11 // All About Track Mattes
It is not uncommon for a special effects stock footage CD to supply both the image (left) and a separate matte created from the image (center). However, if you create a track matte using this matte pass, you may end up with fringing (right). Footage courtesy Artbeats/Cloud Chamber.
In a second comp, applying Remove Color Matting helps remove some of the fringe (top). Applying the Simple Choker effect with a value of +3 tightens up the matte (above).
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Unmultiplying a Separate Matte After Effects’ Track Matte feature assumes straight-style alphas. For instance, when using Luma Track matte, it expects the “fill” image will be larger than the matte, so when the track matte is applied, the edges will appear clean (the excess pixels will be outside the matte edge and therefore transparent). If you’re outputting a separate fill and matte from another application, make sure you render with a Straight Alpha, not a Premultiplied Alpha. Some special effects stock footage collections will supply separate mattes to use for their movies. They are separate files because the most common file format is JPEG, which doesn’t support an alpha channel. However, these mattes are often derived from the original footage (such as an explosion), resulting in a premultiplied, rather than straight, alpha, because some of the background gets mixed in with the semitransparent parts of the image. An example of this is the Artbeats Cloud Chamber footage in [Ex.22-Clouds-1/matte]. This example uses a fairly common scenario in which the movie and the matte have been supplied as separate layers. You might also receive a separate fill and matte from a 3D or editing program. In this case, the AB_CloudChamber.mov was shot on film against a black background. No alpha channel was in sight. By modifying the original movie, Artbeats created an accompanying matte movie. However, because the fill and matte movies share the same edge (premultiplied with black), when a track matte is employed using this matte, the result shows a black fringe around the edges. It might be acceptable if you’re compositing against a dark background, but not against a lighter background. To remove the black fringe, or “unmultiply” it, you need to nest this comp into a second comp [Ex.22-Clouds-2/remove fringe]. In the second comp, the Effect > Channel > Remove Color Matting effect is applied, which helps to remove the black fringe. You cannot do this all in one comp, as the Remove Color Matting effect must occur after the track matte is already composited together because it’s an edge effect (see Building a Track Matte Hierarchy earlier in this chapter). If the movie and matte were created precisely from a 3D program, it’s likely that the edge would look fine at this point. But in this case, it helps to shrink the edge further. Therefore, we added Effect > Matte > Simple