How to Enable Track matte in after effects CC

When you want one layer to show through holes defined by another layer, set up a track matte. For example, you can use a text layer as a track matte for a video layer to allow the video to only show through the shapes defined by the text characters. The underlying layer (the fill layer) gets its transparency values from the values of certain channels in the track matte layer—either its alpha channel or the luminance of its pixels. Defining the transparency of a layer based on the luminance of the track matte’s pixels 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 cases—using alpha channel mattes and using 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 so