Selection Types
You may already be familiar with all of the ways to create
layer transparency; here’s a review just in case.
Matte
Mattes do not only apply when keying out the blue or
green from a visual effects shoot (Figure 3.1); high-contrast
(hi-con) mattes are created by maximizing the contrast of
a particular channel or area of the image. You can effectively
matte an image with itself—with its own highlights or
color areas.
Chapter 6 goes into depth about using mattes—the process
involves using pixel values to create transparency.
Alpha Channel
An alpha or transparency channel is just a matte that has
already been created outside of After Effects and embedded
in an imported image. After Effects itself can, of
course, also create alpha and transparency channels in
rendered images (Figure 3.2).
After Effects is unique among compositing applications in
that alpha channel information persists everywhere. Every
image (and viewer) contains three channels of color and a
fourth transparency channel.
After Effects is also unique in that, internally, it assumes
that any edge premultiplication has been removed. Internally,
all alphas in After Effects are straight (see Chapter 1
for a review of how this is determined on import).
Mask
A mask is a vector shape, drawn by hand (Figure 3.3). As
a vector shape, it can be infi nitely scaled without losing
any defi nition, but because it is drawn by hand, an animated
mask has to be considered costlier than procedural
options. The process of hand-animated selection is known
as rotoscoping and is detailed in Chapter 7.
