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Applying negative scale inverts multires displacement
Confirmed, NormalPublic

Description

System Information
Operating system: Windows 10
Graphics card: RTX 2070

Blender Version
Broken: 2.82a, 2.90 Alpha (542ff416e274)
Worked: 2.79b

Short description of error
When multires-sculpting an object, then mirroring it on any axis and then applying the scale, the multires displacement inverts - turning inside out. Flipping normals (either before or after applying scale) doesn't help.
In 2.79b, multires data is retained correctly after applying scale, although normals need to be flipped afterwards.

Exact steps for others to reproduce the error

  1. Pick the default cube, add a multires modifier to it.
  2. Subdivide a bit for good measure, sculpt something.
  3. In object mode, mirror the object on any axis.
  4. Apply scale.
  5. Observe the sculpt turning inside out.

Event Timeline

Richard Antalik (ISS) changed the task status from Needs Triage to Confirmed.Tue, May 12, 2:58 PM

This looks very much like T74361, but since this worked in 2.79, I will leave this open as separate issue.

I just tested earlier releases - the bug exists since 2.80, so it has probably not been a recent change.

I'm not sure how the displacement is handled internally, but since it's able to invert, would it be possible to have an option to scale the data by an arbitrary number? This would be a workaround at least, and a handy feature at best.