communicates with the netpage printer via a short-range radio link. Tags are sufficiently small and densely arranged that the pen can reliably image at least one tag even on a single click on the page. lt is important= that the pen recognize the page ID
and position on every intenaction with the page, since the interaction is stateless. Tags are error-corroctably encoded to niake them partially tolerant to surface damage.
The netpage page server maintains a unique page instance for each printed netpage, allowing it to maintain a distinct set of user-supplied values for input fields in the page description for each printed netpage.
The relationship between the page description, the page instance, and the printed netpage is shown in'Figure 4. The page instance is associated with both the netpage printer which printed it and, if known, the netpage user who requested it.
12 NervAGE TAGs 12.1 Tag Data Content In a prefeffed form, each tag identifies the region in which it appears, and the location of that tag within the region. A tag may also contain flags which relate to the region as a whole or to the tag. One or more flag bits may, for example, signal a tag sensing device to provide feedback indicative of a funaion associated with the immediate a;ea of the tag, without the sensing device having to refer to a description of the region. A netpage pen may, for exanVle, illunrinate an "active area" LED when in the zone of a hyperlink.
As will be more clearly explained below, in a prefened etnbodimettt, each tag contains an easily rocoprizod invariant structure which aids initial detection, and which assists in niinimizing the effect of any warp induoed by the surface or by the sensing process. The tags preferably tile the entire page, and are sufficiently small and densely arrangeai that the pen can reliably image at least one tag even on a single click on the page. It is important that dte pen recognize the page ID and position on every interaction with the page, since the interaction is stateless.
In a preferred embodiment, the region to which a tag refers coincides with an entire page, and the region ID
encoded in the tag is therefore synonymous with the page ID of the page on which the tag appears. In other embodiments, the region to which a tag refers can be an arbitrary subregion of a page or other surface. For example, it can coincide with the zone of an interactive elenient, in which case the region ID can dinxtly identify the interactive elemettt.
Table 1. Tag data Field Precision (bits) Region ID 100 ag ID 16 lags ota! 120 Each tag cattains 120 bits of infonmation, typically allocated as shown in Table 1. Assunung a maximum tag density of 64 per square inch, a 16-bit tag ID suppofts a region size of up to 1024 squane itdtes. lmpga regions can be mapped continuously without increasing the tag ID procision simply by using abutting regions and maps. The 100-bit region ID allows 210 (-1030 or a million trillion trilGon) different regions to be uniquely identified.
1.22 Tag Data Encoding Tbe 120 bits of tag data are redundantly encoded using a (15, 5) Reed-Solomon code. This yields 360 encoded bits consisting of 6 codewords of 15 4-bit symbols each. The (15, 5) code allows up to 5 symbol ermrs to be corrected per codeword, i.e. it is tolerant of a symbol error rate of up to 33% per codeword.
Each 4-bit symbol is represented in a spatially coherent way in the tag, and the symbols of the six codewords are interleaved spatially within the tag. This ensures that a burst errm (an etror affecting tnxdtiple spatially adjacent bits) damages a minimum number of symbols overall and a minittmm number of symbols in any one codeword, thus maximising the likelihood that the burst error can be fully conected.
1.2.3 Physical Tag Structure The physical representation of the tag, shown in Figure 5, includes fixed target structures 15, 16, 17 and variable data areas 18. The fixed target structures allow a sensing device such as the netpage pen to detect the tag and infer its three-dimensional orientation relative to the sensor. The data areas contain representations of the individual bits of the encoded tag data.
To achieve proper tag reproduction, the tag is rendered at a resolution of 2S6x256 dots. When printed at 1600 dots per inch this yields a tag with a diarneter of about 4 mm. At this resolution the tag is designed to be surrounded by a "quiet area" of radius 16 dots. Since the quiet area is also contributed by adjacent tags, it only adds 16 dots to the effective diameter of the tag.
The tag includes six target structures: a detection ring 1 S; an orientation axis target 16; and four perspective targets 17.
2r'J The detection ring 15 allows the sensing device to initially detect the tag 4. T1%e ring is easy to detect because it is rotationally invariant and because a simple cotrection of its aspect ratio rentoves most of the effxts of petspective distortion. The orientation axis 16 allows the sensing device to deteratine the approximate plaaar orientation of the tag due to the yaw of the sensor. The orientation axis is skewed to yield a unique orientation. The four perspective targets 17 allow the sensing device to infer an accurate two-dimensional perspective transform of the tag and hence an aocttrate three-dimensional position and orientation of the tag relative to the sensor.
All target structures are redundantly large to improve their innrwnity to noise.
The overall tag shape is circular. This supports, amongst other things, optimal tag packing on an irregular triangular grid. In combination with the circular detection ring 15, this makes a circular arrangement of data bits within the tag optimal. To maximise its size, each data bit is represented by a radial wedge 510 in the form of an area bounded by two radial lines 512, a radially inner arc 514 and a radially outer arc 516.
Each wedge 510 has a minimum dintension of 8 dots at 1600 dpi and is designed so that its base (i.e. its inner arc 514), is at least equal to this minimum dintension. The radial height of the wedge 510 is always equal to the minitrtum dimension.
Each 4-bit data symbol is represented by an anay 518 of 2x2 wedges 510, as best shown in Figure 48.
The 15 4-bit data symbols of each of the six codewords are allocated to the four concentric symbol rings 18a to 18d, shown in Figure 5, in interleaved fashion as shown in Figures 49 to 51. Symbols of first to sixth codewords'520-525 are allocated alternately in circular progression around the tag.
The interleaving is designed to maxitnise the average spatial distance between any two sytnbols of the satne codeword.
In order to support "single-click" interaction with a tagged region via a sensing device, the sensing devioe tnust be able to see at least one entire ta,g in its field of view no matter where in the region or at what orientation it is positiotted The required diameter of the field of view of the sensing device is therefore a funaion of tlte size and spacittg of the tags.
Assuniing a circular tag shape, the minimum diameter of the sensor field of view is obtained when the tags are tiled on a equilateral triangular grid. as shown in Figure 6.
1.2.4 Tag Image Processing and Decoding The tag image processing and decoding perfornxd by a sensing device such as the ttetpage pen is ahown in Figure 7. While a captured image is being acquired from the image sensor, the dynamic range of the image is detemrined (at 20). The center of the range is then chosen as the binary threshold for the image 21. 71te image is then thresholded and segmented into connected pixel regions (i.e. shapes 23) (at 22). Shapes which are too small to reprtsent tag target stntctures are discarded. The size and centroid of each shape is also computed.
Binary shape tnotnenu 25 are then computed (at 24) for each shape, and these provide the basis for subsequently locating target structures. Central shape ttmments are by their nature invariant of position, and can be easily made invariant of scale, aspect ratio and rotation.
'ltte ring target.structure 15 is the first to be located (at 26). A ring has the advantage of being very well behaved when perspecdve-distorted. Matching proceeds by aspect-normalizing and rotation-normalizing each shape's tnotnents. Once its second-order moments are notmalized the ring is easy to recognize even if the perspective distortion was significant. The ring's original aspect and rotation 27 together provide a useful approximation of the perspective transfotm.
The axis target structure 16 is the next to be loaaed (at 28). Matching proceeds by applying the ring's normalizations to each shape's naments, and rotation-nonnalizing the resulting moments. Onoe its seoond-order moments are normalized the axis target is easily recognized. Note that one third order moment is required to disaabiguate the two possible orientations of the axis. The shape is deliberately skewed to one side to make this possible. Note also that it is only possible to rotation-notmalize the axis target after it has had the ring's normalizations applied, since the perspeaive distortion can hide the axis target's axis. 7be axis target's original rotation provides a useful approximation of the tag's rotation due to pen yaw 29.
7'he four perspective target structures 17 are the.last to be located (at 30).
Gaod tstimates of their positions are computed based on the'v latown spatial relationships to the ring and axis targds, the aspecx and totation of the ring, and the rotation of the axis. Matching proceeds by applying the ring's nonnalizations to each shape's moments. Once their second-order moments arc normalized the circular perspective targets are easy to recognize, and the target closeat to eaoh estimated position is taken as a match. The original centroids of the four perspective targets are then taken to be the petspective-distorted corners 31 of a square of known size in tag.space, and an eight-degtee-of-fre,edom perspective transform 33 is inferred (at 32) based on solving the well-understood equations relating the four tag-space and image-space point pairs.
The inferred tag-space to itnage-space perspective transform is used to project (at 36) meach known data bit position in tag space into itnage space where the real-valued position is used to bilinearly interpolate (at 36) the four relevant adjacent pixels in the input image. The previously computed image thteshold 21 is used to threshold the resuit to produce the final bit value 37.
Once all 360 data bits 37 have been obtained in this way, each of the six 60-bit Reed-Solotnon codewords is decoded (at 38) to yield 20 decoded bits 39, or 120 decoded bits in total.
Note that the codeword symbols are sampled in codeword order, so that codewords are implicitly de-interleaved during the sampling process.
The ring target 15 is only sought in a subarea of the image whose relationship to the image ,guamtees that the ring, if found, is part of a complete tag. If a complete tag is not found and stucessfully, decoded. then no pen position is recorded for the current fiame. Given adequate processing power and ideally a non-minimal fieM of view 193, an altanative strategy involves seeking another tag in the current image.
The obtained tag data indicates the identity of the region containing the tag and the position of the tag within the region. An accurate position 35 of the pen nib in the region, as well as the overall orientation 35 of the pen, is then inferred (at 34) from the perspective transfarm 33 obscrved on the tag and the known spatial relationship between the pen's physical axis and the pen's optical axis.
1.2.5 Tag Map Decoding a tag results in a region ID, a tag ID, and a tag-relative pen aransform. Before the tag ID and the tag-relative pen location can be translated into an absolute location within the tagged region, the location of the tag within the region must be known. This is given by a tag map, a function which maps each tag ID in a tagged nsgiott to a corresponding location. The tag map class diagram is shown in Figure 22. as part of the netpage printer class diagram.
A tag map reflects the scheme used to tile the surface rrgion with tags, and this can vary according to surface type. When multiple tagged regions share the same tiling scheme and the same tag numbering scheme, they can aiso siare the same tag map.
The tag map for a region must be retrievabla via the region ID. Thus, given a region ID, a tag ID and a pen transform, the tag map can be retrieved, the tag ID can be translated into an absolute tag location within the mgion, and the tag-relative pen location can be added to the tag location to yield an absolute pen location within the region.
1.2.6 Tagging Schemes Two distinct surface coding schemes are of interest, both of which use the tag structure described iarlier in this section. The preferred coding scheme uses location-indicating=. tags as abtady discussed. An alterntlive eoiog scheme uses objeM-indicating tags.
A location-indicating tag eontains a tag ID which, when transtated through the tag inap assaciaoed with the tagged region, yields a unique tag location within the region, The tag-relative locatiori of the pen is added to -this tag location to yield the location of the pen within the region. This in turn is used to determine the location of the pen relative to a user interface elenient in the page description associated with the region. Not only is the user interfaee elentent itself identified, but a location relative to the user interface element is identified. Location-indicating tags therefore trivially support the capuue of an absolute pat path in the zone of a particular user interface element.
An object-indicating tag contains a tag ID which directly identifies a user interface element in the page description associated with the region. All the tags in the zone of the user interface element identify the user interface element, making them all identical and therefore indistinguishable. Object-indicating tags do not, therefae, suppott the capture of an absolute pen path. They do, however, support the capture of a reiatiVe pen path. So long as the position sampling frequency exceeds twice the encountered tag &equency, the displacement from one sampled pen position to the next within a stroke can be unambiguously determined:
Assunc a sampling wavelength of kS and a tag wavelength of with a relationship as defined in EQ 38. For =
two adjacent position samples P, and Põ1, one of EQ 39 and EQ 40 will hold.
Assuming both equations hold leads to the relationship defined in EQ 41.
Since EQ 41 contradicts EQ 38, the assumption that both EQ 39 and EQ 40 hold must be incomact, and dte choice is therefore unambiguous, as stated.
The illustration in Figure 60 shows four tags 500 and a one-dimensional stroke of six sample positions 582 which satisfy EQ 38. Possible aliases 584 of the sample positions art also shown. From inspection, if the distanoe from one saniple position to the next is IS, then the distance from a sample position to the alias of the next sample position exceeds XS.

If the tag wavelength.Xris 4.7 mm, as discussed in earlier, then the sampling wavelength )Ls mast be less than 2.35 mm, lf the temporil sampling frequency is 100 Hz as required for aaatrate handwriting recognition, then the pen speed must be less than 235 mm/s to satisfy EQ 38.
With either tagging scheme, the tags function in cooperation With associated visual elements on the netpage as user interactive elements in that a user can interact with the printed page using an appropriate seosing device in order for tag data to be read by the sensing device and for an appropriate response to be generated in the netpage system.
1.3 DOCUMENT AND PAGE DESCRIPTIONS
A preferred embodiment of a docun-ent and page description class diagram is shown in Figures 25 and 26.
In the netpage system a docnment is =described. at three levels. At the most abstract level the document 836 has a hierarchical structure whose tenainal elements 839 are associated with oontent objects 840 such as text objects, text style objects, image objects, etc. Once the docunient is printed on a printer with a patticular page size and according to a particular user's scale factor preference, the docutrxnt'is paginated and otherwise fotmatted. Forntatted terrninal elements 835 will in sonie cases be associated with content objects which are different from those associated with their cotresponding tenninal elements, particularly where the content objects are style-related. Each printed instance of a 1r'J document and page is also described separately, to aUow input captunal through a particular pege instance $30 to be recorded separately from input captured through other instances of the same page description.
The presence of the most abstract.document description on dte page server allows a usa to requeat a copy of a document without being forced to accept the source documEnt's specific format. 'fhe user may be requesting a uopy through a printer with a different page size, for example. Converseiy, the presence of the formatted doatment description on the page server allows the page server to effrciently interprr.t user actions on a particular pr'nued page.
A fonnatted document 834 consists of a set of formatted page descriptions 5, each of which consists of a set of formatted terminal elements 835. Each fotmatted elemertt has a spatial extent or zone 58 on the page. This defines the active at'ea of input elements such as hyperlinks and input fields.
A docuntent instance 831 corresponds to a fornoatted docume t 834. lt consists of a set of page instances 830, 2'rJ each of which corresponds to a page description 5 of the formatted docunient. Each page instance 830 describes a single unique printed netpage 1, and records the page ID 50 of the netpage. A page instance is not part of a document instance if it represents a copy of a page requested in isolation.
A page instance consists of a set of terminal elenunt instances 832. An element instance only exists if it records instance-specific information. Thus, a hyperlink instance exists for a hyperlink element because it records a transaction ID 55 which is specific to the page instance, and a field instance exists for a field elernent bemse it records input specific to the page instance. An element instance does not exist, however, for static elenxmts such as textflows.

A terminal element can be a static element 843, a hyperlink elenxnt 844, a field element 845 or a page server command element 846, as shown in Figure 27. A static elentent 843 can be a style element 847 with an sssociated style object 854, a textflow element 848 with an associated styled text object 855, an image element 849 with an associated image element 856, a graphic element 850 with an associated graphic objea 857, a video clip element 851 with an associated video clip object 858, an audio clip element 852 with an associated audio clip object 859, or a script element 853 with an associated script object 860, as shown in Figure 28.
A page instance has a background field 833 which is used to trcord any digital ink capturod on the page which does not apply to a specific input element.
in the preferred form of the invention, a tag map 811 is associated with each page instance to allow tags on the page to be translated into locations on the page.
1.4 THE NETFAGE NETwoRtt In a preferred embodiment, a netpage network consists of a distributed set of netpage page servers.10, netpage registration servers 11, netpage ID servers 12, netpage application servers 13, netpage publication servers 14, and netpage printers 601 connected via a network 19 such as the Intemet, as shown in Figure 3.
The netpage registration server 11 is a server which records relationships betvNeen users, pens, printets, appGcations and publications, and thereby authorizes various network activities. It authenticates users and acts as a signing proxy' on behalf of authenticated users in application transactions.
It also provides handwriting recognition services. As described above, a netpage page server 10 maintains persistent information about page descriptions and page ittstances. The netpage network includes any number of page servers, each handling a subset of page instances. Since a page server also maintains user iaput values for each page instana, clients such as netpage printers send netpage input dinxtly to the appropriate page server. The page server interprets any such input relative to the desaiption of the corresponding page.
A netpage ID server 12 allocates document IDs 51 on demattd, and provides load-balancing of page servers via its ID allocation scheme.
A netpage printer uses the Internet Distributed Name System (DNS), or similar, to resolve a netpage page ID
50 into the network address of the netpage page server handling the corresponding page instance.
A netpage application server 13 is a server which hosts interactive netpage applications. A netpage publication server 14 is an ppplication scrver which publishes netpage documents to netpage printers. They are described in detail in Section 2.
Netpage servers can be hosted on a variety of network server platformr from manufacturers such as IBM, Hewlett-Packard, and Sun. Multiple netpage servers can run concurrently on a single host, and a singk server can be distributed over a number of hosts. Some or all of the functionality provided by netpage servers, and in particular the functionality provided by the ID server and the page server, can also be provided directly in a netpage appliance such as a netpage printer, in a computer workstation, or on a local network.
1.5 THE NETPAGE PRINTER
The netpage printer 601 is an appliance which is registered with the netpage system and prints netpage documents on demand and via subscxiption. Each printer has a unique printer ID
62, and is connected= to the netpage network via a network such as the lnternet, ideally via a broadband connection.
Apart from identity and securiry settings in non-volatile memory, the netpage printer contains no persistent storage. As far as a user is concerned, "the network is the computer".
Netpages functirsn interactively across space and time with the help of tlx distributed netpage page servers 10, independently of patticular netpage printers.
The netpage printer receives subscribed netpage documents ftom netpage publieation servers 14. Each document is distributed in two parts: the page layouts, and the actual text and image objects which populate.the pages.
Because of personalization, page layouts are typically specific to a particvlar subscriber and so are pointca.st to the subscriber's printer via the appropriate page server. Text and image objects, on the other hand, are typically shared with other subscribers, and so are multicast to all subscribers' printers and the appropriate page servers.
71te netpage publication server optimizes the segmentation of document content into pointcasts and multicasts. After receiving the pointcast of a docunxnt's page layouts, the printer knows which multicasts, if any, to listen to.
Once the printer has received the complete page layouts and objects that define the document to be printed, it can print the documettt.
The printer rasterizes and prints odd and even pages simultaneously on both sides of the sheet. lt contains duplexed print engine controllers 760 and print engines utilizing MemjetTM
printheads 350 for this purpose.
The printing process consists of two decoupled stages: rastetization of page dcscriptions, and expansion and printing of page images. The raster image processor (RIP) consists of one or more standard DSPs 757 running in paralkl.
The duplexed print engine controllers consist of custom processors which expand, dither and print page images in neal time, synchronized with the operation of the printheads in the print engines.

