While Track Mattes are now used more, you may prefer using the Set Channel or Set Matte filters (with setting shown below), especially when you more access to channel information. These filters are applied to layer which need a new matte or alpha channel. Set Matte, while fully functional, was mainly included for compatability with earlier versions of AE. Set Matte lets you borrow a single channel from another layer to use on the filtered layer. Set Channels is very powerful and allows you to pull a matte or compose a new layer from a number of layer channels or blending modes.

Beacause of the order of how things are caculated in the AE rendering pipeline, here are a few things to remember when using these filters:

1) You may find that when using a Path Text or Basic Text layer as the matte that you'll have to Pre-Compose for AE to pre-calculate the matte.

2) Even if the target layer is scaled, mattes applied with filters should be the same size as the layer with the filter applied to avoid possible artifacts from scaling of the matte (using Stretch Matte to Fit or Stretch Layers to Fit). Below, a movie texture was scaled 50% and Set Matte was applied. The matte was reduced an extra 50% more than needed, even though the "Green Flight" texture was scaled to the same size as the matte.