IK with non uniform scale broke the animation tools
Closed, ResolvedPublic

Description

Blender Version
Broken: 2.78
Worked: 2.77

Non uniform transforms generated by the ik constraint cannot be used by blender operators: Apply visual transform to pose, Animation bake, Copy Attribute, FK/IK snap, none of them will work

Exact steps for others to reproduce the error
Attached blend file contains a very simple ik setup, and a copy with a baked animation, which behaves different. Baking the animation, applying to pose, storing the matrices and setting them, will not end up with the same pose.

This happens since non uniform scale IK has been introduced, and it's fixed disabling USE_NONUNIFORM_SCALE at build time

Bastien Montagne (mont29) triaged this task as "Confirmed" priority.Jul 4 2017, 4:59 PM

Not that familiar with this feature, it is implemented by @Campbell Barton (campbellbarton).

Campbell, mind having a look here?

I think D2088 changed two things:

  • Non-uniform scale is no longer inherited when applying the IK transforms, to match the IK solver normalizing the matrix and making the scale uniform.
  • IK stretch no longer affects the X and Z axes. I think this was an unintentional change. This is demonstrated in ik-stretch.blend, but not necessarily the reason the scripts and tools are broken.

There's a few different scaling cases:

  • No scale or uniform scale: no problem.
  • Non-uniform scale with inherit scale off, as in ik_non_uniform_scale.blend from D2088. This can probably be fixed without breaking the scripts and tools described in this ticket. P513 is an attempt at that but isn't quite right in all cases.
  • Non-uniform scale with inherit scale on. This is when you get shearing transforms which the IK solver can't represent or solve. Arguably shearing bones are never actually what you want, and perhaps not the important case in practice. This works a bit better after D2088 but can still fail.

hi, we need some resolve here, people are waiting for this to be fixed.
Thanks

Sorry I could not dig into coding, busy days. The way I see it, if there is interest in keeping this feature but there is no way to make sure it behaves properly, it should definetely be an optional setting. This way the user can willingly activate non uniform scaling, and renounce the benefit of the animation tools in exchange.

In order to do this, we would need pose.ik_param to return a parameters class rather than a None, even when the solver is set to legacy, so some work would be required on the rna and of course on properties_data_armature.py

Regards