Animating a Walk-Cycle Using Loop Expressions
Dan Ebberts July 27, 2023
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Originally published in 2002
With version 5.5 of After Effects, Adobe has included some new keyframe-looping expressions that can be very useful for cyclic animation and layer-looping. In this tutorial, Dan Ebberts walks us through the basics of how to set up and use these expressions and then we'll use one to loop a walk-cycle as you might do in a simple character animation.
Project file: .sit file not available
The new loop expressions can be a little confusing until you get used to them. Basically, you define the region that you want looped by setting one or more keyframes. Then, the type of loop expression you use determines whether looping occurs before the first keyframe (loop_in) or after the last keyframe (loop_out). The parameters that you provide determine the nature of the looping (cycle, pingpong, offset, or continue) and the duration of the segment that gets looped. An example might make things more clear. Let's say that you have an animated sequence at the start of a layer and you want it to repeat over and over for the duration of the layer. You might use the following expression in each of the animated properties:
loop_out( cycle ,0)
(Note that Adobe's documentation shows these expressions in the form loop_out(type= cycle , num_keyframes= 0), but you can safely leave out the type= and num_keyframes=. I prefer the shorter form and that's what we'll use in this tutorial).
This expression tells After Effects to take the sequence between the first and last keyframes and, starting from the last keyframe, repeat the sequence until the layer's Out point is reached. Note that the second parameter defines the length of the sequence to be looped by telling After Effects how many keyframes (counting backwards from the last one) to loop. A zero in this parameter says to use all keyframes. The expression given above is probably the one you will use most often and it's the one we'll use for the walk-cycle animation.
O.K. Let's Begin Go Ahead and open the loop_project (currently unavailable here) file in After Effects. Then open the intro comp. This comp has layers set up with position keyframes and loop_out expressions applied. You will want to examine the various layers in this comp to see how the loop expressions work. I've set these up so that the keyframes start one second into the comp so that you can see that nothing happens before the first keyframe. Select the cycle_1 layer. As you examine each of these layers, you'll want to make sure that it is the only layer visible (besides grid ) and that you hit the u key to expose the keyframes in the position property and that you open the little twirly so you can see the expression. Step through the keyframe area and beyond. You might expect, since this is supposed to be a cyclic loop, that the segment between the two keyframes would loop over and over. That's not quite what happens as you'll notice as you move past the keyframed segment.
at first frame of new cycle, box does not return to starting position as expected
It seems that the cycle flavor of loop_out works best when the starting and ending positions are the same. Toggle the visibility of cycle_1 off and turn cycle_2 on. You'll see that we've solved this little anomaly by adding one more keyframe and moving the square back to the starting position. Now it works the way you would expect. This is something that you need to keep in mind when using cycle . Toggle off the visibility of cycle_2 and turn on ping_pong . As you step through this layer, you'll notice that it works exactly the way you would expect - the little square moves back-and-forth along its path. Turn off ping_pong and turn on offset . You'll see that the offset version repeats the motion but starts from where the previous cycle left off. Turn off offset and turn on continue . I've changed the path somewhat for this example so you can more easily see the effect. You'll notice that the little square keeps moving with the direction and velocity it had at the last keyframe.
Feel free to experiment with these layers until you feel comfortable with what's going on. I'd recommend changing the expressions to the loop_in variety and seeing that the looping action now occurs before the first keyframe. You might also want to experiment with the num_keyframes parameter, although 0 will probably be what you use the most often. Also try the loop_in_duration and loop_out_duration varieties, where the second parameter is time (in seconds) instead of keyframes.
Go ahead and open the model comp. This comp is an Illustrator file that was imported as a comp. It has a separate body part in each layer. To keep things somewhat simple I made each arm as only one piece and the head is combined with the torso.
exploded view of model showing the pieces
I've taken care of the animation of one cycle of the walk-cycle (since the focus of this tutorial is using the loop expressions, not character animation), but I'll briefly go over what I did. The first step was to move all the anchor points to the center of rotation for that body part. For the feet this would be the ankle, for the lower legs it's the knee, for the upper legs it's the hip joint, and for the arms it's the shoulder joint. I did this in the comp window using the Pan Behind tool.
repositioned anchor points
After the anchor points were in the correct positions, I stitched the character together by establishing parent relationships. Look at the parent column in the time line and you'll see that the torso is the parent of the arms and the upper legs, the upper legs are parents of the lower legs, and the lower legs are parents of the fe
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Animating a Walk-Cycle Using Loop Expressions 
