Adobe After Effects - Manual (Page 409)

No Track Matte
Alpha Matte
Alpha Inverted Matte
Luma Matte
Luma Inverted Matte
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.
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 .)
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 transparency created; next layer above acts as a normal layer.
Opaque when alpha channel pixel value is 100%.
Opaque when alpha channel pixel value is 0%.
Opaque when the luminance value of a pixel is 100%.
Opaque when the luminance value of a pixel is 0%.
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
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