Weird unwanted scaling problem with shapekeys

Greetings! For now I’m stuck using 2.79b since that’s the latest my laptop can run until eventually I can build my workstation. It’s been a few years since I’ve had the opportunity to play around with blender, and - at best - I’ve only ever been an enthusiastic dabbler. I’m not as well versed as many others…

Anywho, I’m getting a weird (and unwanted) scaling action with my shapekeys.

The intent of this project is experimenting with sine wave deformation along a cylinder. It took me a while to figure out that shapekeys is the way to make what I want happen. Here is the starting state of my cylinder…

So I rotated my vertex groups with proportional editing and made a shape key for each group… one plus and one minus. But when I actually scrub through the keys, it’s not only rotating (which is actually perfect) it’s also scaling outward, making weird bulges across the mesh.

However, I found out that if I extend the values to negatives, when it interpolates the deformation… it works exactly the way I want it to! What’s with that?! :astonished:

In the end, it’s actually pretty cool that it’s doing all that, because it gave me ideas to expand what I was originally working towards – animating a audio visualizer like I’ve done in the past. There’s a lot I can work with there. :smiley_cat:

But the main thing I want to know is am I missing something that would make it work properly the right way, without having to use a workaround like negative values? Thanks for your time, peoples! :+1:

EDIT: Forgot to include my blend file…

https://drive.google.com/file/d/1MacryloS17N2aaMJ3u3M7IWTZw8ifks_/view?usp=sharing

First thought: not apllied rotation… but no… this is really weird…maybe this has something to do with the euler/ quaternion rotation problem/solution … nahh this wouldn’t explain the scaling…

AHHHH got it if you make a shape key with no vertices on the y axis then ther seems to be no scaling. So the solution might be transform all vertices slighty on the x axis (or model with no vertices with y=0) ?

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@Okidoki, thanks for the reply, but i’m not going to lie… I only understood half of that. :smile_cat:
It did get me thinking about the math, though.

I’ve figured out a little bit of it. Applying the shape keys to multiple edge loops as a group was throwing off the calculations somewhere between local and global space. I backed up and just keyed each edge loop individually and pretty much got exactly what I was trying to do… just with a bit more effort. It does give me a lot more control, though. If I push the range past the shape key far enough, it starts doing the same weird thing all over again. So it’s an interpolation issue. Either way, the end result actually works out better for what I’m ultimately working towards…

…which is (in theory) going to be a multi-layered audio visualizer. Render time is probably going to be hell when I get there. :scream_cat: :joy_cat:

Current .blend file…

:slight_smile: okay no math: the vertices exactly in the middle (typo: x-positon = 0.0) are bad :wink:
so subdivide different (too much work now) or select all vertices and move slightly to left or right.
Have fun.