Animation with shapekeys looks clunky

Question

I have a swordfish model that I want to make a swimming animation for. Using shapekeys to distort the mesh seemed like a good way to do this, and works for the most part. The issue with this is that the animation doesn't seem to maintain a constant speed, and seemingly slows down towards the end of each key.

During the animation, the tail goes smoothly to the side, but instead of going smoothly to the other side during the animation, it stops momentarily in the middle, then continues to the other side.

Here's a video of what I'm talking about: animation

what i want is for a clean transition from one side to the other, rather than a go-stop-go like I have now, but I don't know what is causing the delay.

Answer 1

By default (unless you change it in the Preferences) the interpolation between keyframes is set to Bézier, which means the changes increase with time and decrease towards the end to have an ease in and ease out effect, which usually helps with smooth transitions.

However when you want constant speed for a looping animation or some other reasons, you might want to switch this to Linear.

You can do this in the Timeline as well when you select all keyframes, the following shortcut is the same, but I'll show it in the Graph Editor where the before/after effect is visible:

First, the green curve for the Y location starting with the slow increase and then slowing down towards the end:

If you now hit T with all keyframes selected (or at least the ones you want to change), this brings up the Set Keyframe Interpolation menu. There you choose Linear.

Afterwards the curve is now a straight line between the two keyframes, i.e. the location is changing at a constant speed.