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Track Mattes

Thus far, you've learned to define transparent areas in a layer by using the Opacity property, alpha channels, masks, and certain Layer modes and effects—all options that are part of the layer itself. Sometimes, however, you won't want to use the transparency provided by the image, or you may find that creating a mask is impractical. This is especially true if you want to use a moving image or one that lacks an alpha channel to define transparency. 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 mattes 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 Timeline window, the matte must be directly above the fill in the stacking order. Because the matte is only included to define transparency, not to appear in the output, 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, which consists of lower layers in the stacking order (Figure 12.47 and Figure 12.48).


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