What are bluescreen, greenscreen and comp A add B effects?
Bluescreen, greenscreen and Comp A add B are effects (transitions in WMM) that ignore a colour of one clip, and overlay the rest onto another clip. So if I had a clip with a blue background and used bluescreen with another clip, the parts of the clip that are not blue will overlay on top of the other clip. Greenscreen ignores the green and Comp A Add B ignores the black background colours of a clip.
