Adobe After Effects CS4 - Manual (Page 388)

382 USING AFTER EFFECTS CS4
Transparency, opacity, and compositing
Last updated 12/21/2009
If you choose an option other than No Track Matte, After Effects converts the next layer above into a track matte, turns
off the video of the track matte layer, and adds a track matte icon
next to the name of the track matte layer in the
Timeline panel.
Note: Although the video is turned off for the matte layer, you can still select the layer to reposition, scale, or rotate it.
Select the layer in the Timeline panel, and then drag the center (indicated by a circle with an X) of the layer in the
Composition panel.
Using a track matte is somewhat similar to using the Preserve Underlying Transparency option, which causes a layer
to get its transparency from the transparency of the composite of the layers below it in the layer stacking order. (See
“
Preserve underlying transparency during compositing ” on page 382.)
Tips for working with track mattes
• Use the Levels effect to increase the contrast between light and dark parts of the matte layer. This reduces the
problem of having a lot of mid-range values, which translate to partial transparency. (Usually, mattes are most
useful when they define areas as entirely transparent or entirely opaque, except at the edges.)
• To use a channel other than the alpha channel of the matte layer as a matte, use one of the Channel effects (such as
the Shift Channels effect) to route the desired channel’s value into the alpha channel.
• To animate a track matte to move with the layer that it’s matting, make the track matte a child of the layer that it’s
matting. (See “
Parent and child layers ” on page 166.)
Online resources about track mattes
Trish and Chris Meyer provide an introduction to mattes in a PDF excerpt from the “All About Track Mattes” chapter
of their book
Creating Motion Graphics with After Effects (4th Edition) .
Harry Frank provides a video tutorial on his graymachine website in which he demonstrates how to use effects on track
matte layers to create custom fade-on transitions.
Preserve underlying transparency during compositing
The Preserve Underlying Transparency option causes a layer to get its transparency from the transparency of the
composite of the layers below it in the layer stacking order. In other words, the opaque areas of the layer with this
option selected appear only when positioned over opaque areas in underlying layers. This behavior is similar to the
behavior of a track matte, except that a track matte can only be a single layer and a track matte must be above the layer
in the layer stacking order. (See “
Track mattes and traveling mattes ” on page 380.)
This option is useful for creating results such as glints or light reflecting off a polished surface.
The behavior of a layer with the Preserve Underlying Transparency option selected is similar to the behavior of a
clipping mask in Adobe Photoshop.
❖ Select the T option in the Modes column for the layer.
Aharon Rabinowitz provides a short video tutorial about the Preserve Underlying Transparency option on the
Creative COW website .
Tim Clapham provides an explanation and demonstration on his website of the Preserve Underlying Transparency
switch.