Alpha Channels and Premultiplication

Alpha Channels and Premultiplication

One major source of confusion and even derision for After
Effects has to do with its handling of alpha channels and
premultiplication. After Effects has a persistent concept of
the alpha channel as part of every image, and this channel
is expected always to be unmultiplied within After Effects,
whether it originated that way or not.
Any color multiplied into edge pixels is to be removed
upon import (in the Alpha section of the Interpret Footage
dialog), and reintroduced only on output. Provided those
Alpha settings are correct, this works surprisingly well, but
at some point you may need to better understand how to
take control of edge multiplication within After Effects.

Opacity

Opacity

Take two identical layers, no alpha/transparency information
for either layer. Set each layer to 50% Opacity, and the
result does not add up to 100%. Here’s why.
Figure 3.9 shows light fi ltering through two overlapping
sheets of paper. Yes, no expense is spared bringing you
these real-world simulations. Let’s suppose that each
sheet is 75% opaque; 25% of the background light passes
through. Add a second sheet and 25% of 25%—roughly
6%—passes through both layers. It’s not a lot of light, but
it’s not zero; it would take a few more layers of paper to
block out the light completely.


After Effects mimics this behavior, adding fractional and
not whole Opacity values of two or more layers. This is not
how it’s handled in most other compositing applications,
such as Shake, and so it takes some veterans by surprise.