DE4229084A1 - Automatic system for adjusting contrast of colour monitor - changes in incident light received by sensor result in computing compensation factor that is used to adjust contrast values - Google Patents

Abstract

The colour graphics computer system provides control of the screen contrast to allow for changes in the ambient lighting level. The system has a lens (1) which focusses the ambient light onto a sensor (2) and the output is digitised (14). Maximum and minimum values are compared (18). The individual values for each colour component (8-10) are corrected by factors based upon the measured ambient brightness and are stored in memory (22-26) for transmission to the screen whenever the incident light received by the screen changes, the system responds to adjust the colour component settings. ADVANTAGE - Corrects for ambient brightness.

Description

The following is an extension for color graphics capable computers to adjust the screen contrast to the ambient brightness of the computer workstation.

A comparable extension is not yet known. A automatic contrast adjustment has so far only been used for Liquid crystal displays (LCDs), for example in quartz watches or portable computers are used. The ambient brightness is measured using a photo sensor and the display backlight accordingly controlled.

The state of the art also includes that with RGB monitors Screen contrast can be adjusted using a controller. The contrast setting is changed by changing the Gain factor of the analog red, green and blue components causes. As the contrast decreases, the distance decreases in the RGB color space the colors shown to each other and to the origin of the RGB color space.

The main advantage of automatic contrast adjustment for RGB monitors is now that the readability and the Visibility of the screen content in the event of changing Lighting conditions at the computer workstation kept constant becomes. This makes work on the computer less tiring. Another advantage is the avoidance of unnecessarily high contrast settings. For cathode ray tubes this leads to a marginally lower emission of X-rays because of the speed and thus the kinetic Energy of the electrons when they hit the fluorescent screen is reduced.

The figure shows the block diagram of the contrast control. The incident light is bundled by the lens ( 1 ) and converted into an electrical parameter (resistance or voltage) by the photosensor ( 2 ). The brightness is digitized by the A / D converter ( 14 ). The sampling rate can be chosen to be very low. It is sufficient if an A / D conversion takes place between two vertical blanking gaps. Function block ( 17 ) averages the brightness values so that short-term brightness fluctuations are ignored. The inertia of the contrast adjustment is controlled by counter ( 31 ). This counts the number of vertical blanking gaps. If the counter reading is "n", the counter is reset and the signal "= 0" is activated during a full screen. The larger "n" is selected, the less often the screen contrast is adjusted. The digital brightness information of the A / D converter ( 14 ) is also fed to the minimum / maximum memory ( 16 ). Two pushbuttons ( 4 ; 5 ) are also connected to this, with which the user can indicate whether the maximum or minimum illuminance at the workplace has been reached. A comparator ( 18 ) is connected downstream of the min / max memory and cancels any erroneous swapping of maximum and minimum. Using the comparator, it is even possible to use just one button. The output values of the comparator are stored in the non-volatile configuration memory ( 19 ). The values saved there are retained after the computer is switched off. EEPROMs or battery-backed CMOS RAMs can be used as memory types. The maximum degree of contrast attenuation can be set using the potentiometer ( 3 ). The resistance value is digitized with the A / D converter ( 15 ). The value can also be stored in the configuration memory. This is particularly useful if the configuration memory ( 19 ) has a system bus connection ( 6 ). The three values can then also be changed or queried under program control.

The averaged brightness values are clipped in block ( 20 ). If the brightness value at the input of block ( 20 ) is lower than the brightness minimum stored in the configuration memory, this value is output. If the maximum brightness is exceeded, the maximum is output and for values between minimum and maximum the input value of block ( 20 ) is output. The clipping function ensures that the brightness values are actually in the interval selected by the user.

The main component of the extension is housed in block ( 32 ). The initialization of the static RAMs ( 22 ; 25 ; 28 ) is controlled from there. When the SRAMs are initialized, the address multiplexers ( 21 ; 24 ; 27 ) apply the digital color components ( 8 ; 9 ; 10 ) to the address inputs of the RAMs. The multiplexers ( 23 ; 26 ; 29 ) ensure that the data lines of the memory modules are connected to the inputs of the video digital / analog converter ( 11 ; 12 ; 13 ). If the RAMs are reinitialized during a vertical blanking interval, the data streams change.

Then the address multiplexers ( 21 ; 24 ; 27 ) connect the outputs of the address counter ( 30 ) to the address inputs of the SRAMs. The data lines of the RAM, which are now switched to input, are connected to the outputs of the RAM initializer ( 32 ).

