Adobe After Effects CS4 - Manual (Page 387)

381 USING AFTER EFFECTS CS4
Transparency, opacity, and compositing
Last updated 12/21/2009
Traveling matte
A. Track matte layer: a solid with a rectangular mask, set to Luma Matte. The mask is animated to travel across the screen. B. Fill layer: a solid
with a pattern effect. C. Result: the pattern is seen in the shape of the track matte. This is then composited over an additional image layer.
A track matte only applies to the layer directly beneath it. To apply a track matte to multiple layers, first precompose
the multiple layers, and then apply the track matte to the precomposition layer.
After Effects preserves the order of a layer and its track matte after you duplicate or split the layer. Within the
duplicated or split layers, the track matte layer remains on top of the fill layer. For example, if your composition
contains layers A and B, where A is the track matte and B the fill layer, duplicating or splitting both of these layers
results in the layer order ABAB.
If you animate the position or other transformations of the track matte layer, it’s called a traveling matte. If you want
to animate the track matte and fill layers using identical settings, consider precomposing them.
More Help topics
“ About alpha channels and mattes ” on page 374
“ Split a layer ” on page 152
“ Set Matte effect ” on page 452
Convert a layer into a track matte
The TrkMat menu shares a column with the blending modes menu. To show the TrkMat menu, make sure that the
Modes column is visible. (See “
Columns ” on page 17.)
1 In the Timeline panel, drag the layer to use as the track matte directly above the layer to use as the fill layer.
2 Define transparency for the track matte by choosing one of the following options from the TrkMat menu for the
fill layer:
No Track Matte No transparency created; next layer above acts as a normal layer.
Alpha Matte Opaque when alpha channel pixel value is 100%.
Alpha Inverted Matte Opaque when alpha channel pixel value is 0%.
Luma Matte Opaque when the luminance value of a pixel is 100%.
Luma Inverted Matte Opaque when the luminance value of a pixel is 0%.
A B
C