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Chapter 8. Effective Motion Tracking > Continuing a Track with Expressions

Continuing a Track with Expressions

Ever have your track point disappear before you're done with it? Perhaps you were tracking a point moving across the screen with the target offset such that the tracked point exited frame before the target. Maybe your track point was obscured at some point in the scene, obscured too badly for the tracker to extrapolate motion.

Other effects software might have an option for continuing the track, but in After Effects, you use a simple expression to do it. This will work only in cases where your tracked item is traveling more or less steadily in more or less a singular direction. A classic example is the sign on the side of a bus moving across frame because whether or not you do it with a corner pin, the point you're tracking is likely to exit frame before your added sign (Figure 8.31).


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