Printers not enabled for IR printing have the option to print tags ttsing IR-absorptive black ink, although this restricts tags to otherwise empty areas of the page. Although such pages have tnore limited fiutctionality than IR-}trinted pages, they are still classed as netpages.
A normal netpage printer prints netpages on sheets of.paper. More specialised netpage printers may print onto more specialised surfaces, such as globes. Each printer supports at.least one surface type, and supports at least one tag tiling schetne, and hence tag map, for each surface type. The tag map 811 which describes the tag tiling scheme actnally used to print a docuntent becomes associated with that docwment so that the document's tags can be cortecdy interpreted.
Figure 2 shows the netpage printer class diagram, reflecting printer-related infonmazion maintained by a registration server 11 on the netpage network.
A preferred embodiment of the netpage printer is described in gteater detail in Section 6 below, with referextce to Figures 11 to 16.
1.5.1 Men>ietTM' Printheads 71m netpage system can operate using printers made with a wide range of digital printing teclmologies, including thermal inkjet, piezoelectric inkjet, laser electrophotographic, and othets. However, for wide consumer acceptance, it is desirable that a netpage printer have the following characteristics:
= photographic quality color printing = high quality text printing = high reliability = low lxlnter cost = low ink cost = low paper cost - simple operation = nearly silent printing = high printing speed = simultanmus double sided printing = conipact fonn factor = low power consumption No comtttercially available printing technology has all of these characteristics.
To enable to production of printers with these characteristics, the present applicant has invented a new print technology, referred to as MemjetTM technology. MemjetTM is a drop-on-demattd inkjet technology that incorporates pagewidth printheads fabricaud using microelectromechanical systems (MEMS) technology. "Figure 17 shows a single printing element 300 of a MemjetTM printhead, The netpage wallprinter incorporates 168960 printing, elements 300 to form a 1600 dpi pagewidth duplex printer. This printar simultanoously prints cyan, magenta, yellow, black, and iti6ared inks as well as paper conditioner and ink fixative.
The printing element 300 is approximately 110 ndcrons long by 32 microns wide.
Arrays of these printing elentents are fon,ned on a silicon substrate 301 that incorporates CMOS logic, data transfer, tinring, and drive circuits (not shown).
Major elenxnts of the printing element 300 are the nozzle 302, the nozzle rim 303, the nozzle chamber 304, the fluidic seal 305, the ink channel rim 306, the ]ever arm 307, the active actuator beam pair 308, the passive actuator beam pair 309, the active actuator anchor 310, the passivc actuator anchor 311, and the ink inlet 312.
The active actnator beam pair 308 is ntechanically joined to the passive actuator beam pair 309 at the join 319. Both beanis pairs are anchored at their respective anchor points 310 and 311. The combination ofelennents 308, 309, 310, 311, and 319 form a cantilevered electrothenztal bend actuator 320.

Figure 18 shows a small part. of an array of printing elements 300, including a cross section 315 of a printing element 300. The cross section 315 is shown without ink. to clearly show the ink inlet 312 that passes through the silicon wafer 301.
Figures 19(a), 19(b) and 19(c) show the operating rycle of a MemjetTM printing element 300.
Figure 19(a) shows the quiescent position of the ink meniscus 316 prior to printing an ink droplet. Ink is retained in the nozzle chamber by surface tension at the ink meniscus 316 and at the fluidic sea1305 formed between the nozzle chamber 304 and the ink channel rim 306.
W.hile printing, the printhead CMOS circttitry distributes data from the print engine controller to the correct printing element, latches the data, and buffers the data to drive the electrades 318 of the active aotuator beam pair 308.
This causes an electrical current to pass through the beam pair 308 for about one microsecond, resulting in Joule heating.
The tetnperature increase resulting from Joule heating causes the beam pair 308 to expand. As the passive actttator beam pair 309 is not heated, it does not expand, resulting in a stress difference between the two beam pairs. This sttess difference is partially resolved by the cantilevered end of the electrotherrrtal bend actuator 320 bending towards the substrate 301. The lever arm 307 transmits this moventettt to the nozzle chamber 304. The nozzle chamber 304 moves about two microns to the position shown in Figure 19(b). This increases the ink pressure, forcing ink 321 out of the nozzle 302, and causing the ink nieniscus 316 to bulge. The nozzle rim 303 prevents the ink mettiscus 316 brom spneading across the surface of the nozzle chamber 304.
As the temperature of the beam pairs 308 and 309 equalizes, the actuator 320 returtts to its original position.
This aids in the break-off of the ink droplet 317 from the ink 321 in the nozzle chamber, as shown in Figure 19(c). The nozzle chamber is refilled by the action of the surface tension at the meniscus 316.
Figure 20 shows a segrttent of a printhead 350. In a netpage printer, the length of the printhead is the full width of the paper (typically 210 nun) in the direction 351. The segment shown is 0.4 mm long (about 0.2% of a complete printhead). When printing, the paper is tnoved past the fixed printhead in the direction 352. The printhead has 6 rows of interdigitated printing elenunts 300, printing the six colors or types of ink supplied by the ink inlets 312.
.25 To protect the fragile surface of the printhead during operation, a nozzle guard wafer 330 is attached to the printhead substrate 301. For each nozzle 302 there is a corresponding nozzle guard hole 331 through which the ink droplets are fired. To prevent the nozzle guard holes 331 from beconring blocked by paper fibers or other debris, filtered air is pumped through the air inlets 332 and out of the nozzle guard holes during printing. To prevent ink 321 from drying, the nozzle guard is sealed while the printer is idle.
1.6 The Netpage Pen The active sensing device of the netpage system is typically a pen 101, which, using its embedded controller 134, is able to capture and decode IR position tags from a page via an image sensor. The image sensor is a solid-state device provided with an appropriate filter to petmit sensing at only near-infrared wavelengths. As described in more detail below, the system is able to sense when the nib is in contact with the surface, and the pen is able to sense tags at a sufficient rate to capture human handwriting (i.e. at 200 dpi or greater and 100 Hz or faster). Infomiation captwed by the pen is encrypted and wirelessly transmitted to the printer (or base station), the printer or base station interpreting the data with respect to the (known) page strocture.
The prefen-ed embodiment of the netpage pen operates both as a normal marking ink pen and as a non-marking stylus. The ntarking aspect, however, is not necessary for using the netpage system as a browsing system, such as when it is used as an intemet interface. Each netpage pen is registered with the netpage system and has a unique pen ID 61.
Figure 23 shows the netpage pen class diagram, reflecting pen-related information tnaintained by a registration server 11 on the netpage network.
When either nib is in contact with a netpage, the pen determines its position and orientation relative to the page. The nib is attached to a force sensor, and the force on the nib is interpreted relative to a threshold to indicate whether the pen is "up" or "down". This allows a interactive element on the page to be 'clicked'.by pressing with the pen nib, in order to request, say, infonmation from a network. Furthermpre, the force is capaned as a oontimtous value to allow, say, the full dynantics of a signature to be verified.
The pen determines the position and orientation of its nib -on the netpage by itneging, in the infrared spxtrttm, an atea 193 of the page in the vicinity of the nib. It decodes the nearest tag and computes the position of the nib relative to the tag from the observed perspective distottion on the imaged tag and the known geometry of the pen optics.
Although the position resolution of the tag may be low, because the tag density on the page is inversely proportional to the tag size, the adjusted position resolution is quite high, exceeding the uainiroum resolution retluied for accunue handwriting recognition.
Pen actions.relative to a netpage are captured as a series of strokes. A
stroke consists of a sequence of tinte-stamped pen positions on the page, initiated by a pen-down event and completed by the subsequent pen-up event. A stroke is also tagged with the page ID 50 of the netpage whenever the page ID
changes, which, under normal circurnstanoes, is at the commencement of the stroke.
Each uetpage pen has a current selection 826 associated with it. allowing the user to perform copy and paste operations etc. The selection is timestamped to allow,the system to discard it after a defined time period. 91rc curnnt selection describes a region of a page instance. It consists of the most.
recent digital ink stroke captuned through the pen relative to the background area of the page. It is interpreted in an application-specific mamter once it is submitted to an application via a selection hyperlink activation.
Each pen has a current nib 824. This is the nib last notified by the pen to the system. In the case of the default netpage pen described above, either the ntarking black ink nib or the non-marking stylus nib is current. Each pen also has a current mb style 825. This is the nib style last associated with the pen by an application, e.g. in ncspome to the ttser selecting a color from a palette. The default nib style is the nib style associated with the current nib. Strokes captured through a pen are tagged with the current nib style. When the strokes are subsequently reproduced, thcy are reproduced in the nib style with which they are tagged.
Whenever the pen is withiq range of a printer with which it can communicate, the pen slowly flashes its "online" LED. When the pen fails to decode a stroke relative to the page, it monterttarily activates its "error" i,ED. When the pen succeeds in decoding a stroke relative to the page, it momentarily activates its "ok" LED.
A sequence of captured strokes is referred to as digital ink. Digital ink fotms the basis for the digital exchange of drawings and handwriting, for online recognition of handwriting, and for online verification of signatures.
7tte pen is wireless and transmits digital ink to the netpage printer via a short-range radio link. The transnritted digital ink is encrypted for privacy and security and packetized for efficient transmission, but is always flushed on a pen-up event to ensure timely handling in the printer.
When the pen is out-of-range of a printer it bufl'ers digital ink in interrtal memory, which has a capacity of over ten nunutes of continuous handwriting. When the pen is once again within range of a printer, it transfers any buffered digital ink.
A pen can be registered with any number of printers, but because aii state data resides in netpages both on paper and on the network, it is largely immaterial which printer a pen is communicating with at any particular tirne.
A preferred embodiment of the pen is described in greater detail in Section 6 below, with referettce to'Figun:s 8 to 10.
1.7 NETPAGE INTERACTION
The netpage printer 601 receives data relating to a stroke from the pen 101 when the pen is used to interact with a netpage 1. The coded data 3 of the tags 4 is read by the pen when it is used to execute a movement, such as a stroke. The data allows the identity of the particular page and associated interactive element to be determised and an indication of the relative positioning of the pen relative to the page to be obtained. 71-e indicating data is transmitted to -1s-the printer, where it resolves, via the DNS, the page ID 50 of the stroke into the network address of the netpage page server 10 which maintains the torrtsponding page instance 830. It then transmits the stroke to the page server. If the page was recently identified in an earlier stroke, then the printer may already have the address of the relevant page server in its cache. Each netpage eonsists of a compact page layout maintained petsistentiy by a netpage page server (see below). The page layout refers to objects such as images, fonts and pieces of text, typically stored elsewhere on the netpage network.
When the page server receives the stroke from the pen, it retrieves the page description to which the sttoke applies, and determines which element of the page description the stroke intersects. It is then able to interpret the stroke in the context of the type of the relevant element.
A "click" is a stroke where the distance and time between the pen down position and the subsequent pen up position are both less than some snall maximum. An object which is activated by a dick typically requires a dick to be activated, and accordingly, a longer stroke is ignored. The failure of a pen action, such as a "sloppy" click, to n.gister is indicated by the lack of response from the pen's "ok" LED.
7ltere are two kinds of input elentents in a netpage page description:
hyperlinks and fonn fields. lnput through a fotm field can also trigger the activadon of an associated hyperlink.
1.7.1 Hyperlinks A hyperlink is a means of sending a message to a renate application, and typically elicits a printed response in the netpage system.
A hyperlink element 844 identifies the application 71 which handles activation of the hyperlink a link ID 54 which identifies the hyperlink to the application, an "alias required" flag which asks the system to uteMde the user's application alias ID 65 in the hyperlink activation, and a description which is used when the hyperlink is neorded as a favorite or appears in the user's history. The hyperlink element class diagram is shown in Figure 29.
When a hyperlink is activated, the page server sends a request to an application sontewhene on the network.
The application is identified by an application ID 64, and the application ID
is resolved in the normal way via the DNS.
Therc are three types of hyperlinks: general hyperlinks 863, form hyperlinks 865, and selection hyperlinks 864, as shown in Figure 30. A general hyperlink can implement a request for a linked docununt, or may simply signal a preference to a server. A form hyperlink submits the corresponding form to the application. A
selection hyperlink subntits the emrent selection to the application. If the current selection contains a single-word piece of text, ;'or example, the application may return a single-page document giving the word's meaning within the context in which it appears, or a transladon into a different language. Each hyperlink type is characterized by what information is submitted to the application.
The corresponding hyperlink instance 862 records a transaction ID 55 which can be specific to the page instance on which the hyperlink instance appears. The aransaction ID can identify user-specific data to the application, for example a "shopping cart" of pending purchases maintained by a purchasing application on behalf of the user:
The system includes the pen's current selection 826 in a selection hyperlink activation. The system includes the content of the associated form instance 868 in a form hyperlink activation, although if the hyperlink has its "submit delta" attribute set, only input since the last form subrnission is included.
The system includes an effective return path in all hyperlink activations.
A hyperlittked group 866 is a group elen-ent 838 which has an associated hyperlink, as shown in Figure 31.
When input occurs through any field element in the group, the hyperlink 844 associated with the group is activated. A
hyperlinked group can be used to associate hyperlink behavior with a field such as a checkbox. It can also be used, in conjunction with the "submit delta" attribute of a form hyperlink, to provide continuous input to an application. lt can iherefore be used to support a "blackboard" interaction model, i.e. where input is captttred and therefore shared as soort as it occurs.
1.7.2 Forms A form defines a collection of related input fields used to capture a related set of inputs through a printed netpage. A form allows a user to submit one or more parameters to an application software program running on a server.
A form 867 is a group element 838 in the document hien3rchy. It uldmately contains a set of terminal field elements 839. A form instance 868 represents a printed instance of a form. It eostsists of a set of field instanees 870 which correspond to the field elements 845 of the fomL Each field instance has an associated value 871, whose type depends on the type of the corresponding field element. Each field value records input through a particular printed form instanoe, i.e.
through one or more printed netpages. The form class diagram is shown in Figure 32.
Each form instance has a status 872 which indicates whether the form is aetive, frozen, submitted, void or expired. A form is active when first printed. A form becomes frozen once it is signed. A form becomes submitted once one of its submission hyperlinks has been activated, unless the hyperlink has its "submit delta" attribute set. A form becomes void when the user invokes a voio form, reset form or duplicate form page command. A fotm expires when the time the form has been active exceeds the form's specified lifetime. Whik the form is active, form input is allowed. htput through a form which is not active is instead captured in the background field 833 of the relevant page instance. When the form is active or frozen, form submission is allowed. Any attempt to subniit a form when the form is not active or frozen is rejected, and instead elicits an form status report.
i5 Each form instance is associated (at 59) with any form instances derived trom it, thus providing a vetsion history. This allows all but the latest version of a form in a particular time period to be excluded from a seareh.
All input is captured as digital ink. Digital ink 873 consists of a set of timestamped stroke groups 874,each of which consists of a set of styled strokes 875. Each stroke consists of= a set of tiniestariiped pen positions 876, eaeh of which also includes pen orientation and nib fora. 'ltte digital ink class diagram is shown in Figure 33.
A field element 845 can be a checkbox Tield 877, a text field 878, a drawing field 879, or a signature field 880. The field element class diagram is shown in Figure 34. Any digital ink captured in a field's zone 58 is assigned to the field.
A checkbox field has an associated boolean value 881, as shown in Figure 35.
Any nuvlt (a tick, a cross, a stroke, a fill zigzag. etc.) captured in a checkbox field's zone causes a true value to be assigned to the field's value.
A text field has an associated text value 882, as shown in Figure 36. Any digital ink captured in a text field's zone is automatically converted to text via online handwriting recognition, and the text is assigned to the field's value.
Online handwriting nxognition is well-understood (see for example Tappert, C., C.Y. Suen and T. Wakahara, "7be State of the Art in On-Line Handwriting Recognition", IEEE Transactions on Pattem Analysis and Machine Intelligenoe, Vol.12, No.8, August 1990).
A signature field has an associated digital signature value 883, as shown in Figure 37. Any digital ink capumed in a signature field's zone is automatically verified with respect to the identity of the owner of the pen, and a digital signature of the content of the form of which the field is part is generated and assigned to the field's value. The digital signature is generated using the pen user's private signature key specific to the application which owns the fonn.
Online signature verification is well-understood (see for example Plamondon, R. and G. Loreue, "Autotnatic Siguature Verificadon and Writer Identification - The State of the An", Pattern Recognition, Vol.22,1Vo.2, 1989).
A field element is hidden if its "hidden" attribute is set. A hidden -field element does not have an input zone on a page and does not accept input. It can have an associated field value which is included in the form data when the form containing the field is submitted.
..Fditing" commands, such as strike-duoughs indicating deletion, can also be recognized in form fields.
Because the handwriting recognition algorithm works "online" (i.e. with access to the dynanucs of the pen movenient), rather than "offline" (i.e. with access only to a bitmap of pen markings), it can recognize run-on discretely-written characters with relatively high accuracy, without a writer-dependent training phase. A writer-dependent model of handwriting is automatically gcnerated over time, however, and can be generated up-front if netessary.
Digital ink, as already stated, consists of a sequence of strokes. Any stroke which starts in a particular element's zone is appended to that element's digital ink stream, ready for interpretation. Any stroke not appended to an object's digital ink streatn is appended to the background field's digital ink stream.
Digital ink captured in the background field is interpreted as a selection gesture. Ciraumscription of one or more objects is generally interpreted as a selection of the circvmscribed objects. although the acdral interpratation is application-specific.
Table 2 summarises these various pen interactions with a netpage.
Table 2. Summary of pen interactions with a rtetpage Object Type Pen input Action Hypeiiink 3enerai iick ubmit action to application orm lick ubmit form to appisaation eiectlort lick ttbmit selection to application orm field hedtbox y mark ign tnie to fieki ext andwriting nvett digital ink to text; assign text to field rawing igital ink ign digital ink to field ignature ignature erify digital ink signature; generate digital ignature of form; assign digital signature to d e ircumscription ign digitai ink to current selection The system niaintains a current selection for each pen. The selection consists simply of ahe n-ost recent stroke captured in the background field. The selection is cleared after an inactivity tinteout to ensure predictable behavior.
The raw digital ink captured in every field is retained on the netpage page server and is optionally transmitted with the form data when the form is submitted to the application. This allows the application to interrogate the raw digital ink should it suspect the original conversion, such as the conversion of handwritten text. This can, for example, involve human intervention at the application level for fonns which fail certain application-specific consistency checks. As an extension to this, the entire background anea of a fotm can be designated as a drawing field. The applicMion can then decide, on the basis of the presence of digital ink outside the explicit fields of the form, to route the form to a human operator. on the assumption that the user may have indicated amendments to the filled-in fields outside of those fields.
Figure 38 shows a flowchart of the process of handling pen input relative to a netpage. The process consists of receiving (at 884) a stroke from the pen; identifying (at 885) the page instance 830 to which the page ID 50=in the stroke refers; retrieving (at 886) the page description 5; identifying (at 887) a formatted element 839 whose zone 58 the stroke intersects; determining (at 888) whether the fonnatted element corresponds to a field elemertt, and if so appending (at 892) the received stroke to the digital ink of the field value 871, interpreting (at 893) the accumulated digital ink of the field and determining (at 894) whether the field is part of a hyperlinked group 866 and if so aetivating {at 895) the associated hyperlink; alternatively detem ning (at 889) whether the formatted element corresponds to a hypsrlink element and if so activating (at 895) the coTresponding hyperlink; alternatively, in the absence of an input field or hyperlink, appending (at 890) the received stroke to the digital ink of the background field 833; and copying (at 891) the received stroke to the current selection 826 of the current pen, as maintained by the registration server.
Figure 38a shows a detailed flowchart of step 893 in the process shown in'Figure 38, where the accumulated digital ink of a field is interpreted according to the type of the field. The process consists of detennining (at 896) whether the field is a checkbox and (at 897) whether the digital ink represents a checkmark, and if so assigning Kat 898) a ttue value to the field value; altematively determining (at 899) whether the field is a text field and if so converting {at 900) the digital ink to computer text, with the help of the appropriate registration setver, and assigning (at 901) the converted computer text to the field value; altematively determining (at 902) whether the field is a signature field and if so verifying (at 903) the digital ink as the sigoaatre of the pen's owner, with the help of the appropriate rtgistruion server, cieating (at 904) a digital signature of the contents of the corresponding forat, also with the help of the registration server and using the pen owner's private signature key relating to the corresponding application, and assigning (at 905) the digital signature to the field value.
1.7.3 Page Server Commands A page server command is a conunand which is handled locally by the page serva. It openstes dinxtly on fmrn, page and document instances.
A page server command 907 can be a void form command 908, a duplicate form conunand 909, a reset form command 910, a get form status command 911, a duplicate page eommattd 912, a reset page command 913, a get page status command 914, a duplicate document command 915, a reset document command 916, or a get document status command 917, as shown in Figure 39.
A void fotm comtnautd voids the corrosponding form instancx. A duplicate form command voids the corresponding form instance and then produces an active printed copy of the current form instattce with field values preserved. The copy contains the same hyperlink transaction IDs as the original, and so is indistinguishable from the original to an app6cation. A reset form command voids the corresponding form instance and then produces an active.
printed copy of the form instance with field values discarded. A get form status comtnand produces a printed report on the status of the crorresponding form instance, including who published it, when it was printed, for whom it was printed, and the form status of the form instance.
Since a form hyperlink instance contains a transaction ID, the application has to be involved in producing a new form instance. A button requesting a new form instance is therefore typically implemented as a hyperlink.
A duplicate page command produces a printed copy of the corresponding page instance with the background field value preserved. If the page contains a forTn or is part of a fonm, then the duplicate page command is interpretod as a duplicate form command. A reset page command produces a printed copy of the correspottding page instance with the background field value discarded. If the page contains a form or is part of a fortn, then the reset page command is interpreted as a reset form conunand. A get page status command produces a printed .report on the status of the eorresponding page instance, including who published it, when it was printed, for whom it was printod, and the status of any forms it contains or is part of.
The netpage logo which appears on every netpage is usually associated with a duplicate page element.
When a page instance is duplicated with field values preserved, field values are printed in their native form, i.e. a checkmark appears as a standard checkinark graphic, and text appears as typeset text. Only drawings and signatures appear in their original fonm with a signature accompanied by a atattdard graphic indicating successful signamre verification.
A duplicate docuntent conunand produces a printed copy of the corresponding document instance with background field values preserved. If the document contains any forms, then the duplicate document command duplicates the fotms in the same way a duplicate form command does. A reset document cornmand produces a printed copy of the corresponding document instance with background field values discarded. If the document contains any forms, then the reset docuntent command resets the forms in the sanie way a n.set form connttand does. A get document status eonnnand produces a printed report on the status of the corresponding document instance, including who published it, when it was printed, for whom it was printed, and the status of any forms it contains.
If the page server command's "on selected" attribute is set, then the eommand operates on the page identified by the pen's current selection rather than on the page containing the con nartd. This allows a ntenu of page server commands to be printed. If the target page doesn't contain a page server command element for the designated page server -'20 -command, then the command is ignored.
An app6ca[ion can provide application-specific handling by embedding the relevant paAge server command element in a hyperlinked group. The page server activates the hyperlink associated with the hyperlinked group rather than executing the page server command.
A page server command element is hidden if its "hidden" attribute is set. A
hidden command element does not have an input zone on a page and so cannot be activated directly by a user. It can, however, be acdvated via a page server command embedded in a different page, if that page setver command has its "on selected" attribute set.
1.8 STANDARD FEA7uREs CF NETPnGES
In the preferred fona, each netpage is printed with the netpage logo at the bottom to indicate that it is a netpage and therefore has interactive properties. Tte logo also acts as a copy button. In most cases pressing the logo produces a copy of the page. In the case of a form, the button produces a copy of the entire form. And in the case of a secatre document, such as a ticket or coupon, the button elicits an explanatory note or advertising page.
The default single-page copy function is handled dinxtly by the relevant netpage page server. Special copy flmctions are handled by linking the logo button to an application.
1'rJ 1.9 USER HELP SYSTEtIt In a preferred embodiment, the netpage printer has a single button labelled "Help". When pressed it elicits a single page of information, including:
= status of printer connection = status of printer consumables = top-level help menu = document function menu = top-level netpage network directory The help menu provides a hierarchical manual on how to use the netpage system.
The document function menu includes the following funetioris:
= print a copy of a document = print a ckan copy of a form = print the status of a docunient A document function is initiated by simply pressing the button and then touching any page of the document.
The status of a document indicates who published it and when, to whom it was delivered, and to whom and when it was subsequendy submitted as a form.
The netpage network directory allows dx user to navigate the hierarchy of publicaiions and services on the network. As an altemative, the user can call the netpage network "900" number "yellow pages" and speak to a human operator. The operator can locate the desired document and route it to the user's printer. Depending on the document type, the publisher or the user pays the small "yellow pages" service fee.
71te help page is obviously unavailable if the printer is unable to print. In this case the "error" light is lit and the user can request remote diagnosis over the network.
2 PERSONALIZED PUBUCATION MoDEL
In the following description, news is used as a canonical publication example to illustrate personalization mechanisms in the netpage system. Although news is often used in the limited sense of newspaper and newstnagazine news, the intended scope in the present context is wider.
In the netpage system, the editorial content and the advertising content of a news publication are personalized using different mechanisms. The editorial content is personalized according to the reader's explicitly stated and intplicitly captured interest profile. The advertising content is personalized according to the reader's locality and demographic.

