Overlapping Transparency

Overlapping Transparency

“Density” is traditionally a fi lm term describing how dark
(opaque) the frame of fi lm is at a given area of the image:
the higher the density, the less light is transmitted. Masks
and alpha channels are also referred to in terms of “density,”
and when two masks or mattes overlap, density can
build up when it should not (with masks) or fail to build
up when it should (with mattes).
Figures 3.21 and 3.22 show the simple solution to a common
problem; the Darken and Lighten mask modes
prevent any pixel from becoming more dense than it is in
the semi-transparent areas of either matte. These modes
should be applied to the masks that are below overlapping
masks in the stack in order to work.