When you want one layer to show through a hole in another layer, set up a track matte. You’ll need two layers—one to act as a matte, and another to fill the hole in the matte. You can animate either the track matte layer or the fill layer. When you animate the track matte layer, you create a traveling matte. If you want to animate the track matte and fill layers using identical settings, consider precomposing them.

Define transparency in a track matte using values from either its alpha channel or the luminance of its pixels. Using luminance is useful when you want to create a track matte using a layer without an alpha channel or a layer imported from a program that can’t create an alpha channel. In both alpha channel mattes and luminance mattes, pixels with higher values are more transparent. In most cases, you use a high-contrast matte so that areas are either completely transparent or completely opaque. Intermediate shades should appear only where you want partial or gradual transparency, such as along a soft edge.

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.

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.

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 track matte’s shape and added to the image layer, which is below the track matte layer.

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 Work with parent and child layers.)