2.1 Ent7oAU-t. PEasotiAt[u7toH
A subscriber can draw on two kinds of news sources: those that deliver news publications, and those that deliver news streams. While news publications are aggregated and edited by the publislw, news streams are aggregated either by a news publisher or by a specialiud news aggregator. News publications typically eorrespond to tiaditional newspapers and newsmagazines, while news streams can be many and varied:
a"raw" news feed from a news sarvice, a cartoon strip, a freelance writer's column, a friend's bulletin board, or the reader's own e-mail.
The netpage publication server supports the publication of edited news publications as well as the aggregation of multiple news streanu. By handling the aggregation and hence the formatting of news streams selected directly by the reader, the server is able to place advertising on pages over which it otherwise has no editorial control.
The subscriber builds a daily newspaper by selecting one or nmre contributing news pubGcations, and cneadng a personalizod version of each. The resulting daily editions are printed and bound together into a single newspaper. The various members of a household typically express their different interests and tastes by selecting differatt daily publications and then customizing them.
For each publication, the reader optionally selects specific sections. Some sections appeat daily, while others appear weekly. The daily sections available from The New York Times online, for example, include "PageOne Plus", "National", "International", "Opinion", "Business'", "Arts/l.iving", "Technology", and "Sports". 71te set of available sections is specific to a publication, as is the default subset.
The reader can extend the daily newspaper by creating custom sections; each one drawing on any number of news streams. Custom sections nright be created for e-mail and friends' announcements ("Personal"), or for monitoring news feeds for specific topics ("Alerts" or "Clippings").
For each section, the reader optionally specifies its size, either qualitatively (e.g. short, mediuss, or long), or numerically (i.e. as a limit on its number of pages). and the desired proportion of advertising, either qualitatively (e.g.
high, normal, low, none), or numerically (i.e. as a percerttage).
3?n mder also optionally expresses a preference for a large number of shorter atticles or a small numba of longer articles. Each article is ideally written (or edited) in both short and long forms to support this preference.
An article niay also be written (or edited) in different versions to match the expected sophistication of the reader, for example to provide children's and adults' versions. The appropriate version is selected aeoording to the reader's age. The reader can specify a"reading age" which takes precedence over their biological age.
The articles which make up each section are selected and prioritized by the editors, and each is assigned a useful lifetime. By default they are delivered to all relevant subscribers, in priority order, subject to space constraints in the subscribers' editions.
In sections where it is appropriate, the reader may optionally enable collaborative filtering. This is then applied to articles which have a sufficiently long lifetime. Each anicle which qualifies for collaborative filtering is printed with rating buttons at the end of the atticle. The buttons can provide an easy choice (e.g. "liked" and "disliked'), making it nwre likely that readers will botha to rate the article.
Articles with high priorities and short lifetimes are therefore effectively considered essential reading by the editors and are delivered to most relevant subscribers.
The reader optionally specifies a serendipity factor, either qualitatively (e.g. do or don't surprise me), or numerically. A high serendipity factor lowers the threshold used for matching during collaborative filtering. A high factor makes it more likely that the corresponding section will be filled to the reader's specified capacity. A different serendipity factor can be specified for different days of the week.
The reader also optionally specifies topics of particular interest within a section, and this nmdifies the priorities assigned by the editors.
The speed of the reader's Internet connection affects the quality at which images can be delivered. The reader optionally specifies a preference for fewer images or smaller images or both.
If the number or size of images is not reduced, then images may be delivered at lower quality (i.e. at lower resolution or with gneater compression).
At a global level, ahe reader specifies how quantities, dates, times and monetary values are localized. This involves specifying whether units are imperial or metric, a local timezone and time format, and a local curnettcy, and whether the localization consist of in situ translation or annotation. These preferences are derived from the reader's locality by default.
To reduce reading difficulties caused by poor eyesight, the reader optionally specifies a global preference for a larger presentation. Both text and images are scaled accordingly, and less information is accommodated on each page.
The language in which a news publication is published, and its corresponding text encoding, is a property of the publication and not a pteference expressed by the user. However. the rtetpage system can be conftgared to provide automatic translation setvices in various guises.
2.2 ADVeansO+G LocAL~ATtoPt AND TARCiETtNc The personalization of the editorial content d'trectly affects the advertising content, because advertising is typically placed to exploit the editorial context. Travel ads, for example, are more likely to appear in a travel section than elsewhere. The value of the editorial content to an adveniser (and therefore to the publisher) lies in its ability to attract large numbers of readers with the right demographics.
Effective advertising is placed on the basis of locality and ddnographics.
Locality determines proximity to particular services, retailers etc., and particular interests and cortcems associated with the local community and environment. Demgraphics determine general interests and preoccupations as well as likely spending patterns.
A news publisher's most profitable product is advertising "space", a multi-dimensional entity determined by the publication's geographic coverage, the size of its neadership, its readetship demographics, and the page area available for advertising.
In the netpage system, the netpage publication server compuses the approximate multi-dimensional size of a publication's saleable advertising space on a per-section basis, taking into account the publication's geographic coverage, the section's readership, the size of each readei s section edition, each reader's advertising proportion, and each reader's demographic.
In comparison with other media, the netpage system allows the advertising space to be defined in,greater detail, and allows smaller pieces of it to be sold separately. It therefore allows it to be sold at closer to its tiue value.
For example, the same advertising "slot" can be sold in varying proportions to several advertisers, with individual readers' pages randomly re,ceiving the advertisement of one advertiser or another, overall preserving the proportion of space sold to each advertiser.
The netpage system allows advertising to be linked directly to detailed product information and online purchasing. It therefore raises the intrinsic value of the advertising space.
Because personalization and iocalization are handled automatically by netpage publication servers, an advertising aggregator can provide arbitrarily broad coverage of both geography and demographics. The subsequent disaggregation is efficient because it is automatic. This mlkes it more cost-effective for publishers to deal with advertising aggregators than to directly capture advertising. Even though the advertising aggregator is taking a proportion of advertising revenue, publishers may find the change profit-neutral because of the greater efficiency of aggregation. The advertising aggregator acts as an intennediary between advertisers and publishers, and may place the same advertisement in multiple publications.
It is wonh noting that ad placement in a netpage publication can be more complex than ad placxment in the publication's traditional counterpart, because the publication's advertising space is more complex. While ignoring the full complexities of negotiations between advertisets, advertising aggregators and publishers, the prefetxd'form of the netpage system provides some automated support for these negotiations, including support for automated auctions of advertising space. Automation is particularly desirable for the placement of advertisements which generate small tunonnts of income, such as small or highly localized advartisements.
Onoe placement has been negotiated, the aggregator captures and edits the advertisement and records it on a netpage ad server. Correspondingly, the publisher records the ad placement on the relevant netpage publication server.
'rJ When the netpage publication server lays out each user's personalized publication, it picks the relevant advertisements from the netpage ad server.
2.3 USER PROFILES
2.3.1 Information Filtering The personalization of news and other publications relies on an assortment of user-specific profile information; including:
= publication cvstonrizations = collaborative filtering vectors = contact details = presentation prefenences The custontization of a publication is typically publication-specific, and so the custoniization information is maintained by the relevant netpage publication server.
A collaborative filtering vector consists of the user's ratings of a number of news items. It is used to correlate different users' interests for the purposes of making recomtnendations.
Although there are benefits to maintainint a single coliaborative filtering vector independentiy of any particular publication, there are two reasons why it is more practieai to maintain a separate vector for each publication: there is likely to be more overlap between the vectors of subscribers to the same publication than between those of subscribers to different publications;
and a publication is likely to want to prosent its users' collaborative filteriag vectors as part of the value of its brand, not to be found elsewhere. Collaborative filtering vectors are therefore also maintained by the relevant netpage publication server.
Contact details, including name. street address, ZIP Code, state, country, telephone numbers, are globa) by nature, and are maintained by a netpage n:gistration server.
Presentation preferences, including those for quantities. dates and times, are likewise global and maintained in the same way.
The localization of advertising relies on the locality indicated in the user's contaa details, while the targetittg of advertising relies on personal information such as date of birth, gender.
marital status, income, profession, eduatioa, or qualitative derivatives such as age range and income range.
For those users who choose to reveal personal information for advertising purposes, the informstion is maintained by the relevant netpage registration server. In the absence of such information. advertising can be targeted on the basis of the demographic associated with the user's ZIP or ZIP+4 Code.
Each user, pen, printer, application provider and applic.ation is assigned its own unique identifier, and the netpage registration server maintains the relationships between them, as shown in 'Figutes 21, 22, 23 ttnd 24. For registration purposes, a publisher is a special kind of application provider, and a publication is a special kind of application.
Each user 800 may be authorized to use any number of printers 802, and each printer may allow any number of users to use it. Each user has a single default printer (at 66), to which periodical publications are delivered by default, whilst pages printed on demand are delivered to the printer through which the user is interacting. The server keeps track of which publishers a user has authorized to print to the user's default printer.
A publisher does not record the ID of any particular printer, but instead resolves the ID when it is required.
When a user subscribes 808 to a publication 807, the publisher 806 (i.e.
application provider 803) is authorized to print to a specified printer or the user's default printer. This authorization can be revoked at any tinie by the user. Each user may have several pens 801, but a pen is specific to a single user. If a ttsa is autlwrized to use a particular printer, then that printer recognizes any of the user's pens.
The pen ID is used to locate the corresponding user profile maintained by a patticuiar netpage regiuration serva, via the DNS in the usual way.
A Web terminal 809 can be authorized to print on a particular netpage printer, allowiog Web pages and netpage documents encounterod during Web browsing to be conveniently ptinted on the neanen netpage printer.
The netpage system can collect, on behalf of a printer provider, fees and commissions on inootoe earned tltmugh publications printed on the provider's printers. Such income an include advettising fees, click-ttuvugh fees, e-commeroe oommissions, and transaction fees. If the ptinter is owned by ghe user, then the usa is the ptinta provider.
Each user also has a netpage account 820 which is used to aocumulate micro-debits aod credits (sudt as those described in the preceding paragraplt); contact details 815, including name, address and telephone numbers; giobal preferences 816, including privacy, delivery and bealization settings; any number of biometric ttioords 817, containiog the user's encoded signstute $18, fingerprint 819 etc; a handwriting modei 819 automatically maintained by the sysoem;
and SET payment cand accounts 821 with which e-commerce payments can be made.
2.3.2 Favwiles List A netpage user can maintain a list 922 of "favorites" - links to useful documents etc. on the netpage network.
The list is maintained by the system on the user's behalf. It is organized. as a hierarchy of foiders 924, a prefemed embodiment of which is shown in the class diagram in Figure 41.
2.3.3 Hisstoty Ust 71te system maintains a history list 929 on each user's behalf, containing links to documents etc. accessed by the user through the netpage system. It is organized as a date-ordenad list, a pekmd embodiment of which is ahown in the class diagram in Figure 42.
2.4 INTELLttEtlT PAGE LAYOUT
The netpage publication server automatically lays out the pagas of each user's pztsonalized publication on a section-by-saxion basis. Since most advertisements are in the form of pre-formatted tectangks, they ane placed on the page before the editorid content.
The advertising ratio for a section can be achieved with wildly varying advertising ratios on individual pages within the seaion, and the ad layout algorithm exploits this. 7be algorithm is con'figured to attempt to eo-locate ciosely tied editorial and advertising content. such as placing ads for toofmg mataial specifically within the publication beeause of a special feature on do-it-yourself roofing repairs.
The editoria! content selected for the user, including text and associated images and graphics, is then laid out according to various aesthetic ruies.
The entire process, including the selection of ads and the selection of editorial eontent, must be itetaled once the layout has converged, to attempt to more closely achieve the user's stated section size prefen:ncx. The section size preferenoe can, however, be matched on average over titne. allowing significant day-to-day variatioos.
2.5 DoCl1MENT FtXiIiAAT
Once the document is laid out, it is encoded for efficient distribution and periiatent storage on the netpage network.
7'he ptimary efficiency mechanism is the separation of information specific to a single user's edition and infommtion shated between multiple users' editions. The specific information consists of the page layout. 'i7te shared infonaation consists of the objects to which the page layout refers, including images, gnspitics, and pioces of text.
A text object contains fully-fonnatted text represented in the Extensible Markup Language (XML) using the Extensible Stylesheet Language (XSL). XSL provides precise control over text formatting indepettdeotly of the region into which the text is being set, which in this cxse is being provided by the layout. The text object eontains embedded language codes to enable automatic translation, and embedded hyphenation ltints to aid_ with paragtaph fomatting.
An itnage object encodes an image in the JPEG 2000 wavelet-based compressed image format. A graphic object encodes a 2D gnaphic in Scalable Vector Gnsphics (SVG) format.
The layout itself consists of a series of placed itnage and graphic objects, linked textflow objects throttgh which text objects flow, hyperlinks and input fields as described above; and watermark regions. These layout objects are summarized in Table 3. The layout uses a compact format suitable for efficient distribution and atorage.

