A foreground live-video image, made up of a "blue back" part, a "hand/arm" part, and a "shadow" part of the "hand/arm" part, is chroma-key composited with a background live-video image. The "blue back" part is displayed to be transparent on the display screen. The "hand/arm" part is displayed to be opaque....http://www.google.es/patents/US5500684?utm_source=gb-gplus-sharePatente US5500684 - Chroma-key live-video compositing circuit
