Once you know all about masking and adjustment layers, you are ready to move on to Track Mattes!
A track matte is an invisible layer that is used to control the opacity of the layer directly beneath it. In essence, they replace the alpha channel of the layer they are being applied to.Let's assume you have 2 layers of footage on top of each other. The top layer will, if its opacity is set to 100% and you have not applied any masks, obscure the layer below it entirely.
However, you can add a track matte to the top layer to control its opacity. First, you will need to add a new layer directly on top of the layer you want to use it as a matte for. This layer can be anything you want: text, shapes, a video or another composition. In the following example I simply added a few shapes onto my new layer:
The next thing you will have to do is go to your 'TrkMat' column in the layer properties window and enable the track matte on the layer below your matte. If you cannot see this column, you may have to click on the 'Toggle Mode/Switches' button at the bottom of the screen. Or you can hit F4 on the keyboard
The column shows a combo box with a number of options for the track matte:
I will discuss all these options below, but for now just note that you need to set the track matte option on the layer below your track matte layer. This will turn the layer above it into a track matte and automatically make it invisible (because in the most cases you do not want to see your track matte).
Now let's look at the different type of track mattes in Adobe After Effects.