A method for compositing a graphic image with a background image to produce translucent effects identifies a key signal for each portion of the graphic image. The key signal is varied according to the amount of translucency desired for the graphics image from completely transparent to completely opaque....http://www.google.com.au/patents/US4827253?utm_source=gb-gplus-sharePatent US4827253 - Video compositing using a software linear keyer
