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Adobe After Effects CS5 : Expressions - Looping Keyframes

6/19/2013 7:39:36 PM
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The expression language provides two convenient ways to loop a sequence of keyframes: loopOut() and loopIn().

Suppose you keyframed a short animation and you want that sequence to repeat continuously. Simply add this expression to the keyframed property

loopOut("cycle")

and your animation will loop for the duration of the comp (Figure 1).

Figure 1. The solid line in the graph represents the keyframed bounce action. The dotted line represents the subsequent bounces created by loopOut("cycle").

Tip

A small glitch in the cycle version of loopOut() drops the first keyframe from each of the loops. If you want the frame with the first keyframe to be included, add a duplicate of the first keyframe one frame beyond the last keyframe.


There are three other variations of loopOut(), as well:

  • loopOut("pingpong") Runs your animation alternately forward, then backward.

  • loopOut("continue") Extrapolates the animation beyond the last keyframe, so the value of the property keeps moving at the same rate (and in the same direction, if you’re animating a spatial property such as Position) as the last keyframe. This can be useful, for example, if you’re tracking an object that has moved offscreen and you want After Effects to extrapolate where it would be if it kept moving at the same speed and in the same direction.

  • loopOut("offset") Works similarly to "cycle" except that instead of returning to the value of the first keyframe, each loop of the animation is offset by an amount equal to the value at the end of the previous loop. This produces a cumulative or stair-step effect.

loopIn() operates the same way as loopOut(), except that the looping occurs before the first keyframe instead of after the last keyframe. Both loopIn() and loopOut() will accept a second, optional parameter that specifies how many keyframes to loop. Actually, it’s easier to think of it as how many keyframed segments to loop. For loopOut() the segments are counted from the last keyframe toward the layer’s In point. For loopIn() the segments are counted from the first keyframe toward the layer’s Out point. If you leave this parameter out (or specify it as 0), all keyframes are looped. For example, this variation loops the segment bounded by the last and next-to-last keyframes:

loopOut("cycle",1)

Two variations on the expressions—loopOutDuration() and loopInDuration()—enable you to specify the time (in seconds) as the second parameter instead of the number of keyframed segments to be looped. For loopOutDuration(), the time is measured from the last keyframe toward the layer’s In point. For loopInDuration(), the time is measured from the first keyframe toward the layer’s Out point. For example, this expression loops the two-second interval prior to the last keyframe:

loopOutDuration("cycle",2)

If you leave out the second parameter (or specify it as 0), the entire interval between the layer’s In point and the last keyframe will be looped for loopOutDuration(). For loopInDuration(), the interval from the first keyframe to the Out point will be looped.

 
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