Apple Final Cut Pro 7 - Manual (Page 1204)

Screen
Screen 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 below 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 and is an alternative to using a luma key. Screen is
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
Overlay
Whites and blacks in the foreground image become translucent and interact with the
color values of the background image, causing intensified contrast. Whites and blacks in
the background image, on the other hand, replace overlapping areas in the foreground
image.
Overlapping midrange values are mixed together differently depending on the brightness
of the background color values. Lighter background midrange values are mixed by
screening. Darker background midrange values, on the other hand, are mixed together
by multiplying.
1204 Chapter 71 Compositing and Layering