Apple Final Cut Express HD - Manual (Page 786)

786 Part IX Effects
Screen
Like Lighten, Screen also emphasizes the lightest parts of each overlapping image,
except that the midrange color values of both images are mixed together more evenly.
Blacks in either image allow the overlapping image to show through completely.
Darker midrange values underneath a certain threshold allow more of the overlapping
image to show. Whites from both images show through in the resulting image.
The order of two clips affected by the Screen composite mode does not matter.
Suggested uses: The Screen composite mode is especially useful for knocking out the
blacks behind a foreground subject, instead of using a Luma Key. It’s mainly useful
when you want the rest of the foreground subject to be mixed with the background
image as well, based on its brightness. It’s good for glow and lighting effects, and for
simulating reflections. You can also use the Add and Lighten composite modes to
create variations of this effect.
Top layer Bottom layer Screen composite