Add and Screen

Add and Screen

Add and Screen modes both brighten the foreground image
while making darker pixels transparent. Screen yields a subtler
blend than Add in normal video color space but does
not work correctly with linear color (details in Chapter 11).
Add mode is every bit as simple as it sounds; the formula is
newPixel = A + B
where A is a pixel from the foreground layer and B is a
background pixel. The result is clipped at 1 for 8- and
16-bit pixels.
Add is incredibly useful with what After Effects calls a
Linearized Working Space, where it perfectly re-creates the
optical effect of combining light values from two images.
It is useful for laying fi re and explosion elements shot in
negative space (against black) into a scene, adding noise or
grain to an element, or any other element that is made up
of light and texture.

Screen mode has an infl uence similar to Add mode’s,
but via a slightly different formula. The pixel values are
inverted, multiplied together, and the result is inverted:
newPixel = 1–((1–A) * (1–B))
Once you discover the truth about working linearized with
a 1.0 gamma, you understand that Screen is a workaround,
a compromise for how colors blend in normal video space.
Screen is most useful in situations where Add would blow
out the highlights too much—glints, fl ares, glow passes.