Why doesn’t Set Matte just work like a Track Matte?

Since Adobe added selectable Track Mattes to After Effects, the Set Matte effect has become less useful. But it still comes in handy sometimes, and it still has some quirks that mean it doesn’t work the same way a track matte does, and that confuses people.

Rather than explain all the ins and outs of how Set Matte works, I’ll just explain what you have to do to get predictable results. And that is - both the layer it's applied to and the layer it takes the matte from need to be either the same size as the comp (and centered in it) or a shape layer, text layer or continuously rasterized layer.

Let’s just go over that again. Are both of the two layers involved, one of these types of layer:

• comp-sized-and-centered

• a shape layer

• a text layer

• an Adobe Illustrator layer, with the 🔆 switch turned on

• a pre-composition, with the 🔆 switch turned on

If they are, things should be fine. If not, things probably won’t work how you expect.

Why does it behave like this? It’s because effects (Set Matte is an effect) treat live vector layers (shape, text etc) as though they were exactly comp-sized-and-centered, regardless of how you move, scale or rotate the layer. If both your layers are effectively the same size and in the same position, Set Matte behaves how you expect it to.