Scaling object by -1 (to mirror) leads to odd sculpting brush behavior #118726
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Reference: blender/blender#118726
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** System Information **
Operating system: Win11
Graphics card: RTX3080
** Blender Version **
tested in 3.6 and 4.0, same behavior
** Short description of error and 2 workarounds **
Context is sculpting using additive/subtractive brushes, suppose we create an arm for a character (or just add an UV Sphere and make some visible changes to it to see the mirroring applied later), and then go into Object mode, shift-d to duplicate, then mirror it using scale: s > x > "-1" (to scale in X axis), then go back into sculpt mode for this mirror object and when using draw or any other additive/subtractive brushes, the brush is actually subtracting instead of adding in the mirrored axis, but keeps adding/subtracting in the other axes like it is supposed to.
Remeshing, cleaning up or anything else I tried didn't help, first fix involved exporting as obj and reimporting it back, that fixed the problem.
Other workaround I found that fixes the issue:
Object mode: reset scale using ctrl+a (apply) > "Scale", then back to sculpt mode, remesh, and now the brush is adding/subtracting in all axes on that mirrored object like it is supposed to. Just resetting the scale doesn't help, remeshing has to be performed.
Other brushes, like grab, work just fine just after mirroring the scale, the problem is only with the additive/subtractive brushes and that particular mirrored axis, that led me to think that this is an actual bug rather than just an undocumented feature.
When scaling -1 your normals get inverted. Brushes are subtracting because youre sculpting on back faces.
In overlays enable "Face Orientation" and youll see red faces. Select them and press Shift N (or Ctrl Shift N)
if I create new project, place an uv sphere in it, enable face orientation overlay, all surfaces on the sphere show blue, perform the S > X > -1, all faces remain blue, yet when switching to sculpt mode and using add brush in X direction, it performs as subtractive brush
Hm it seems normals are indeed not shown as red, but anyway. That is the problem and its expected as far as I know. You can simply apply scale so that scale becomes 1;1;1 again and brushes will work correctly.
yes, that is what I expected and described in detail in the original post, except without remeshing resetting the scale to 1:1:1 alone doesn't fix the problem, it starts to work like it should only after resetting the scale AND remeshing, hence I opened this ticket, I obviously don't know how the code handles these actions, but it seems like a possible bug to me, not a major one, since it is easy to work around, but still
In fact, the problem occurs due to normals that are inverted.
The sculpt could detect this case and invert the brush.
Whether it is intentional or not, the
Sculpt, Paint & Textureteam can decide, but for now I am confirming the issue.