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(Page 2 of 3 pages for this article < 1 2 3 >) Tuesday, July 05, 2005Visual Rhythm, Part 1: After Effects TricksTechniques for easily creating and coordinating multiple objects.
The Slinky EffectSometimes you don’t want the pure randomness of the wiggle expression; you just want the animation of slave layers to trail behind a master layer - as if they’re slow to react, and then playing catch up. This trick requires the lesser-known expression valueAtTime. To make one layer precisely follow another, first keyframe the master layer, and then use the pick whip tool to connect the position (or again, any other parameter) of the slave layer to the position of the master. This expression should look like: thisComp.layer("master").position You can then add the valueAtTime expression to look at the position of the master layer at a time other than the current time. Say you want the slave to be where the master was at the current time minus 0.1 seconds. Add the following text onto the end of the above expression: .valueAtTime(time - 0.1) If you were to duplicate the slave layer with the above expression, all of the slaves would be delayed by the same amount. To get the slaves to spread out over time, you need to use the layer’s number in the timeline stack to determine how much of a delay each slave should use. Assuming the master was layer #1 and the first slave was layer #2, the complete expression would be: thisComp.layer("master").position.valueAtTime(time - (index-1) * 0.1) Then duplicate the slave as many times as you want to create your slinky effect. Before doing so, you might want to add a Slider Control to the master layer (as we did for the wiggle trick), select “0.1” in your expression, and drag the pick whip to the new slider to replace this hardwired-value with something you can adjust. Beyond that, all of the suggestions we mentioned above about replacing the source for some of the layers and using blending modes applies. Play with the velocity of the master layer’s animation using Easy Ease and the such to see the slinky expand and contract (see below).
The valueAtTime expression can be used to delay the animation of layers, creating slinky-style effects.What if you want to combine the swarm and slinky effects? Take your slinky expression for the slave layer, and add to its end: + wiggle(1, 50) - value
Click here (16 kB .zip) to download an After Effects project that demonstrates these swarming and slinky techniques.
(Page 2 of 3 pages for this article < 1 2 3 >) 
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The final result is shown in at the right. Of course, you can replace the speed value of 1 and the amount value of 50 with Expression Controls to make them easier to adjust later.