Table 3. Notpage layout objects Layout Attttibute Fonnat oi objsct linked objsct mage osidon mage object ID PEG 2000 raphic osition 3raphic object ID VG
etditow extfiow ID
one ptional text object ID ML/XSL
ink ype lication ID, etc.
ield ype atemtark e 2.6 DocurtErr DtsMunON
As described above, for purposes of efficient distribution and persistent storage on the netpage network, a user-specific page layout is separated from the shared objects to which it refers.
When a subscribed publication is ready to be distributed, the netpage publication server allocates, with the help of the netpage ID server 12, a unique ID for each page, page inatanex, document, and document instanoe.
The server computes a set of optiniized subsets of the shared content and creates a muldcast channel for each subset, and then tags each user-specific layout with the names of the multicast channels which will exny the shated content used by that layout. The server then pointcasts each user's layouts to that user's printer via the appropriate page server, and when the pointcasting is contplete, multicasts the shared content on the specified channels. After receiving its pointcast, each page server and printer subscribes to the multicast channels specified in the page layouts. During the multicasts, each page server and printer extracts from the multicast streams those objects referred to by its page layouts.
The page servers persistently archive the received page layouts and shared content.
Once a printer has received all the objects to which its page layouts refer, the printer re-eteates the fully-populated layout and then rasterizes and prints it.
Under normal cinvrnctances, the printer prints page's faster than they can be delivered. Assuming a quarter of each page is covened with iniages, the average page has a size of kss than 400KB. The printer can thertfore hold in ewes.c of 100 such pages in its intemal 64MB memory, allowing for temporary buffers etc. The printer prints at a rate of one page per sewnd. This is equivalent to 40QKB or about 3Mbit of page data per second, which is similar to the highest expa:ted, tau of page data delivery over a broadband network.
Even ander abnormal ciroamgtances, such as when the printer nms out of papor, it is likely that the user will be able to replenish the paper supply before the pr9nter's 100-page intemal storage capscity is exhausted.
Howeva, if the printer's intarnal menwry does fiil up, then the printer wip be unable to make use of a multicast when it fnst occurs. The netpage publication setver therefore allows printers to submit nequests for re-mtdticsists.
When a critical number of nequests is reoeived or a timeout occurs, the setver re-mtlticasts tbe oonespood'mg altn+ed objeas.
Once a document is printed, a printer can produee m exact duplicate at any time by retrieving iis pW layouts and contents from the relevant page server.
2.7 Ow-DessnND DoctmEmrs When a netpage document is requested on demand, it can be personalized and deGverad in mtch the same way as a periodical. However, since there is no ahaned content, ddivery is made direaiy to the roquesting pinter witbout the use of multicm.
When a non-netpage document is requested on demand, it is not personaliud. and it is delivened via a designated netpage formatting server which reformats it as a netpage docunznt.
A netpage formatting server is a special instanoe of a netp ge publication server. The netpage fotuwtting server has knowledge of various lntemet dowtnent formats, including Adobe's Portable Document Format (PDF), and Hypenext Markup langua e (H'PML). la tite oase of R7ML, it can make use of the higher ruolution of the printed page to present Web page's in a anild-cwhmm forntat, with a table of contents. It can automatically include all Web pages directly linked to the requested page. The usa can tune this behavior via a prefereuce.
The netpage forntatting server makes standard ne"ge behavior, inciuding interactivity and persistence, available on any Intemet document, no matter what its origin and foimat. It hidts knowlcdge of difl'erent document fonnats from both the netpage printer and the netpage page server,'and hides knowledge of the netpage system from Web servers.
S SECtWanr 3.1 CRtrProonAPav Cryptography is used to protect sensitive information, both in storage and in transit, and to authenticate panies to a transaction. There are two classes of cryptography in widespread use: secret-key cryptography and public-key cryptography. The netpage network uses both classes of cryptography.
Secret-key cryptography, also referrcd to as sytnmetric cryptography, uses the. ssme key to enarypt and decrypt a message. Two parties wishing to exchange nxasages must first arrange to securely exchange the secret key.
Public-key cryptography, also referred to as asymmetric cryptography, uses two eneryption keys. The two keys arc mathematically related in such a way that any.message encrypted using one key can only be deerypted using the other key. One of these keys is then published, while the other is kept private. The public key is used to euaypt any message intended for the holder of the private key. Once encrypted using the public key, a message exn only be decrypted using the private key. Thus two parties can securely exchange messages without first having to exchmge a secret key. To ensure that the private key is secure, it is normal for the holder of the private key to generate the key pair.
Public-key cryptography can be used to create a digital signtuute. The holder of the private key can eneste a known hash of a message and then encrypt the hash using the private key.
Anyone can then verify that the atctyptal hash constitutes the "signature" of the holder of the private key with respect to that particular message by decrypting the encrypted hash using the public key and verifying tlx hash against the message. lf the signatute is appended to the message, then the recipient of the message can verify bodt that the message is genuine and that it has not been alwed in tiansit.
To make public-key cryptography work, there has to be a way to distribute public keys which prevents impersonation.llris is notmally done using certificate,s and eertificsite authorities. A catificue authority is a ttvsted tltird party which authenticates the connection between a public key and someone's identity. 71te certificate authority verifie.s the person's identity by examining identity documents, and then creates and signs a digital certificate containing the petson's identity details and public key. Anyone who trusts the certificate authotity can use the public key in the cxrtificate with a high degttie of cenainty that it is genuine. 71uy just have to verlfy that the -certt"ficate has indeed been signed by the cxrtificate authority, whose public key is well-known.
ln most transaction environments, public-key cryptograplty is only used to creaie digital signatures and to securely exchange secret session keys. Secaet-key cayptography is used for all other purposes.
In the following discussion, when reference is made to the semre transmission of information between a netpage printer and a server, what actttally happeas is that the printer obtains ahe server's cern'ficxte, authenticates it with reference to the certificate anthority, uses the public key-exclunge key in the oeartificate to exchange a secret session key with the server, and then uses the secret session key to encrypt the message data. A session key, by definition, can have an arbitrarily short lifetime.
32 Nf7FACE PRtNtER SEcuRtrtt Each netpage printer is assigned a pair of unique identifiers at time of manufacture which ase storcd in read-only memory in the printer and in the netpage registration server database.
The first ID 62 is public and uniquely identifies ahe printer on the netpage network. The second ID is secret and is used when the printer is first registered on the network.
When the ptinter connects to the netpage network for the first titne after installation, it creates a signature.
public/private key pair. It transmits the secru ID and the public key securely to the netpage registration server. The server eompares the sa.ret ID against the printer's secret ID recorded in its database, and accepts the registration if the 1Ds match. lc then aeates and signs a certificate containing the printer's public ID and public signaturc key, and stores the certifiexte in the ngistration database.
'llie netpage registration server acts as a oertificate authority ftx netpage printers, since it has aooess to seeret information allowing it to verify printer identity.
When a user subscribes to a publication, a nxs,rd is created in the netpage :egistration server database authorizing the publisher to print the publication to the user's default printer or a specified printer. Every document sent to a printer via a page server is addressed to a particular user and is signed by the publisher using the publisher's private signature key. The page server verifies, via the regisnation databa,se, that the publisher is authorized to deliver the publication to the specified user. The page server verifies the signature using the publisher's public key, obtained frum the publisher's certificate stored in the regisnation database.
The netpage registration server accepts request4 to add printing authorizations to the databa.se. so long as those reqttests ate initiated via a pen rcgistered to the printer.
3.3 NETPAGE PEN SEWRttY
Each netpage pen is assigned a unique identifier at time of msnufacture which is stored in nead-only memory in the pen and in the netpage registnation server database. The pen ID 61 uniquely identifies the pen on the netpage network.
A netpage pen can =7Qrow" a number of netpage printets, and a printer can "know" a number of pens. A pen communicates with a printer via a radio frequency signal whenever it is within range of the printer. Once a pen and printer are registered, they regularly exchange session keys. Whenever the pen transnrits digital ink to the printer, the digital ink is always encrypted using the appropriate session key. Digital ink is never transmitted in the clear.
A pen stores a session key for every printer it knows, indexed by printer ID, and a printer stores a session key for every pen it knows, indexed by pen ID. Both have a large buCfnite storage capacity for session keys, and will fcxget a session key on a least-recently-used basis if necessary.
When a pen comes within range of a printer, the pen and printer diwover wlwther they know eseh othet. if .28-they don't know each other, then the pnnter detemiines whetha it is supposed to know the pen. 7his might be, for example, because the pen belongs to a usa who is registered to use the printer. If the printer is meant to know the pen but doesn't, then it initiates the automatic pen registradon ptoacdure. If the printa isn't meaot to know tbe pen, then it apees with the pen to ignore it until the pen is placed in a dtargittg cup, at which time it initiates the n:gistratlon prooediue.
In addition to its public ID, the pen contains a secnetkey-exchange key. The key-exchange key is also recmded in the netpage registration server database at time of manufacture.
During regist.ration, the pen transnbts its pen ID to the ptinter, and the printer transmits the pen ID to the netpage registration server. The server getterates a session key for the printer and pen to use, and securely transmits the session key to the printer. It also traosmits a copy of the session key encrypted with the pen's key-exchange key. The ptinter stotes the seoion key internally, indexed by the pai ID, and transmits the atcrypted session key to the pen. The pen atotes the session key intarnally, indexad by the printer ID.
Although a fake pen can impersonate a pen in the pen registration protoeol, only a real pen an decrypt the session key tranatrottod by tlu printer.
When a previously unrngistered pen is ftrst registerod, it is of limited use until it is liuked to a user. A
registered but'un-owned" pen is only allowed to be used to request and fill in netpage user and pen registration forms, to register a new user to which the new pen is automatically linked, or to add a new pen to an existing user.
The pen uses secret-key nther than public-key encryption=because of hardware performanee constraints in the pen-$.4 SECt1RE DoCUSAEPIT8 The netpage system supports the delivery of secnre documents such as tickets and coupons. The netpage printer includes a faality to print watenmarks, but will only do so on tequest from publishers who are suitably authorized.
The publisher indicates its authority to print watermarks in its cettificate, which the printer is able to authendatt.
The "watenmark" ptinting process uses an alternative dither matrix in specified "watermark" n:gions of the page. Back-to-back pages contain mirror-image watermark regions which coincide when printed. The ditber matrices used in odd and even pages' watermark regions are designed to produce an intetfetence effect when the regions are viewed together, achieved by looking through the printed sheet.
The effect is sintilar to a watemark in that it is not visible when looking at only one side of the page, and is lost when the page is copied by nortnal means.
Pages of secune documents cannot be c.opied using the built-in netpage copy ntechamsm described in Scetion 1.9 above. This extends to copying netpages on netpage-aware photocopiers.
Secure docatments are typically generated as part of e-commeroe trsosections.
They can thmfose include the user's photograph which was captured when the user registered biometric infomuation with the netpage regiattation server, as described in Section 2.
When presented with a secure netpage document, the recipient can verify its authenticity by rcquKSting its status iq the usual way. The unique ID of a secure document is only valid for the lifetime of the document, and-secure document IDs are allocated non-contiguously to prevent their prediction by opportunistic forgers. A secure document verification pen can be developed with built-in feedback on verification failum to support easy point-of-presentation document verifiadon.
Clearly neither the watermark nor the user's photograph ate secure in a cryptographic arnse. They simply provide a significant obstacle to casual forgery. Online document verification, particularly using a vcrifica[ion pen, provides an added level of security where it is needed, but is still not entirely immune to forgeries.
3.5 Now-REnuou-Tm In the netpage system, forms subnritted by users are delivered reliably to fonns handkts and are persistently archived on netpagc page servers. It is therefore impossible for recipients to repudiate delivery.
E-commerce payments made through the system, as described in'Section 4, are also impossible for the payee to rtpudiate.

