Fix double-sided normals for negative scale meshes#24395
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Objective
Fix incorrect PBR lighting for double-sided meshes with negative-scale transforms.
#8859 fixed culling for single-sided glTF materials by detecting inverted scale in the loader. Double-sided materials still need a shader-side fix, because culling is disabled and raw
front_facingis reversed by negative-scale winding.Solution
Add a winding-corrected front-facing helper in the PBR shader path.
Testing
Tested with Khronos NegativeScaleTest
The bottom-right
-1.0column should match the expected lighting/reflection behavior for double-sided materials.To validate non-regression for normal/tangent handling, reviewers can also test:
NormalTangentMirrorTest
NormalTangentTest
Showcase
Before
After
