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Track Mattes

So far, you've explored several ways to define transparent areas in a layer: using the opacity property, alpha channels, masks, and certain layer modes and effects. Yet all of these options are part of a layer itself. Sometimes, you don't want to use the transparency the image provides, or find that creating a mask is impractical. This is especially true if the image you want to use to define transparency is a moving image, or one that doesn't have an alpha channel. Whatever the case, you may simply want to use a separate image to define transparency. Any image used to define transparency in another image is called a matte. Because they use alpha channel or luminance information to create transparency, mattes are usually grayscale images (or images converted into grayscale).

In After Effects, track matte refers to a method of defining transparency using an image layer, called the fill, and a separate matte layer. In the Time Layout window, the matte and fill must be arranged so that the matte is directly above the fill in the stacking order. The matte is only included to define transparency, not to appear in the output. Therefore, its video switch is turned off. The track matte is assigned to the fill layer using the Track Matte pull-down menu in the Switches panel. Transparent areas reveal the underlying image, consisting of lower layers in the stacking order (Figure 12.47 and Figure 12.48).


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