4.1 SECURE ELECTftorotc TnANSACTtorr (SFT) 7he netpage system uses the Secure Electmnic Transaction (SET) system as one of its payment systeus. SET, having been developed by MasterCard and Visa, is organized aronnd paymettt cards, and dds is rn8ected in the terminology. However, much of the system is independent of the type of aooounts being used.
In SET, cardholders and nxrchants register with a certi5cate authority and are issued with certificates containing their pubGc signattue keys. The certificate authority verifies a cardholder's registration details with the card issuer as appropriate, and verifies a merchant's registracion details with the acquirer as approprime. Cardholders and merchaats store their respective private signature keys securely on their computers. During the payment prooess, these certificates are used to mutually authenticate a merchant and cardholder, and to authenticate them both to the payment gateway.
SET has not yet been adopted widely, partly because cardholder maintenance of keys and certif'icases is considered burdensome. Interim solutions which maintain cardholder keys and certificates on a server aod give the cardholder access via a password have niet with some success.
4.2 SET PAYMENTS
In the netpage system the netpage registration server acts as a proxy for the netpage user (i.e. the cardholder) in SET paymcnt transactions.
The netpage system uses biometrics to authenticate the user and authorite SET
payments. Because the system is pen-based, the biotnetric used is the user's on-line signature, consisting of time-varying pen position and pressure. A
fingerprint biometric can also be used by designing a fingerprint sensor into the pen, although at a higher cost. The type of biometric used only affects ahe capture of the biometric. not the authoiization aspccts of the system.
The first step to being able to make SET paymarts is to register the user's biometric with the netpage registration server. This is done in a controiled environment, for example a bank, where the biometric can be copturtd at the same time as the user's identity is verified. The biotnetric is captuted and stoted in the registration database, linked to the user's record. The user's photograph is also optionally captuned and linked to the record. 7he SET cardholder registration process is completed, and the resulting private signature key and oatificm an: stot+od in the database. The user's payment card information is also atored, giving ahe netpage registration server enough information to act as the user's proxy in any SET payment aransaction.
When the user eventually supplies the biomettic to complete a paymeot, for ezample by signing a netpage order form, the printer sccutely transmits the order information, the pen ID
and the biometric data to the netpage registrationserver. The server verifies the biometric with respect to the user identified by the pen ID, and from ahen on acts as the user's proxy in completmg the SET paytnont transatxion.
4.3 MKMO-PAYMEDRB
The netpage system includes a nKChanism for micro-payments, to allow the user to be conveniently chatged for printing low-cost documents on demand and for copying eopyright documems, and possibly also to allow the n~ to be reitnbursed for expenses incurrcd in printing advenising material. The latter depends on the level of subsidy alns.dy provided to the user.
When the user registers for e-commerce, a network account is established which aggtegates tnicro-paymeuts.
The user receives a statement on a regular basis, and can settle any outstanding debit balance using the standard payment mechanism.
The network account can be extended to aggregate subscription fees for periodicals, which would also otherwise be presented to the user in the form of individual statewmts.

4.4 TRANSAt:'nONs When a usa requests a netpage in a particular application context, the application is able to embed a user-specific transaction ID 55 in the page. Subsequent input through the page is tagged with the transaction ID, and the application is thereby able to establish an appropriate context for the user's input.
When input occurs through a page which is not user-specific, however, the application must use the user's unique identity to establish a context. A typical example involves adding items from a pre-printed catalog page to the user's virtual "shopping cart". To protect the user's pfivacy, however, the unique user ID 601mown to the netpage system is not divulged to applications. This is to prevent different application providers from easily correlating indepettdently aocumulated behaviaral data The netpage regisnation server instead maitttains an attonymous rr.lationship betwaen a usa and an applicaqon via a unique alias ID 65, as shown in Figure 24. Wheneva the usa activates a hypetiinlt tagged with the registered" aaribute. the netpage page server asks the n.etpage agistradon server to traasLte -tbe associated appl'rcation ID 64, together with the pen ID 61, into an alias ID 65. The alias ID is then submitted to the hyperlinlc's application.
The application maintains state infonamuion indexed by alias ID, and is able to retrieve user-specific state infommtion without knowledge of the globai ideatity of the usa.
The system also maintains an independent certificate and private sigruture key for each of a user's applications, to allow it to sign application tnmsactions on behalf of the user using only application-specific inionaation.
To assist the system in routing product bar code (UPC) "hyperlink"
acxivatioas, the system raoosds a favorite application on behalf of ehe user for any number of product types.
Each application is associated with an applicatiott provider, and the system maintains an aoeount on bebalf of each appliea<ion provider. to allow it to credit and debit the provider for click-through fees etc.
An application provider can be a publisher of periodical subscribed content.
7he system reeads the user's willingness to receive the subscribed publieation, as weU as the expected froquenry of publiation.
4.5 R6SOURCe DESCRIPTtONB AND COPYRlOHT
A preferred embodiment of a resource description class diagram is shown in Figure 40.
Each docvment and content object may be described by one or more resouroe descriptions 342. Resource descriptions use the Dublin Core nietadata element set, which is designed to facilitate discovery of elecxronic. rcsotvces.
Dublin Core rnetadata conforms to the World Wide Web Consertium tW3C) ResourceDescription Framewak (RDF).
A resource description may identify rights holders 920. The netpage system automatically traasfers copyright fees from users to rights holders wlxu users print copyright content.

A communications protocol defines an ordered ezchange of inessages between entities. In the netpage syssem, entities such as pens, printers and servers utilise a set of defined protocols to cooperatively handle user interaction with the netpage system.
Each protocol is illustrated by way of a sequence diagram in which the horit:ontal dimension is used to represent message flow and the vertical dimension is used to n:ptesont time.
Each entity is repnesented by a r+eetangle containing the name of the entity and a verticat column representing the lifeline of the endty. During the time att etuity exists, the lifeline is shown as a dashed line. During the time an entity is active, the lifeline is shown as a double line.
Because the protocols considered here do not create or destroy entities, lifelines are generally cut short as soon as an entity ceases to participate in a protocol.
5.1 SUBSCRIPTION DELIVERY PROTOCOL
A preferred embodiment of a subscription delivery protocol is shown in Figure 43.
A large number of users may subscribe to a periodical publication. Each user's edition may be laid out differently, but marty users' editions will share conunon content such as text objects and image objects. The subscription delivery protocol therefore deGvers document strucWues to individual printers via poiptcast. but delivers shared eonoent objects via multicast.
The application (i.e. pubGsher) fust obtains a document ID 51 for each docament fiom an ID server 12. It then sends each document structure, including its document ID and page descriptions, to the page server 10 responsible for the doatment's newly allocatod ID. It includes its own application ID 64.
the subscriber's alias ID 65, and the relevant set of tratlticast channel names. It signs the message using its private signature key.
The page server uses the application ID and alias ID to obtain from the registration server the corresponding usa ID 60, the ttser's selecxed printer ID 62 (which may be explici8y selected for ahe application. or may be the nsa's default printer), and the applitation's certificate.
'Itte application's certificate allows the page strver to verify the message sigttature.ltte page server's request to the registration serva fails if the application ID and alias ID don't together identify a subscription 808.
The page server then allocates document and page instance IDs and forwards the page descriptions, including page IDs 50, to the printer. It includes the relevant set of multicast channel nantes for the printer to listen to.
It then returns the newly allocated page IDs to the application for future referettce.
Once the application has distributed all of ihe documettt strucNres to the subscribers' selecxed printas via the relevant page servers, it multicasts the various subsets of the shared objects on the previously selected multicast channels.
Both page servers and printers ntomtor the appuopriate multicau channels and receive their required oorttent objects.lhey are then able to populate the previously pointcast document structures. 7his allows the page servers to add complete documents to their databases, and it allows the printers to print the documents.
5.2 HYPt71LINK ACTIVATION PRGTOcoL
A preferred embodiment of a hyperlink activation protocol is shown in Figure 45.
When a user clidcs on a netpage with a netpage pen, the pen conanunicates the click to the nearest netpage printer 601. 7tte click identifies the page and a location on the page. The printer already knows the ID 61 of the pen from the pen connection protocol:
The printer detennines, via the DNS, the network address of the page server l0a handling the particular page ID 50. The address may already be in its cache if the user has recently interacted with the sante page. The printer then forwards the pen ID, its own printer ID 62, the page ID and click loeation to the page server.
The page server loads the page description 5 identified by the page ID and detetmines which input element's zone 58, if any, the click lies in. Assuming the relevant input element is t hyperlink element 844, the page server then obtains the associated application ID 64 and link ID 54, and determines, via the DNS, the network address of dte application server hosting the application 71.
The page server uses the pen ID 61 to obtain the corresponding user ID 60 from the regisuation server 11.
and then allocates a globally unique hyperlink request ID 52 and builds a hyperlink request 934. The hyperlink request class diagram is shown in Figun: 44. The hyperlink rcqttest records the IDs of the requesting user and prirtla, and identifies the clicked hyperlink instance 862. The page server then sends its own server ID 53, the hyperlink request ID, and the link ID to the appGcation.
The application produces a response document according to application-specific logic, and obtains a document ID 51 from an ID server 12. It then sends the document to the page server IOb responsibk for the document's newly allocated ID, together with the requesting page server's ID and the hyperlink request ID.
The second page server sends the hyperlink request ID and application ID to the first page server to obtain the corresponding user ID and printer ID 62. 71te first page server rejects the request if the hyperlink request has expired or is for a different application.
The second page server allocates document instance and page IDs 50, tetums the newly allocated page IDs to the application, adds the complete document to its own database; and finally sends the page descriptions to the requesting Pnnter:
7M hyperlink instancx may include a mesmingful transactiott ID 55. in which pse the first paae server includes the transaction ID in the niessage sent to the application. This allows the application to establish a transacdon-specific context for the hyperlink activation.
If the hyperlink requites a user alias, i..c its "alias required" attribute is set, thea the first page server sends both the pen ID 61 and the hyperlink's applicxtion ID 64 to the regis[radon server 11 to obtain not just the user ID
corresponding to the pen ID but also the alias ID 65 corresponding to the application ID and the usa ID. It includes the alias ID in the message sent to the application, allowing the application to establish a usa-spect'fic context for the hyperlink activation.
5.3 HAtuowRmNc RECOoNmon PROTOCOL
When a user draws a stroke on a netpage with a netpage pm the pea communicates the stroke to the nearest netpage pnnter. 7lte stroke ideubfies the page and a path on the page.
The printer forwards the pen ID 61, its own printer ID 62, the page ID 50 and stroke path to the page server 10 in the usual way.
The page server loads the page description 5 identified by the page ID and detetutines which input element's zone 58, if any, the stroke intersects. Assun ng the relevant input element is a sext field 878, the page server appends the stroke to the text field's digital ink.
After a period of inactivity in the zone of the text field, the page server sends the pen ID and the pending strokes to the registration server 11 for interpretation. The registration server identifas the user conespOndiag to the pen, and uses the user's accunmlated handwtiting ntodel 822 to interpret the strokes as handwritten text. Once it lna emvened the strokes to text, the registration server retutns the text to the requesting page server. The page server appends ehe text to the text value of the text field.

6.4 SKiru-TUitE VERIFtCATtoM PROTOCOL
Assuming the input element whose zone the stroke intersects is a signature field 880, the pa6e server 10 appends the stroke to the signature field's digital ink.
After a period of inactivity in the zone of the signature field, the page server sends the pen ID 61 and the pending strokes to the registration server 11 for verification. It also sends the application ID 164 associated with the form of which the signature field is part, as well as the form ID 56 and the current data content of the form. 'Itte re.gistration aerver identifies the user corresponding to the pen, and uses the user's dynandc signature biomettie 818 to verify the strokes as the user's signature. Once it has verified the signature, the registnrtion server uses the application ID 64 and user ID 60 to identify the user's application-specific private signature key.
It then uses the key to generate a digital signature of the form data, and returns the digital signature to the requesting page server. The page server assigns the digital signature to the signature field and sets the associated form's status to fmzen.
The digital signature includes the alias ID 65 of the corresponding user. This allows a single fortn to cwhue multiple users' signatures.
5.5 FORM SUBMISSION PROTOCOL
A prefetred embodiment of a form submission protoool is shown in Figure 46.
Fotm submission occurs via a form hyperlink activation. It thus follows the protocol defined in Section 3.2, with some form-specific additions.
In the case of a form hyperlink, the hyperlink activation message sent by the page serva 10 to the application 71 also contains the form ID 56 and the current data content of the form. If the form contains any signature fields, then the appiication verifies each one by extracting the alias ID 65 associated with the corresponding digital signature and obtaining the corresponding certificate from the registration server 11.

5.6 COMMISBION PAYMEtR PROTOCOL
A prefenrcd embodimeat of a commission payment protocol is shown in Figure 47.
In an e-commerce environment, fees and commission.s may be payable from an appliration pmvider to a publisher on dick-throughs, transactions and sales. Commissions on fees and eommissions on commissions may also be payable fmm the publisher to dw provider of dw ptinter.
The hyperlink request ID 52 is used to route a fee or eommission credit from the target application provider 70a (e.g. meadumt) to the souroe application provider 70b (i.c pubGsher), and 6om the sooroe applicmion provida 70b to the printer provider 72.
'Ihe target application reeeives the hyperlink request ID from the page server 10 when the hyperlink is fttat activated. as described in Section 5.2. When the target application needs to credit the souree applicatiott provider, it sends the application provider cmdit to the original page server together with the hypedink request ID. 'tlte page server uses the hyperlmlt request ID to identify the source appliation, and aends the credit on to the relevant n:gistnuion server I I
together with the source application ID 64, its own server ID 53, and the hyperlink request ID. The registration server aedits the corresponding application provider's account 827. It also notifies the application provider.
If the application provider needs to credit the printer provider, it sends the printa provider credit to the original page server together with the hyperlink requestID. The page server uses the hyperlink request ID to identify the printa. and sends the cnedit on to the relevant registra6on server together with the printer ID. The registtation server credits the conrcsponding printer provider account 814.
The source application provider is optionally notified of the identity of the target applicxtion provider, and the printa provider of the identity of the source application provider.
6. NETPAGE PEN DEtiCRWT10N
6.1 PEN MECwwtcs Refetring to Fgutes 8 and 9, the pen, generally designated by refenence numeral 101, includes a housing 102 in the form of a plastics moulding having walls 103 defining an interior space 104 for mounting the pen oomponeats. 7Le pen top 105 is in operation rotatably mounted at one end 106 of the housing 102. A semi-transparent cover 107 is seaued to the opposite end 108 of the housing 102. The cover 107 is also of moulded plastics. and is formed hom aend-transparent material in order to enable the user to view Bie staws of the LED
mounted within the housing 102. The eover 107 includes a main part 109 which substantially surrounds the end 108 of the housing 102 and a projecting portion 110 which projects back from the main part 109 and 5ts within a eomesponding slot l11 formed in the walls 103 of the housing 102. A radio antenna 112 is mounted behind the projecting portion 110, within the housing 102. Screw thovads 113 surrounding an aperture 113A on the cover 107 are arranged to receive a metal end piece 114, including corresponding screw ehreads 115. The metal end piece 114 is rn,movable to enable ink camidge replacement.
Also mounted within the cover 107 is a tri-color status 1.ED 116 on a flex PCB
117. The antenna 112 is also mounted on the flex PCB 117. 7he status LED 116 is mounted at the top of the pen 101 fwgood all-aoatnd visibiGty.
The pen can operate both as a normal marking ink pen and as a non-marking stylus. An ink pen cartridge 118 with nib 119 and a stylus 120 with stylus nib 121 we mounted side by side within the housing 102. Either the ink camidge nib 119 or the stylus m'b 121 can be brought forward through open end 122 of dw metal end piece 114, by rotation of the pen top 105. Respective slider bloc#s 123 and 124 are mounted to the ink cartridge 118 and stylus 120, respectively. A rotatable cam barrel 125 is secured to the pen top 105 in operation and arranged to rotate ahenewith.llte cam barrel 125 includes a cam 126 in the form of a slot within the walls 181 of the cam barrel. Cam followers 127 and 128 projecting 6om siider blocks 123 and 124 fit within the cam slot 126. On rotation of the cam barrel 125, the slider blocks 123 or 124 move relative to each other to project either the pen nib 119 or stylus nib 121 out thtough the hole 122 in the ntetal end piece 114. The pen 101 has three states of openuion. By tunning the top 105 through 900 steps, the three states are:

.34-= Stylus 120 nib 121 out;
= ]nk cartridge 118 nib 119 out; and = Neither ink cartridge 118 nib 119 out nor stylus 120 nib 121 out.
A second flex PCB 129, is mounted on an electronics chassis 130 which sits within the housing 102. 7tte second flex PCB 129 mounts an infrared LED 131 for providing infrared radiation for projection dnto the surface. An image sensor 132 is provided mounted on the second flex PCB 129 for receiving reflected radiation from the surface. The scoottd flex PCB 129 also motmts a radio frequettcy chip 133. which includes m RF transndtta and RF receiver, and a controller chip 134 for controlling operation of the pen 101. An optics block 135 (fonmed from moulded clear plastics) sits within the cover 107 and projects an infrared beam onto the surface and nxeives images onto the image sensor 132.
Power supply wires 136 connect the components on the seeond flex PCB 129 to batseay eontacts 137 which are mounted within the cam batrel 125. A terminal 138 connects to the battery contacts 137 and the cam barrel M. A three volt rechargeable battery 139 sits within the cam barrel 125 in contact with the battery corttaots. An induction charging coil 140 is mounted about the seoond flex PCB 129 to enable recharging of the battery 139 via itdutxioti. 'Ibe second flex PCB 129 also mounts an infrared 1.ED 143 and infrared photodiode 144 for detecting displacement in the cam batrel 125 when either the stylus 120 or the ink cartridge 118 is used for writing, in order to enable a deterxWnation of the foroe being applied to the surface by the pen nib 119 or stylus nib 121. 'Ilte IR
photodiode 144 detects light from the IR LED
143 via reflectors (not shown) mounted on the slider blocks 123 and 124.
Rubber grip pads 141 and 142 are provided towards the end 108 of the housing 102 to assist gripping the pen 101, and_ top 105 also includes a clip 142 for clipping the pen 101 to a pocket.
5.2 PEN COPftROL.t.ER
The pen 101 is arranged to detern ne the position of its nib (stylus nib 121 or ink cartridge nib 119) by imaging, in the intrared spectrum, an area of the surface in the vicinity of the nib. It records the loatiwt data from tlte nearest location tag, and is ananged to calculate the distance of the nib 121 or 119 from the location tab utilising optics 135 and controller chip 134. The controller chip 134 calculates the orientation of the pen and the nib-to-tag distance'from the perspective distortion observed on the imaged tag.
Utilising the RF chip 133 and antenna 112 the pen 101 can trattsmit the digita) ink data (which is enerypted for security and packaged for efficient transmission) to the computing system.
When the pen is in range of a receiver, the digital ink data is tnansmittcd as it is farmed. When the pen 101 nwves out of range, digital ink data is buffered within the pen 101 (the pen 101 cira ery includes a buffer arranged to store digital ink data for approximately 12 minutes of the pen motion on the surface) and can be transmitted lata.
The controller chip 134 is mounted on the second flex PCB 129 in the pen 101.
Figure 10 is a block diagram illustratirtg in ntore detail the architeeture of the controller chip 134.
Figure 10 also shows reptesentations of the RF chip 133, the image sensor 132, the i-color status LED 116, the IR illuniination LED 131, the 1R force sensar LED 143, and the force sensor photodiode 144.
71re pen cmttroller chip 134 includes a controlling ptooes.wr 145. Bus 146 enables the exchan.ge of data between epmponents of the controller chip 134. Flash menwry 147 and a 512 KB
DRAM 148 are also included. An analog-todigital converter 149 is amnged to convert the analog signal from the force sensor photodiode i44 to a digital sigoal.
An image sensor interfatx 152 interfaces with the image sensor 132. A
transoeiver controller 153 and base band circuit 154 are also included to interface with the RF chip 133 which includes an RF circuit 155 and RF resonators and inductors 156 connected to the antenna 112.
7Tte controlling processor 145 captures and decodes location data from tags from the surface via the image sensor 132, monitors the force sensor photodiode 144, controls the LEDs 116,131 atd 143, aed handles short-range radio commnication via the radio transceiver 153. It is a medium-perfortnattce J-40MHz) general-purpose RISC proeessor.

