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I understand that Absolute Shape Keys are for creating a deformation sequence, and are more practical than Relative Shape Keys when many deformations need to happen one after the next. The trouble is that I'm not sure how to set the timing to match my desired animation length.

1. Why do absolute shape keys increment by 10?

What does the 10 represent? Am I correct in understanding that it is not keyframes, nor seconds, but rather some arbitrary value that gets calculated as a result of the keyframing of the Evaluation Time slider. Why 10 instead of 1?

Furthermore, can this value be nudged to make a shape key come earlier or later relative to the others? The active/selected shape key shows two triangular arrows to the left and right of the value, but they can't be clicked.

2. Why do absolute shape keys need to have their timing reset? When to reset and when not to reset?

3. how can we keyframe a looping animation of absolute keyframes? How can we find out what frame number needs to be keyframed in order to make that animation loop smoothly? And does the first shape key in the list need to be duplicated down to the last position in the list in order for the tweening/morphing to loop?

The best documentation I've been able to find on absolute shape keys is in The Blender Manual, but it's pretty vague. It basically says "reset and drag the slider". And none of the shape key tutorials around the web that I've seen cover absolute to a useful extent.

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Controlling the timing of absolute shape keys

You can add keyframes for the "Evaluation Time". The easiest way in the UI is to hover over the Evaluation Time slider and press "i". This will map your defined shape keys to particular frames in your animation.

Attempted answers to your other questions

  1. The "frame" for absolute keys does seem to be separate from the usual meaning of "frame" in Blender and not related to keyframes. Not sure why it increments by 10 or why it's not editable, but it ends up not mattering much because of the fact that you can set the eval time and make keyframes for it.
  2. Resetting timing seems to be an organizational tool. If you delete one of the shape keys, you'll have a gap of 20 in the eval time from one to the next, and resetting lets you get back to a list that increments by 10. If there's a deeper reason, I don't know it.
  3. As far as I can tell, the best way to loop is, as you suggest, duplicating the basis keyframe for the final keyframe. I tried setting a final keyframe with evaluation time set to zero, and the result was my object rapidly undoing all of its shape changes, one at a time, on its way back to the basis shape key. To connect the loop after making a keyframe for your final shape key, I'd try making one more keyframe for your basis shape key in the very next frame to stop the intermediate values of eval time from getting animated. (If you want to avoid the doubling of a frame, you could also set eval time to a fractional value corresponding to what the shape of the object would be at that frame if it were continuously morphing.)
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