The arithmetic operations of the RAM initializer are as follows shown:

First the relative ambient brightness is determined:

The values of rBrightness are in the closed interval of zero to one.

So that the following equations become more transparent, the following assumptions are made:

• - The resolution of the A / D converter ( 15 ) is 8 bits, this 8-bit value is called the MinFactor
• - The red, green and blue components also have a depth of 8 bits.

MinFactor / 255 is the factor by which the color components are maximal may be weakened.

The factor by which all color components are to be multiplied (MulFactor), now follows:

so:

R ′ = R _* Mul factor
G ′ = G _* Mul factor
B ′ = B _* Mul factor.
(R ', G' and B 'denote the RGB output signals ( 11 ; 12 ; 13 ).

So that integer arithmetic can be used, the color components multiplied by the fraction counter (MulFaktorZ) and then divided by the denominator, so:

R ′ = R _* MulFaktorZ / 255 ("/" is the division sign)
G ′ = G _* MulFaktorZ / 255
B ′ = B _* MulFaktorZ / 255.

In order not to have to divide each incoming color value by 255, offers a trick:

R ′ = R _* MulFaktorZ / 255 _* 256/256
G ′ = G _* MulFaktorZ / 255 _* 256/256
B ′ = B _* MulFaktorZ / 255 _* 256/256.

R ′ = R _* (256 _* MulFaktorZ / 255) / 256
G ′ = G _* (256 _* MulFaktorZ / 255) / 256
B ′ = B _* (256 _* MulFaktorZ / 255) / 256.

The term "256 _* MulFaktorZ / 255" is referred to below as "MulFaktor2".

The integer division by 256 is trivial because it is just the bottom ones 8 bits must be ignored.

The multiplication is avoided by using the SRAMs. The need to initialize the RAMs during a vertical Blanking interval goes like this: In a latch, the above Assumptions must be 16 bits wide, the variable x is held. This is initially set to zero.

The counter ( 30 ) now addresses the memory cell 0 of the three memories and writes the value of x into them. Then x is increased by MulFaktor2 and the higher byte is written into memory cell 1 etc. The initialization is finished when the value 255 _* MulFaktor2 / 256 has been written into memory cell 255.

After initialization, the memory content of the three SRAMs would look like this based on the above assumptions:
Address 0: 0 _* MulFaktor2 / 256
Address 1: 1 _* MulFaktor2 / 256
Address 2: 2 _* MulFaktor2 / 256
Address 3: 3 _* MulFaktor2 / 256
. . .
Address 255: 255 _* MulFaktor2 / 256.

The greater the depth of color, the better the contrast adjustment. A color depth of 8 bits per color component is practical the optimum because it changes the screen contrast very much "Soft" and can be done almost unnoticed by the user. Of course, the color depth can also be chosen lower and the red, green and blue color components can too have different numbers of bits.

In the case of pallet-oriented color graphics systems, the described one can Extensions are mainly program-controlled, since it’s enough after a few frames in the vertical blanking gaps to change the color palette. Of the Circuit requirements for the expansion would then affect the Photo sensor and the associated A / D converter with system interface restrict.  

With true color systems, a more extensive relocation is the Tasks in a program are not possible because only with the additional SRAMs the contrast can be adjusted quickly enough. The access time of the SRAMs to be selected depends entirely on the maximum resolution.

Some functional blocks of the extension (except 21-29) could be well accommodated in a micro-controller.

Claims (1)

Circuit extension for RGB color graphics systems with any color depth to adjust the contrast of the monitor image to the ambient brightness of the computer workstation, characterized by the following features:

• - The contrast is controlled so that it increases in proportion to the ambient brightness.
• - The contrast is controlled by the fact that the digital red, green and blue components, which come from the video logic of the respective computer, are reduced by a certain factor before being forwarded to the video digital / analog converter. By reducing the RGB components, the colors are drawn closer to the origin of the RGB color space and are increasingly less different from each other. In order to achieve an analog effect in the YUV color space, only the V component has to be reduced.
• - The degree of attenuation depends on the value (here: resistance) of a photo sensor, in front of which a convex lens can be mounted for focusing.
• - The scaling down of the color components is done with three (for red, green and blue each) fast memories, preferably with static RAMs. One color component each is connected to the address bus of a memory. The scaled-down color component is then present on the data bus. The three memories are initialized in the vertical blanking interval.