The processor 145, digital transceiver components (transceiver controller 153 and baseband circuit 154), image sensor interface 152, flash memory 147 and 512KB DRAM 148 are integrated in a single controller ASIC.
Analog RF components (RF circuit 155 and RF resonators and inductors 156) are provided in the separate RF chip.
The image sensor is a 215x215 pixel CCD (such a sensor is produced by Matsushita Electronic Corporation, and is described in a paper by Itakura, K T Nobusada, N Okusenya, R Nagayoshi, and M Ozaki, "A 1 nun 50k-Pixel IT CCD Image Sensor for Miniature Camera System", IEEE Transactions on Electronic Devices, Volt 47, number 1, January 2000) with an IR filter.
The controller ASIC 134 enters a quiescent state after a period of inactivity when the pen 101 is not in contact with a surface. It incorporates a dedicated circuit 150 which monitors the force sensor photodiode 144 and wakes up the controller 134 via the power manager 151 on a pen-down event.
The radio transceiver conununicates in the unlicensed 900MHz band nomnally used by cordless telephones, or alternatively in the unlicensed 2.4GHz industrial, scientific and medical (ISM) band, and uses frequency hopping and collision detection to provide interference-free communication.
In an altemative embodiment, the pen incorporates an Infrared Data Association (IrDA) interface for short-range convnunication with a base station or netpage printer.
In a further embodiment, the pen 101 includes a pair of orthogonal accelerometers mounted in the nomzal plane of the pen 101 axis. The accelerometers 190 are shown in Figures 9 and 10 in ghost outline.
The provision of the accelerometers enables this embodiment of the pen 101 to sense motion without reference to surface location tags, allowing the location tags to be sampled at a lower rate. Each location tag ID can then identify an object of interest rather than a position on the surface. For example, if the object is a user interface input element (e.g. a command button), then the tag ID of each location tag within the area of the input elemont can directly identify the input element.
The acceleration measured by the accelerometers in each of the x and y directions is inkgrated with respect to time to produce an instantaneous velocity and position.
Since the starting position of the stroke is not known, only relative positions within a stroke are calculated.
Although position integration accumulates errors in the sensed acceleration, accelerometers typically have high resolution, and the time duration of a stroke, over which errors accumulate, is short.

7. NETPAGE PRINTER DESCRIPTION
7.1 PRINTER MECHANICS
The vertically-mounted netpage wallprinter 601 is shown fully assembled in Figure 11. It prints netpages on Letter/A4 sized media using duplexed 8'/Z" MemjetT'" print engines 602 and 603, as shown in Figures 12 and 12a. It uses a straight paper path with the paper 604 passing through the duplexed print engines 602 and 603 which print both sides of a sheet simultaneously, in full color and with full bleed.
An integral binding assembly 605 applies a strip of glue along one edge of each printed sheet, allowing it to adhere to the previous sheet when pressed against it. This creates a final bound document 618 which can range in thickness from one sheet to several hundred sheets.
The replaceable ink cartridge 627, shown in Figure 13 coupled with the duplexed print engines, has bladders or chambers for storing fixative, adhesive, and cyan, magenta, yellow, black and infrared inks. The cartridge also contains a micro air filter in a base molding. The micro air filter interfaces with an air pump 638 inside the printer via a hose 639. This provides filtered air to the printheads to prevent ingress of micro particles into the MemjetT"' printheads 350 which might otherwise clog the printhead nozzles. By incorporating the air filter within the cartridge, the operational life of the filter is effectively linked to the life of the cartridge. The ink cartridge is a fully recyclable product with a capacity for printing and gluing 3000 pages (1500 sheets).

Referring to Figure 12, the motorized media pick-up roller assembly 626 pushes the top sheet diretxly from the media tray past a paper sensor on the fitst }xint engine 602 into the duplexed Meayetn printir.ad a4sembly.lbe two MemjetTM print engines 602 and 603 are mounted in an opposing in-line sequential cotttiguration along the sttaight paper path. The paper 604 is drawn into the first print engine 602 by integral, powencd pick-up rollers 626.11te position atd size of the paper 604 is sensed and full bleed printing commences. Fixuive is printed simultatmously to aid drying in the siwstest posolk time The paper exits the first MemjetT" print engine 602 through a set of powered exit spike wheels (aligned along the straight paper path), which act against a rubberined naller. These spike wheels oontact the 'wet' printed snr6ioe and continue to feed the sheet 604 into the second Me4et"' print engine 603.
Refearing to Figures 12 and 12a, the papa 604 passes firom the dupiexad pruu eagines 602 and 603 mto the binder assembly 605. 'Ihe printed page passes betweett a powerrd spike wheel axle 670 with a fibroua support rrolier and another movable txle with spike wheds and a moRnauary acfaa Elue whcel. The movabk axle/ghse assembly 673 is mounted to a metal support bracket and it is transported forward to interface with the powered axk 670 via gears by action of a camsltaR. A separate motor powers this camahaR.
The glue wheel assembly 673 consists of a partially hollow axle 679 with a rotating coupling for the glue supply hose 641 from the ink cartridge 627. This axle 679 eonects to a glue wheel; which absorbs adhosive by eapillary action through radial holes: A molded housing 682 wrrounds the glue wheel, with an opening at the front. Pivoting side moldings and sprung outer doors are attaclied to the metal bracket and hinge out sideways when the rest of the assentbly 673 is thrust forward. This action exposes the glue wheel through the front of the molded housing 682. Tension springs close the assembly and effectively cap the glue wheel during periods of inactivity.
As the sheet 604 passes into the glue wheel assembly 673, adhesive is applied to one vertical edge on the front side (apart from the first sheet of a docttmettt) as it is transported down into the binding assenibly 605.
7.2 PRwrow CoNMot.t.eR Anc.wtECftatE
The netpage printer controller consists of a controlling prooessor 750, a factory-installed or field-installed network interfaoe module 625. a radio transceiver (transceiver controlkr 753, baseltattd circuit 754. RF circuit 755, and RF resonators and inductors 756), dual raster inmge processor (RIP) DSPs 757, duplexed print engine controlkrs 760a and 760b, flash memory 658, and 64MB of DRAM 657, as illuatnftd in Figure 14.
The controlling processor handles communication with the network 19 and with local wireless netpage pens 101, senses the help button 617, controls the user intarfaee LEDs 613-616. and feeds and synchronines tlte RIP DSPs 757 and print engine controllers 760. It consists of a ntedium-perfonmance general-purpose nricroprooeasar. The caterolling pracassor 750 eomawnicates with the princ engine cotwoqers 760 via a high-speed serial bus 659.
The RIP DSPs rasterize and compress page descriptions to the netpage printer's compressed page fomtat.
Each print engine controller cxpands, dithers and prints page inages to its associated Memjet"' ptintltad 350 in real time (i.e. at over 30 pages per minute). The duplexed print engine controllets print both sides of a sheet simultancously.
The master print engine congroller 760a cxmtmis ehe paper trmport and monitots ink usage in conjuntxiott with the master QA chip 665 and the ink cartridge QA chip 761.
The printer controlkr's flash memory 658 holds the softwane for both the prooessor 750 aed the DSPs 757, as well as configuration data. 7tis is copied to main ns;mory 657 at boot dme.
The processor 750, DSPs 757, and digital Iransceiver components (transceiver controller 753 and baseband circuit 754) are integrated in a single controller ASIC 656. Analog RF
components (RF circuit 755 and RF resonatoss and inductors 756) are provided in a separate RF chip 762. The network intcrfaoe module 625 is sepatate, since netpAge printers allow the network connection to be factory-selected or field-selected. Flash meanoty 658 md the 2)056Mbit (64MB) DRAM 657 is also off-chip. The print engine controllers 760 are provided in separue ASICs.

A variety of network interface tnodules 625 are provided, eachproviding a netpage network interface 751 and optionally a local computer or network interface 752. Netpage network Interttet interfaces include POTS tnodertu, Hybrid Fiber-Coax (HFC) cable tnodems. ISDN modems, DSL modems, satallite transoeivers, current and next- etteration ceDular telephone transceivers, and wireless local loop (WLL) transceivers.
Local interfaces include IEEE 1294 (parallel port), lOBase-T and 100Base-T Ethemet, USB and USB 2.0, IEEE 1394 (Firewire), and various emerging home networking interfaas. If an intetnet eonnecxion is available on the local network, then the local network interface can be used as the netpage network interface.
The radio transceiver 753 communicates in the unlicensed 900MHz band nonmally used by cordless telephottes, or alternatively in the unlicensed 2.4GHz industtial, scientific and medical (ISM) band, and uses frequency hopping and oollision detec.tion to provide interfaenae-ftee ootnmunication.
The printa controller optionally incorporates an Infrared Data Association (IrDA) interface for raxiving data "squirted" from devices such as netpage can>cras. In an alternative entbodiment, the printer uses the 1rDA interface for ahort-raage coautwnication with stptably configured netpage pens.
7.2.1 RAsTERtznrnoN AND PRtNTM
Once the main processor 750 has received and verified the document's page layouts and page objects, it runs die appcopriate RIP softwate on the DSPs 757.
The DSPs 757 rasterize each page description and compress the rasterized page image. The ntain processor stores each compressed page intage in memory. The sintplest way to load-balance multiple DSPs is to let each DSP
rasteriu a separate page. The DSPs can always be kept busy since an arbitrary number ol' rastaized pages ~can, in general, be stored in memory. This strategy only leads to potentially poor DSP
utilization when rasterizing short documents.
Watenmark regions in the page description are rasterized to.a corttone-tesolution bi-level bitmap which is losalessly compressed to negligible size and which forms part of the compressed page image.
The inftared (IR) layer of the printed page contains coded netpage tags at a density of about six per inch.
Each tag encodes the page ID, tag ID, and control bits, and the data content of each tag is generated during rasterization 2'rJ and stored in the compressed page image.
The main pmassor 750 passes back-to-back page images to the dupkxed pnnt engne eontrollers 760. fach print engine controller 760 stores ahe compressed page image in its local memory, and starts the page expansion and printing pipeline. Page expansion and printing is pipelined because it is itnpraaical to store an entite 114MB bi-level CMYK+IR page image in memory.
7.2.2 PRIPIT ENCiINE CoN7ROU.ER
7!u page expansion and printing pipeline of the print engine controller 760 consists of a high speed 1EEE
1394 serial interface 659, a standud JPEG decoder 763, a standard Group 4 Fax decoder 764, a custom halftoner/compositor unit 765, a custom tag encoder 766, a line loader/formatter u it 767, and a custom interfaee 768 to the MemjetT" printhead 350.
The print engine controller 360 operates in a double buffered mmner. While one pmge is lotded into DRAM
769 via the high speed serial interface 659, the previously loaded page is read tmm DRAM 7+69 and passed through the prlnt engine controller pipeline. Once the page has finished printing, the page just loaded is printed while another page is loaded Ttte first stage of ahe pipeline expands (at 763) the JPHC'roomprcssed contone CMYK Isyer, expands (at 764) dte t.,mttp 4 Fax-compressed bi-level black layer, and renders (at 766) the bi-level netpage tag layer according to the tag format dcfined in section 1.2, all in parallel. The second stage dithers (at 765) the contone+CMYK layer and composites (at 765) the bi-level black layer over the resnlting bi-level CMYK layer. The tesultant bi-levelCMYK+IR dot data is buffered and fortnatted (at 767) for printing on the Memjet"' printhead 350 via a set of line buffers. Most of these line bufkrs are stored in tha off-chip DRAM. 7be final stage prints the six ctwtnels of bi-kvel dot data (includiitg futative) to the MemjetTM printhead 350 via the printhead interface 768.
When several print engine controllers 760 ane used in tmison, such as in a duplexed configtuation, @tey ue synchronized via a shared line sync signa1770. Only one print engine 760, selected via the extemal master/slave pin 771, generates the line syac signal 770 onto the ahared line.
The print engine controller 760 contains a low-speed processor 772 for'synchmnizing the page expansion and rendering pipeline, configuring the printhead 350 via a low-speed serial bus 773, and controlling the stepper motors 675, 676.
In the 8%" versions of the netpage printer, the two print engines each prints 30 I.etter pega per minute along the long dimension of the page (11"), giving a line rate of 8.8 kliz at 1600 dpi. In=the 12" veraions of the napaae printer, the two print engines each prints 45 Letter pages per minute along the short dimension of the page (8%"); giving a line rate of 10.2 kHz. 7tiesa line rates are well within the operating 6rquency of the MemjetT" prindtead, which in the cturent design exceeds 30 kHz.

