IK-Stretching no longer stretches on Y-Axis
Closed, InvalidPublic


System Information
OS: Windows 10 64-bit
GPU: NVIDIA GeForce GTX 1070 x2
CPU: AMD FX-8350

Blender Version
Broken: 2.79 (6f633de)
Worked: Latest stable 2.78 release. Also worked in the 2.78.5 (7b25ffb) experimental build that included the 2.79 features for Denoising and the Principled Shader

Short description of error
Introduced in 2.78, enabling stretching in the IK settings (the bottom slider) allowed bones to stretch only on the Y-Axis. I took advantage of this heavily when I found out about this, but in any 2.79 version released up to this point, the stretching seems to have reverted back to how it worked before 2.78 where bones will enlarge on all axis. This can be seen in this Youtube Video. This stretching reverting back is breaking all of my animations I have done in the past year as the limbs now awkwardly enlarge when I originally had them stretch only on the Y-Axis.

Exact steps for others to reproduce the error

  • Create a simple IK rig. Only two bones are necessary. The deform bone, and the IK bone.
  • Enable IK stretching by moving the stretching slider to be above 0.00.
  • In most versions of 2.78, the base deform bone should only stretch along the bone's local y-axis. In the latest stable and development version of 2.79, the base deform bone will simply just enlarge (scale) on all local axis.


Joshua Leung (aligorith) claimed this task.

Thank you for your report. Unfortunately, we've had to change the way that the IK-stretching with non-uniform scale is working in order to fix several critical bugs the 2.78 method was causing.

The new (2.79) behaviour intentionally aims to be more similar to the old (pre-2.78) behaviour. This is because the 2.78 behaviour was causing significant breakage issues with multiple rigs in critical ways (e.g., IK <-> FK switching was broken on those rigs, along with several other built-in tools). For more details, see T51971 and T51930, and the related commits fixing those issues.

Apologies for the file breakage that these changes have caused. As noted on the release notes, we did expect that some rigs would be broken by this change.

As for your rigs, here is a setup that you could use instead:

The "B1-Copy" and "B2-Copy" bones copy the transforms from the IK chain bones, while excluding the X and Z scaling. The geometry in turn is only deformed by these two new "deformer" bones, instead of by the IK bones directly. Hope this helps.