Funky behavior with scales and rotation when changing actor hierarchy.

[Menotdan]

Issue description:
I have a rotated and non-uniformly scaled table, which I would like to child some objects to. However, when I do so, their position and scaling is messed up and becomes unusable.
https://streamable.com/ijnjet

Steps to reproduce:

1. Create an empty actor.
2. Rotate it around the Y axis by some small amount, maybe 15 degrees.
3. Scale the actor by some non-uniform set of numbers.
4. Create a cube actor and rotate it a bit.
5. Add the cube as a child of the empty actor, the transformation will break.

Minimal reproduction project:
break scale.zip
(under "Repro" in the main scene, take the empty actor with children and child it to the empty actor without children)

Flax version:
master e68c736

[Menotdan]

@mafiesto4 is there a reason this is on 1.9? I think this is a pretty big bug considering it's basic transformation operations, those should probably be stable for a release version lol

[mafiesto4]

Ah okay. This will account for more delay on the update tho.

[Menotdan]

This might be a regression from 9a21cfd.

[mafiesto4]

I don't know how to fix this. Clearly Actor::SetParent should calculate _localTransform differently when worldPositionsStays=true but Transform is limited to a single Translation+Rotation+Scale so when changing transformation hierarchies this might not be possible to preserve transformation properly. We would need to migrate to matrix-only usage representing any linear transformation. I tested this and Unity seems to not handle it properly as well so it's not an obvious issue.

[Menotdan]

I tested this and Unity seems to not handle it properly as well so it's not an obvious issue.

Oh yeah, you are right lol
Weird. I guess I will have to find some other way to organize my meshes then? Or just not use scale

[Menotdan]

I've asked in the Unity discord about this issue, and apparently it is expected behavior or at least a known problem that needs to be worked around. Maybe this can be closed.