8 NETPAOe Tacs a.1 TAG TwNo 8.1.1 Planar Surface Tag Tiling In ordet to support "single-click" interaction with a tagged region via a sensing devim the sensing device must be able to see at least one entire tag 4 in its field of view no matter where in the region or at what orientation it is positioned. The required diameter of the field of view of the sensing device is therefore a function of the size and spacing of the tags 4.
In the case where the tag shape is circular, such as the preferred tag 4 describcd earlier, the ntinimum diameter m of the sensor field of view is obtained when the tags 500, of diametar k. aae tiled on an equilateral triaagitlar grid, as shown in Figure 52 and defined in EQ 1. This is achieved when the center-to-center .tag spacing is the sone as the tag diameter k With a tag dianteter k of 256 dots (-4 mm at 1600 dpi), m is therefore 552 dots (-8.8 tnm). With a quiet area of 16 dds, i.e. an effective tag diameter k of 272 dots (-4.3 nun). m increases to 587 dots (-9.3 amt).
When the tags 4 are moved a distance s apart, where s is at least as large as k, then the minimum field of view is given by EQ 2.
When no overlap is desired in the horizontal direction between suooessive lines of tags 300, for exanqte to make tag rendering easier, the tags must be moved apart by a minimum amount given by EQ 3. For a 256-dot diameter tag, N is therefore 40 dots (-0.6 mm at 1600 dpi). Since this exceeds the quiet area requind t'a the tag, the quiet areavan be ignored if tag lines are rendered to not overlap.
Setting s= k+ u in EQ 2 gives EQ 4. For a Z56-0ot dianseoer tag, s is therefore 296 dots.t-4.7 avn at 1600 dpi). and m is 598 dots (-9.5 mm).
8.1.2 Spherical Surface Tag Tiling A regular icosahedron is often'used as the basis for generating an aln-ost regular triangular tiling of a sphere.
A regular icosahedron, such as icosahedron 526 in Figure 53, is composed of twenty equal-siaed equilateral triangular faces 528 sharing thirty edges 530 and twelve ventices 532, with I'ive of the edges 530 meeting at each of the vertioes 532.
To achieve the required tiling, the icosahedron 526 is inscribed in a target sphere, and each triangle 328 of the icosahedton 526 is subdivided into an equal nuntber of equal-sited equilateral subdivision triaagks to yield the desired total number of triangles. If each edge '530 of the icosahedron is divided into v equal intervals, defining a set of v-1 points along each edge, and each pair of corresponding points along any two adjacent edges is joined by a line parallel to the other shared adjaoent edge. the lines so drawn intensect at the vertices of the desired equal-sized and equilateral subdivision triangles, resulting in the creation of v2 triangles per triangular face 528 of the icosahedron 526, or 20v2 triangles in all. Of the resulting 10Y2 + 2 vertices, five triangular faces meet at each of the twelve original vertices of the icosahedron 526, and six triangular faces meet at the each of the remaining vertices. The twelve original vertices532 already lie on the sphere, while the remaining vertices lie inside the sphere.
Each caeated vertex is therefore centrally ~J projected onto the sphere, giving the desind tiling.
A sphere approximated by a regular polyhedron in this way is referred to as a geodesic, and the parameter v is referred to as the frequency of the geodesic. Figure 54 shows an icpsahedral geodesic 534 with v=3. i.e. with 180 fatxs 528.
The closer a subdivision triangle is to the center of a face of the icosahodron 526, the further it is from the surfacx of the sphere, and hence the larger it is when projected onto the sphere. To nrinimise variation in the size of projected subdivision triangles, subdivison vertices can systematically be displaced prior to projection (Tegmatit, M.. "An Icosahedron-Based Method for Pixelizing the Celestial Sphere", Ap) Lett4rs, 470, L81, October 14, 1996). If v = I then no vertices are created and the angle subtended by a triangular face at a vertex remains 60 . As v incaesses, however, the surface defined by the five triangular faces surrounding each original vertex becomes increasingly flat, and the vertex angie of each triangular face converges on 72 (i.e. 360 / 5). This defines the worst case for a tag tiling of a sphere. In a 72 isosceles triangle the base length is 1.18 times the length of the two sides. The maximum tag spacing s for the purposes of calculating the sensor field of view is therefore close to 1.18k.
With a tag diameter of 256 dots md aquiet area of 16 dots, i.e. an effective tag diameter k of 272 dots (-4.3 mm), m is therefore 643 dots (-10.2 mm) aceording to EQ2.
The angle subtended by each edge of an icosahedron at the center of the circumscribing sphere is given by EQS
For a sphere of radius r the arc length of each centrally projected edge is r9. Given a tag diameter of K in the same units as r, the number of tags n required to cover the sphere is given by EQ 6.
For a given n, r is limited by EQ 7.
If n is limited to 216, to allow the use of a 16-bit tag ID without requiring multiple regions to cover the sphere, and K is taken to be 4.3 nun as above, then r is liniited to -310 mm.
A typical globe has a radius of 160 mm. Its projeaed arc length of -177 mm fits 41 evenly spaced tags with negligible additional spacing. Such a globe uses 16812 tags in total.
e.1.3 Arbitrary Curved Surface Tag T'ilinS
A triangle mesh can approximate a surface of arbitrary topography and topology without intmducing discontinuities or singularities, with the local scale of the mesh being dictated by the local curvature of the surfaoe and an error bound. Assuming the existence of a triangle mesh for a particular surface, an effective non-regular tiling of tags can be produced as long as each mesh triangle respects a nrinimum vertex angle and a minimum edge length. A tilipg is considered effective with n;spect to a particular sensing device if the field of view of the sensing device is guaranteed to 3'rJ include at least one complete tag at any position of the sensing device on the surface.
The tiling procedure starts by placing a tag at each vertex of the mesh, so the minimum edge length is the same as the tag diameter k. The tiling procedure proceeds by inserting a tag at the ntidpoint of any edge whose length exceeds a maximum tag separation s. As illustrated in Figure 9, the maximum tag spacing s is calculated so that if two adjacent tags 4a and 4b are a distance s + E apart, then there is room for another tag 4c between them, i.e. EQ S.
However, if the vertex angle between two edges of length s+ E is less than 60 , then the inserted tags will overlap.
To prevent inserted tags from overlapping, a minimum tag separation t is introduced, where t>_ k. The nvnimum vertex angle a then becomes a function of k and t, as shown in EQ 9.

Clearly, when t= k, 0 is constrained to be 60 . i.e. the mesh is constrained to be equilateral. But as illtutrated in Figme 56, when t> k. 0 catt be kss than 60 without inserted tags overlapping.
The maximum tag separation s must be based on the new minimum tag separation t, in aooordance with EQ
10.
When considering a particular mesh triangle, there are four distinct tag insertion scenarioa. By assumiag t6at the ntinirnum vertex angle is no less than 30 (i.e. half of 60 ), it can be shown that whenever a mesh triangle has at least one edge less than or equal to s in length, the remaining two edges at+e less than 2s in length.. In praaioe the nrinimttm vertex angle is typically at least 45 .
In the first scenario (Figure 57) no edges of a triangle 546 exoeed s in length, so the tagging of the triaagk is already complete.
In the second scenario (Figure 58) one edge 548 of a triangle 550 exceeds s in length. A tag 552 is inserted at the midpoint of the edge 548 to complete the tagging of the triangle 550.
In the third scenario (Figure 59) two edges 554, 556 of a triangie 558 esceed s in iengtlt. Tags 560. 562 are inserted at the midpoint of each of the two long edges 554, 556 and this may complete the tagging of the triangle 558.
Centers of the two inserted tags 560, 562 together with the two vertices 564, 566 of the short edge 568 of tlte original triangle 558 form a trapezoid. If either diagonal of the trapezoid exceeds s in length then a fmal tag 570 is inserted at the centa of the trapezoid to complete the tagging of the triangle.
In the fourth scenario (Figure 60) all three edges 572 of a triangk 573 exceed s in length. A togged vertex 574 is inserted at the midpoint of each edge 572 and the three new vertices 574 are joirted by edges 576. The tagging procedim is then recursively applied to each of the four resuitant triangles 577, 578, 579 and 580. Note that the new triangles respect the minimum vertex angle because they have the same shape as the original triangle 573.
The tag tiling variables are summarized in Table 4.
Table 4. Tag tiling variables variable Meaning inimum vertex angle k diameter m inimum diameter of sensor field of view on surface s 'mum center-to-center tag spacing t inimum center-to-center tag spacing 8.2 TAc SENotc 8.2.1 Pen Orientation To allow a pen-like sensing device to be used as a comforuMe writing insnumt:nt, a range of pea orkntations must be supported. Sinee the pen nib is consuained to be in eotttact with the surface, the orieutation of the pen can be characterized by the yaw (z mtation), pitch (x rotation) and roll (y rotation) of the pen, as illuauated in Pigure 61. While the yaw of the pen must be unconstrained, it is reasonable to constrain the pitch and roll of the pen as well as the overall tilt of the pen resulting from the eombination of pitch and roll.
Yaw is conventionally applied after pitch, such that. for example, in the case of a pen device it would define a twist about the physical axis rather than a direction in the aurface plane. In a pen with a marking nib, hoowever, the image sensor is mounted off the axis of the pen and the pen's image sensing ability (and hence its yaw sensing ability) is therefore constrained unless the pen is held almost vertically, as diswssed below. Yaw is tltemfore applied before pltch.
allowing the full yaw range to be specified by rotating the pen relative to the surface while keeping pitch and roll constant.

Pitch and roll are conventionally defined as y and x rotations, respectively.
Here they are defined as x and y rotations, respectively, because they are defined with respect to the x-y coordinate system of the surface, where the y axis is the natural longitudinal axis and the x axis is the natural lateral axis when viewed by a user. In a right-handed 3D
coordinate system, roll is conventionally defined as positive when anticlockwise, while pitch and yaw are conventionally defined as positive when clockwise. Here all rotations are defined as positive when anticlockwise.
The pen's overaU tilt (0) is related to its pitch (t) and the roU (W) in accordance with EQ 11.
The pen's tilt affects the scale at which surface festures are imaged at different points in the field of view, and therefore affects the resolution of the image sensor. Since it is impractical to sense the area dinxtly under the pen nib, the pen's tilt also affects the distance from the nib to the center of the imaged area. This distance must be known to allow a precise nib position to be derived from the position determined from the tag.
8.2.2 Image Sensing The field of view can be rnodeled as a coru defined by a solid half-angle a(giving an angular field of view of 2a), and an apex height of D above the surface when the optical axis is vertical. Although the image sensor is typically rectangular, only the largest elliptical subarea of the image sensor is relevant to guaranteeing that a sufficiently large part of the surface is imaged, a5 quantified earlier.
The intersection of the field of view cone with the surface defines an elliptical window on the surface. This window is circular when the optical axis is vertical.
Figun: 62 illustrates the geontetric relationship, for a given pitch-related tilt 0 of the pen's optical axis, between the pen's nib (point A), the pen's optical axis (CE), and the field of view window (FH). The tilt is defined to be clockwise positive from the vertical. The equations which follow apply to both positive and negative tilt.
When the pen is not tilted, the window diameter (i.e. I BD I) is given by EQ
12.
lf, when the pen is not dlted, the distancx from the nib to edge of the window (i.e. I AB I) is T. then the distance S from the nib to the center of the window (i.e. I AC I ) is given by EQ 13.
When the pen is tilted by 0, the distance from the viewpoint to the surface along the optical axis is reduoed to d(i.e. I GE ~), given by EQ 14.
71u width of the window (i.e. I FH is then given by EQ 15.
D and a must be chosen so that an adequately large area is imaged throughout the supported tilt range. The required nrinimum diameter m of the area is given by EQ 4, while the width of the actual imaged area is given by EQ 15.
This then gives EQ 16.
Once D and a are deterrnined, an image sensor resolution must be chosen so that the imaged area is adequately sampled, i.e. that the maximum feature frequenry is sampled at its Nyquist rate or higher.
When imaged, the scale of the surface decreases with increasing distance from the viewpoint and with increasing inclination relative to the viewing ray. Both factors have maximum effect at point F for positive tilt and point H
for negative tilt, i.e. at the point in the window fwthest from the viewpoint.
Note that references to F in the following discussion apply to H when the tilt is negative.
The distance of point F from the viewpoint (i.e. I EF is given by EQ 17.
Scaling due to the inclination of the surface relative to the viewing ray through F (EF) is given by EQ 18.
If the surface feature frequency isf, then the angular surface feature frequency m at F (i.e. with respect to the field of view) due to both factors is given by EQ 19.
When there is no object plane tilt (i.e. 0 = 0), this reduces to EQ 20.
The image sensor is, by definition, required to image at least the entire angular field of view. Since the pixel density of the image sensor is uniforni, it must image the entire field of view at maxinwm grequency. Given an angular field of view in iniage space of 2a', an image sensor tilt (i.e. in-age plane tilt) with respect to the optical axis of 0', and a =42-sampling rate of n(where n 2 2 according to Nyquist's theorem), the minimum image sensor resolution q is given by EQ
21 and EQ 22.
Ihe cos-squared term in the numerator in EQ 22 results from the same neasoning as the cos-squared term in the denominator in EQ 19.
When there is no image plane tilt (i.e. 0' = 0), and the image sp9oe and object space augttla fields of view are equal (i.e. a' = a), this reduces to EQ 23 and EQ 24.
When then: is no object plane tilt (i.e. 0= 0) this reduces futther to EQ 25.
Whoe the image plane tilt and the object plaoe tilt are equal (i.e. 0' = 6).
and the image space and object space angular 8eld.c of view are equal (i.e. a' = a), EQ 22 mduoes to EQ 24C
Matching the image plane tilt to the object plane tilt therefore yields a smalkr required image sensor siae than when the image sensor tilt is fixed at zero, and eliminates perspective distortion from the captured image. Variablrimage sensor tilt is, however, a relatively cody option in ptactice, aad also requires geater depth of field.
Figure 63 illustrates the geometric reladonship, for a given mll-related tilt 0 of the pen's optical axis, between the pen's nib (point A), the pen's optical axis (CE), and the field of view window (FH). 71te tilt is again deflned to be clockwise positive from the vertical. With the exception of EQ 13, the preceding equations apply equaqy to roll-induced t91t. For roll-induced tilt the distance S from the nib to the eenter of the window (i.e. I AC I) ia zero rathet than as defined by EQ 13.
For pitch-induced tilt, the magnitude of the tilt range is maximised by choosing a minimum-(ne6ative) tilt and a maximum (positive) tilt which have the sante image sensor requirement.
Since, for pitch-induced dlt, the surface is more distant for negative ttlt than for positive tilt of the same magnitude, the mittimum has a smalkr magttitude than the maximum: For roll-induced tilt they have the same magnitude.
As described above, the smallest features of the tag 4 are the structares which encode the data bits, and tbese have a minimum diameter of 8 dots. This gives a maximum feature fiequency f of about 7.9 per mm at 1600 dpi.
As calculated according to EQ 4 above, an equilatera) triangular tiling of 2S6-dot diametertags with no overlap between successive lines of tags requires a nvnimutn field of view window diameter on the surface oT598 dots, or about 9.5 mm at 1600 dpi.
Most people hold a pen at about +30 pitch and 0 roll. The inking ball of a ball-point nib loses effective eontact with the surface beyond about +50 pitch (i.e. 40 from the horizontal). A reasonable target pitch range is theiefore -100 to +50 , and a reasonabie roll range -30 to +300, bearin' in mind greater limitations on oombiaed pitch and roll as given by EQ 11.
The higlily compact (1.5 nnn') Matsushita CCD image settsor (Matsushita Electronic Corponation, and is described in a paper by ltelcura, K T Notmsada. N Okusenya, R Nagayoshi, and M
Ozaid, "A lmm SOlc Fixel' IT CCD
Image Sensor for Miniature Camera System". IEEE Transactions on Electronic Devices, Volt 47, number 1, January 2000) is suitable for use in a compact device such as a pen. It has an available resolution of 215)Qt5 pixels. Assuming equal image and object space angular fields of view, no image plane tilt, and a nib-to-window distanoe T of 4 mm, optimizing the geometry using EQ 16 and EQ 24 to achieve the desired pitch and roll ranges stated above yields a pitch tange of -16 to +480 (640) and a roll range of -280 to +28 (56 ) with a viewing distattce D of 30 mm and an angular field of view of 18.8 (a = 9.4 ). The availabk pitch range is actaally 21 to +43 . atd tfiis is mapprjd to close to the desired range by pitching the optical axis at -5 relative to the physical axis. Note that the tilt range can be expanded slighdy by optimizing a non-zero tilt of the image plane.
The overall pen tilt is thus confined to an elliptical cone whose major angk is 64 in the pitch plane and whose minor angle is 56 in the roll plane.
The image sensing variables are summarized in Table 5.

.43-Table S. Image sensing variables variable meaning a ' Object space field of view half-artgle oe image space field'of view half-angie y Pon yaw 6 Object plane tiR (i.e. overall pen tift) A' lmage plarte tift ~ Pen pitoh ~r Pen roD
w Angular frequency in field of view D Normal viewing distance d TV" viewing distance Surface feature frequency n Sampling rate q Inmage sensor resolution Distance from nib to center of field of view on surface (when 8 S
= 0) T Distance from nib to edge of field of view on surface (when 8 = 0) 8.3 TAG I)Ecootwo a.s.I Tag Image Processing and Decoding Tag image processing is described earlier in Section 1.2.4. It culminates in knowledge of the 2D perspective ttattsform on the ta& as well as the decoded tag data.
8.3.2 Inferring the Pen Transform Once the 2D perspective tntnsform is obtained which pccounts for the.perspective distmtio.n of the, tag in the captured image, as described earlier, the cotresponding discrete 3D tag transfonn with respect to the pen's optical axis can be inferted, as described below in Section 8.4.
Once the discrete 3D tag transform is known, the cotresponding 3D pen transform can be inferred. i.e. the transform of the pen's physical axis with respect to the surface. The pen's physical axis is the axis which is embodied in the pen's shape and which is experienced by the pen's user. It passes through the nib. The relationship betweea the physical axis and the optical axis is illustrated in Figure 64.
It is convenient to define three coordinate spaces. In sensor space the optical axis coincida with the z axis and the the viewpoint is at the origin. In pen space the physical exis coincides with the z axis and the nib is at the origin.
In tag space the tag 4 lies in the x-y plane with its center at the origin.
The tag transform transforms the tag 4 from tag space to sensor space.
Sensor space is illustrated in Figure 64. 'llte labelling of points in Figure 64 is consistent with the labelling in Figure 62. The viewpoint is at E, the sensed point is at G, and the nib is at A. The intersection point G between the optieal auis and the surfatx is refeffed to as the sensed point. In contrast with the geometry illustrated in Figure 62 vhem the nib is oonsidered as a point, here the nib is considered as a small sphere. If the nib is curved, then the tilt of the physical axis affects the offset between the sensed point and the contact point between the nib and the surface. The center point K of the sphericai nib, about which the physical axis pivots, is refemed to as the pivot point.

