Effect

Effect

Many effects generate transparency: some (such as Levels
and Curves) by offering direct adjustment of the alpha
channel, others (in the Channel folder) by creating or
replacing the alpha channel, and still others that generate
images from scratch that can optionally include an alpha
channel.

Combined Techniques

Even an ordinary effects shot will typically combine more
than one of the above techniques. You will typically apply a
garbage matte prior to a color key, or enhance the effect of
a blending mode with a matte.
The art is in knowing which approach to apply for a given
situation, and for this, there is no substitute for a deep
understanding of how they work.

Compositing: Science and Nature

What exactly is happening in a simple A over B composite?
In After Effects, it seems nearly as natural as laying one
object on top of another does in the real world. In most
other compositing applications even A over B is a compositing
operation and that is closer to the truth—a truth that
After Effects obscures in order to make the process more
natural to an artist (particularly one who understands
Photoshop).
Not only that, but there is more to what is going on in the
real world than might be obvious, because of the phenomena
of optics. The three realms of virtual images—the
physical world, human vision, and optics—are interdependent;
if you’re shaky in your understanding of one of them,
it will probably affect all three.
You as a compositor are not supposed to re-create reality
on your computer screen, but rather re-create the way the
camera (and the eye) sees the world. This affects something
even so fundamental as how the edges of things
should look in order for the eye to accept them.