Drag the layer that will be the track
matte directly above the layer that will be the fill layer.
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 a pixel’s luminance value is 100%.
Luma Inverted Matte
Opaque when a pixel’s luminance value is 0%.
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 track matte layer’s name in the Timeline panel. The Composition
panel displays the fill layer viewed through the alpha channel of
the matte layer.
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.