The nib makes nominal contact with the surfacx at point A when the optical axis is vestical. KA is defined to be parallel to the optical axis. When the pen is tilted, however, contact is at point L, as shown in Figure 65. Given the radius R of the nib, the distance of the pivot point K from the surface, e.g.
at A or I, is always R.
The discrete tag transform includes the translation of the tag center from the sensed point, the 3D tag rotation, and the translation of the sensed point from the viewpoint.
Given the translation d of the sensed point from the viewpoint in the discrete tag trtiosform, and according to EQ 14, the sensed point is given by EQ 27.
Since the physical axis only differs from the optical axis by a y tnmsladon and x rotatiao (i.e. pitch), the physical axis lies in the y-z plane. With referctroe to F'igme 64, where I AC
S and1EC I= D(just as in Figuse=6~.'), it is clear that in sensor spaee the position of the pivot point is given by EQ 28.
'tlte vector from the sensed point to the pivot point is therefoio given by EQ
29.
711c vector from the pivot point to the aontact point is by definition a siuface nonmal of length R. It is constructed by applying the 3D tag rotadon M to a tag space surface normal, normalizing the result, and scaling by R. as showninEQ30andEQ31.
The vector from the sensed point to the contact point is then obtained in accordance with EQ 37., This is transformed into tag space by applying the, inverse of the tag transform 3D rotation, and is then added to the vector from the tag center to the sensed point, to yield the vector from the tag center to the contact point in tag space, i.e. on the surface, in accordance with EQ 33.
This is finally added to the tag's absolute location, as implied by its tag ID, to yield du nib's desintd absolute location in the tagged mgion: see EQ 34.
The finai step is to infer the pen's 3D oiientation from the tag's 3D
orientation. The pett's discrete rotations are simply the inverses of the tag's discrete rotations, with the pen's pitch also including the effea of the pitch (~,) of the optical axis with respect to the pen's axis, as defined in EQ 3S, EQ 36 and EQ 37.
8.4 INFERRWG THE TAG TRANSFORIr1 The image of the tag 4 captured by the image sensor contains perspective distortion due to the position and orientation of the image sensor with respect to the tag. Once the perspective targets of thc tag are found in image spaoe, an eight-degree-of-freedom perspective transform is inferred based on solving the well-undcrstood equations relating the four tag-space and image-space point pairs. The discrete transform steps which give rise to the image of the tag are concatenated symbolically, and a set of simultaneous non-linear equations is obtained by equating==conesponding terna in the concatenated transform and the perspective transform. Solving these equations yields the discrete tranaform steps, which include the desired tag offset from the nib, 3D tag rotation, and viewpoint offset from the surface.
8.4.1 Modeiing the Tag Transtorm The transform of the tag 4 from tag space to image space can be modeled as a concatettation of the following transform steps:
= x-y translate (by tag-to-viewpoint offset) = z rotate (by mg yaw) = x rotate (by tag pitch)*
== y rotate (by tag roll) = z translate (by tag-to-viewpoint offset) = perspoctive proja:t (with specified focal length) = x-y scale (to viewport size) These are concatenated symbolically to produce a single trausfofm testrix which effects the tag tnnsform Table 7 summarizes the discrete transform variables used in the following sections. together with the lafte of each variable.

Table 7. Discrete transform variables and their ranges Variable Abbrev. Meaning Unh Range transfornt y - yaw 0 0<-ys2x pitch 0 -zt/2 < 0 < a!2 ~ - roll 0 --r/4<yr<rN4 tx A tag-to-viewpoint x offset 0 tr B tag-to-viewpoint y offset 0 COSY C cosine of yaw 1 -1 5 C 51 siny D sine of yaw 0 -1 5 D 51 coo E cosine of pitch 1 0< E 51 sin* F sine of pitch 0 -1 < F< 1 cosw G cosine of roll 1 0< G 51 sinyr H sine of roll 0 -1 < H < 1 t, I tag-to-viewpoint z offset 1<0 1/X J inverse focal length - J> 0 S viewport scale S> 0 Translate in x-y plane by t, and t, according to EQ 42 (where A=t, and B=t,).
Rotate about z by y according to EQ 43 (where 'C=cos(Y) and D=sin(y)), giving EQ 44.
Rotate about x by # according to EQ 45 (where E=cos(y and F=sin(*)). giving EQ
46.
Rotate about y by W according to EQ 47 (where-G=cos(W) and H=sin(W)), giving EQ 48, where K and L are defined by EQ 49 and EQ 50.
Translate in z by t= according to EQ 51 (where /--t=), giving EQ 52.
Perspective project with focal length X and projection plane at z=O according to EQ'S3 (where J=1/A), giving EQ 54.
Scale to viewport by S according to EQ 55, giving EQ 56.
Transform a point in the x-y plane (z. 0) according to EQ 57, giving EQ'S8.
Finally, expand K and L, giving EQ 59.
8.42 2D Perspective Transfonr Given an inferred eight-degree-of-freedom 2D perspective transform matrix as defined in EQ 60, multiply by an unknown f to obtain the general nine-degree-0f-freedom form of the matrix, as shown in EQ 61.
Transform a 2D point according to EQ 62, giving EQ 63.
8.4.8 Inferring the Tag Transform 8.4.3.1 Equating Coefficients Equating the coefficients in EQ 59 with the coefficients in EQ 63 results in EQ 64 to EQ 72, being nine non-linear equations in I I unknowns.
These equations are augmented as required by the trigonometric identity relating the sine and cosine of an angle (i.e. the sine and cosine of any one of yaw, pitch and roll), as shown in EQ 73.

.46-Given the sine and cosine of an angle, the comesponding angle is obtained using a two-erguntent antan as shown in EQ 74.

8.4.32 Solving for X-Y Offset EQ 66 can be simplified using EQ 64 and EQ 65 to give EQ 7S and then EQ 76.
EQ 69 can be simplified using EQ 67 and EQ 68 to give EQ 77 and then EQ 78.
EQ 72 can be simplified using EQ 70 and EQ 71 to give EQ 79 and then EQ 80.
EQ 76 can be re-written as EQ 81, and EQ 78 can be re-written as EQ 82..
Equating EG 81 and EG 82 and solving for B yields EG 83 through EG 85 and finally EQ 86, which defines S.
Substituting the value for B into EG 82 and simplifying yields EC 87 through EG 90 and BnaRyeG 91, which defines A.
This therefore gives the x-y otfset of the tag 4 from the viewpoint, since A=t, and B=t, 8A.3.3 Solving for Pftch From EQ 68, EQ 92 can be obtained.
From EQ 67, EQ 93 can be obtained.
From.EQ 64, EQ 92 and EQ 93, EQ 94 can be obtained.
From EQ 65, EQ 92 and EQ 93. EQ 95 pn be obtained.
From EQ 70, EQ 92 and EQ 93, EQ 96 can be obtained.
From EQ 71, EQ 92 and EQ 93, EQ 97 can be obtained.
From EQ 94, EQ 98 can be obtained.
From EQ 95, EQ 99 can be obtained.
From EQ %, EQ 100 can be obtained.
From EQ 97, EQ 101 can be obtained.
From EQ 98 and EQ 99, EQ 102 and dien EQ 103 can be obtained.
From EQ 100 and EQ 101, EQ 104 and then EQ l0S can be obtained.
From EQ 103 and EQ 105, EQ 106 and then EQ 107 can be obtained.
EQ 107 only has a valid basis if G and H are both non-zero. Since hyj < tr/2;
the cosine ~(G) of the roll is always positive and hence non-zero. The sine of the roll (H) is only non-zero if the roll is non-zero. Specific handling for zero pitch and roll is described in Section 6.7.3.10.
This therefore gives the ntagnitude of the sine of the pitdt, since F=sin(0), and hence the cosine (E) of the pitch by EQ 73, according to EQ 108.
Since M < rr/2, the cosine (E) of the pitch is always positive, so there is no antbiguity when taking the squate root. 7he sign of the sine (F), however, must be determined by other nteans, as described in Sestion 6.73.9.
Given E and F, the pitch is then obtained, according to EQ 109.
8A.3.4 Solving for Roll From EQ 103, EQ 110 can be obtained:
From EQ 73, EQ 111 and then EQ 112 can be obtained.
71tis therefore gives the magnitude of the sine of the roll, since H=sin(yr), and hence the cosine {0 of the roll by EQ 73, according to EQ 113.
Since IyrI <W4, the cosine (G) of the roll is always positive, so there is no ambiguity when taking the square root. The sign of the sine (H), howcver, must be determined by other ttteans, as described in Section 6.7.3.9.
Given G and H, the roll is then obtained according to EQ 114.

8.4.3.5 Solving for Yaw From EQ 73, EQ 92 and EQ 93, EQ 11S and then EQ 116 can be obtained.
From EQ 92 and EQ 116, EQ 117 and then EQ 118 can be obtained.
From EQ 92 and EQ 116, EQ 119 and then EQ 120 can be obtained.
In EQ 116, and hencx EQ 118 and EQ 120, the sign of the square mot is determined by the sign of i, which can be determined from EQ 80, giving EQ 121.
Since! (tz) is negative, J(1A) is positive, and /J < -1 (because then EQ 123 holds.
Given C and D. the yaw is then obtained according to EQ 123.
8.4.9.6 Solving for Viewport Scale The cosine (C) and sine (D) of the yaw are by definition never simultanoously zero. Sinoe the cosine (E) of the pitch is never zero, either EQ 67 or EQ 68 can therefore always be used to detenmine the viewpmt stale jS).
If D is non-zero, then from EQ 67, EQ 124 can be obtained.
Otherwise, if C is non-zero, then from EQ 68, EQ 125 caii be obtained.
8.4.5.7 Solving for Focal Length Similaay, since the cosine (G) of the roll is never zero, either EQ 70 or EQ
71 can be used to determine the inverse focal length (J), so long as either the pitch or roll is non-zero.
However, the signs of the sines (F and Il) of the pitch and roll may not be known. However, the sign of the product (FH) of the sines of the pitch and roU is given by EQ
103, as shown in EQ 126.
The sign can be assigned arbitrarily to F. since the sign of J is known a priori. If gi is non-zero, then from EQ
20. 70, EQ 127 can be obtained.
If hi is non-zero, then from EQ 71, EQ 128 can be obtained.
In practicx, the choice between using EQ 127 and EQ 128 is based on which of gi and hi has the lner magnitude. The inverse focal length is unknown if gi and hi are both zero, i.a if the pitch and roll are both zero.
8.4.3.8 Solving for Z Offaet 25. Once the inverse focal length (J) is known, the z offset (1) is obtained from EQ 80, aooordiag to EQ 129.
Again, the z offset (1) is unknown if the inverse focal length (J) is unknown, i.e. if the pitch and roU are both zero.
8.4.3.9 Determining Direction of Pitch and RoU
The sign of the p r o d u c t ( F H ) of 1he sines of t h e pitch a n d r o ll is given b y E Q 126. Since - A / 4 < w< tt/4, a 30 roll adjustment of +x/4 can be introduced to ensure the roll is aiways positive, without invalidating any other assumptions.
Once the toll adjustment is introduced, EQ 126 gives the sign of the sine (F) of the pitch alone.
The roll adjustment is inunduced as follows. The viewport scale (S), inverse focal len6th {J), rod z offset t/) are all computed as described. A 3D transfonn matrix is cneated from the 2D
perspecxive transform natrix. The.inverses of the viewport scale, focal length projection and z tnanslation are applied to the 3D matrix in reverse otda. The roll 3'rJ adjustmem is then appUed by pre-nwltiplying the matrix by a tt/4 y rotation matrix. The roll, pitch and yaw are coniputed as described. Since the roll is positive, the pitch direction is now known.
'the x/4 roll adjustment is finally subtracted from the roU to give the actuai roil.
When the roll and pitch are both zero, the focal length and z offset are both unknown as described above.
However, in this case there is no need to adjust the roli since the pitch and roU are already krawn.
40 8.4.3.10 Handling Zero Pitch and Roll When either the pitch or roll is zero, the generai solution based on EQ 107 beoomes invalid. The table of Figure 85 shows the 12 degenerate forms of EQ 64 through EQ 71 which result when the yaw is vatiously zero (or A), W2 (or 3a12), and non-zero, and the pitch and roll are variously zero and non-zero.l7te table ofl=igures 86 and 87 sets out the required logic for detecting and handling cases where the pitch and/or roll are zero, with each case motivated by zeros appearing in the table of Figure 85. The cases in the table of Figure 85 are labelled with the case numbers from the table of Figures 86 and 87.
CONCLUSION
The present invention has been described with reference to a preferred embodiment and number of specific alternative embodiments. However, it will be appreciated by those skilled in the relevant fields that a number of other embodiments, differing from those specifically described, will also fail within the spirit and scope of the present invention. Accordingly, it will be understood that the invention is not intended to be limited to the specific embodiments described in the present specification. The scope of the invention is only limited by the attached claims

Claims (59)

1. A region defined in relation to a surface, coded data being disposed within the region, wherein the coded data includes identity data for identifying the region.

2. A region according to claim 1, wherein the coded data takes the form of at least one tag.

3. A region according to claim 2, wherein the tags substantially fill the region.

4. A region according to claim 3, wherein the tags within each region are identical to each other, but are distinct from tags in a plurality of other regions on the surface of on other surfaces for which other regions are defined.

5. A region according to claim 2, wherein the tags within each region are positioned stochastically within that region.

6. A region according to claim 2, wherein the tags within each region are positioned in a regular array.

7. A region according to claim 2, wherein the tags within each region are substantially uniformly distributed within that region.

8. A region according to claim 2, wherein the tags are disposed on the surface such that the relative spacing of their centres is less than about 12mm.

9. A region according to claim 8, wherein the relative spacing is less than about 3mm.

10. A region according to claim 9, wherein the relative spacing is less about 1mm.

11. A region according to claim 9, wherein the region is defined as the entire surface.

12. A region according to claim 2, wherein the surface is defined by a substrate.

13. A region according to claim 12, wherein the substrate is laminar.

14. A region according to claim 2, wherein the tags are disposed at predetermined positions on the surface.

15. A region according to claim 14, wherein the tags are disposed on the surface within a tessellated pattern comprising a plurality of tiles, each of the tiles containing a plurality of the tags.

16. A region according to claim 15, wherein the tiles interlock with each other to substantially cover the surface.

17. A region according to claim 16, wherein the tiles are all of a similar shape.

18. A region according to claim 17, wherein the tiles are triangular, square, rectangular or hexagonal.

19. A region according to claim 15, wherein the tags are disposed stochastically within each of the tiles.

20. A region according to claim 1, wherein the region is identified with sufficient precision to distinguish the region from 1.5 × 10 14 other regions.

21. A region according to claim 2, wherein each of the tags includes at least one common feature in addition to the identity data.

22. A region according to claim 21, wherein the at least one common feature is configured to assist finding and/or recognition of the tags by associated tag reading apparatus.

23. A region according to claim 21, wherein the at least one common feature is represented in a format incorporating redundancy of information.

24. A region according to claim 23, wherein the at least one common feature is rotationally symmetric so as to be rotationally invariant.

25. A region according to claim 23, wherein the at least one common feature is ring-shaped.

26. A region according to claim 2, wherein each of the tags includes at least one orientation feature for enabling a rotational orientation of the tag being read to be ascertained.

27. A region according to claim 26, wherein the at least one orientation feature is represented in a format incorporating redundancy of information.

28. A region according to claim 27, wherein the at least one orientation feature is rotationally asymmetric.

29. A region according to claim 27, wherein the at least one orientation feature is skewed along its major axis.

30. A region according to claim 2, wherein each of the tags includes at least one perspective feature for enabling a perspective distortion of the tag being read to be ascertained.

31. A region according to claim 30, wherein the at least one perspective feature includes at least four sub-features which are not coincident.

32. A region according to claim 2, wherein each tag includes a plurality of tag elements, the identity data being defined by a plurality of the elements.

33. A region according to claim 32, wherein the tag elements are disposed in at least one arcuate band around a central region of each tag.

34. A region according to claim 33, wherein there are a plurality of the arcuate bands disposed concentrically with respect to each other.

35. A region according to claim 34, wherein each element takes the form of a dot having a plurality of possible values.

36. A region according to claim 35, wherein the number of possible values is two.

37. A region according to claim 35, wherein when representing one of the possible values, the tag elements absorb, reflect or fluoresce electromagnetic radiation of a predetermined wavelength or range of wavelengths to a predetermined greater or lesser extent than the surface.

38. A region according to claim 35, wherein the possible values of the tag elements are defined by different relative absorption, reflection or fluorescence of electromagnetic radiation of a predetermined wavelength or range of wavelengths.

39. A region according to claim 35, wherein the tags are not substantially visible to an average unaided human eye under daylight or ambient lighting conditions.

40. A region according to claim 35, wherein the tags are slightly visible to an average unaided human eye under daylight or ambient lighting conditions.

41. A region according to claim 32, wherein the tags are visible to an average unaided human eye under daylight or ambient lighting conditions.

42. A region according to claim 1, wherein the identity data is represented in a format incorporating redundancy of information.

43. A region according to claim 2, wherein the tags are printed onto the surface by means of a printer.

44. A region according to claim 43, wherein the printer is an ink printer.

45. A region according to claim 44, wherein the tags are printed using ink that is absorbent or reflective in the ultraviolet spectrum or the infrared spectrum.

46. A region according to claim 43, wherein the printer also prints additional information onto the surface.

47. A region according to claim 46, wherein the additional information is printed onto the surface using colored or monochrome inks.

48. A region according to claim 47, wherein the additional information is printed onto the surface using one of the following combinations of colored inks:
CMY;
CMYK;
CMYRGB; and spot colour.

49. A region according to claim 2, wherein at least a plurality of the tags are disposed stochastically upon the surface.

50. A region according to claim 49, wherein the identity data of each of the tags includes position data indicating the tag's position in relation to either the surface or a plurality of the other tags.

51. A region according to claim 50, wherein the tags are disposed in a regular array on the surface.

52. A region according to claim 51, wherein the array is triangular.

53. A region according to claim 52, wherein the tags are tiled over the surface

54. A region according to claim 53, wherein the array is rectangular.

55. A region according to claim 54, wherein the tags are tiled over the surface.

56. A region according to claim 2, further including additional non-tag information disposed on the surface.

57. A region according to claim 1, wherein any 10 millimetre diameter subregion of the region includes sufficient coded data to identify the region.

58. A region according to claim 58, wherein any 10 millimetre diameter subregion of the region includes sufficient information to identify at least one point of the region.

59. A surface, including a region according to any one of claims 1 to 58.