How can a computer replace a green backdrop with another scene?

A performer can appear beneath a starry sky while standing in front of a plain green backdrop. The computer does not move the performer into another place. It combines parts of two images, a process called compositing.

The illustration uses a tiny foreground containing 48 square pixels. A pixel is one small image sample. The toy rule marks every pixel with the exact color RGB (0,200,0) as transparent. RGB names its red, green and blue amounts. At each marked position, the composite takes a pixel from the replacement sky instead. Forty green pixels are replaced, while eight performer pixels remain.

The rule selects color, not people. A matching green shirt would also become transparent. A darker green patch would remain under this exact-match rule. Real color-keying tools handle a range of colors and soften edges; lighting, shadows and reflected green can make a convincing composite harder.