Why negative scales don't work?

Questions
,
1

I’m trying to scale some objects, but I find that they become weird when negatives are used… So I’ve been trying to figure out a way to fix the weirdness (objects flipping back to positive scales when they should be negative).

Why do objects not work with negative scaling?
For example setFromMatrixScale() will cause all negatives to become positive.

                gizmo.groupInstance[i].instance.scale.setFromMatrixScale( gizmo.groupInstance[i].oldMatrix );
				
				matrix.makeScale(   gizmo.group.scale.x * Math.sign( gizmo.groupInstance[i].oldScale.x ), 
									gizmo.group.scale.y * Math.sign( gizmo.groupInstance[i].oldScale.y ), 
									gizmo.group.scale.z * Math.sign( gizmo.groupInstance[i].oldScale.z ) );				
				gizmo.groupInstance[i].instance.scale.applyMatrix4( matrix );

I can’t seem to figure out how to keep it correctly negative… If I scale one axis negative… it seems okay… if I scale two axis, then it doesn’t work and acts weird… if I scale 3 axis then it becomes invisible and weird… what is going on?

2

It doesn’t work because the normals get flipped when scaling by a negative number. This means that the faces turn inside-out, and the materials, lighting, etc. get all wonky.

3

why would a negative scale become positive though?

4

I thought that was fixed before:

Was it reverted?

5

Oh wait, does it work with scaling a mesh, but not scaling a geometry directly? Maybe that’s the difference. Oh also maybe not supported for individual instances of InstancedMesh?