Gotchas

Gotchas

Even an advanced user has to pay attention when working
in a composition with track mattes. Unlike parented

layers, track mattes do not stay connected with their target
if moved around; they must occupy the layer directly above
in order to work.
After Effects does at least help you in certain ways. Duplicate
a layer (Ctrl+D/Cmd+D) with a track matte activated
and it moves up two layers, above the track matte
layer. Include the track matte when you duplicate and
it also moves up two layers, so layer order is preserved.

There is a work-around that allows a matte layer to be
anywhere in the Timeline, but it contains its own perils.
Effect > Channel > Set Matte not only lets you choose any
layer in the comp as a matte, it keeps track if that layer
moves to a different position. It also offers a few custom
matte handling options regarding how the matte is scaled
and combined. However, nothing you add to the other
layer, including Transform keyframes, is passed through.

Render Order

This brings us to render order with track mattes. In most
cases, adjustments and effects that you apply to the matte
layer are calculated prior to creating the target matte. To
see how this can break, however, try applying a track matte
to another track matte? It works… sometimes, but not
often enough that it should become something you try
unless you’re willing to troubleshoot it.

Conclusion

Now that we’ve examined selections in detail, the next
chapter looks in depth at solving issues such as those
described above for render order; you’ll begin to see how
to use the Timeline as a visual problem-solving tool for
such situations.