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.