Image keying is a method which merges two images. The two input images are connected to a switch which is turned according to some information (key) attached to image pixel. Chroma-key is the standard video keying method used in TV industry to select part of real images - e.g. a weather man in front of a blue screen - by using chromaticity as the means of differentiation. However, chroma-key implicitly assumes that the real objects are always in front of the synthesized objects or scene.
In contrast, the z-key uses pixel-by-pixel depth information in the form of a depth map as a switch, thus it is able to separate based on the detailed shape and position of surfaces. Therefore, we can place real and virtual objects into a synthesized image according to their local and global positional relationship.
The z-key method will extend the possibilities of synthesizing virtual reality and make applications such as tele-operation, tele-presence, training systems with simulation, and games playing in virtual reality, possible and/or more effective.