The Chroma Key effect keys out all image pixels that are similar to a specified key color. When you key out a color value in a clip, that color or range of colors becomes transparent for the entire clip. Control the range of transparent colors by adjusting the tolerance level. You can also feather the edges of the transparent area to create a smooth transition between the transparent and opaque areas.

View full size graphic

Chroma Key effect

A.

Original image

B.

Blue color keyed out

C.

Image on second track

D.

Final composite image