Incorrect normal map with material.side = THREE.BackSide + vertex tangents

[donmccurdy]

Description

Mentioned by @WestLangley in #25580 (comment), normal maps are displayed incorrectly when material.side === THREE.BackSide and the geometry uses vertex tangents.

Reproduction steps

1. Create a geometry with vertex tangents
2. Create two material with the same normal map, one front-sided and one back-sided
3. Extrusions on one side should appear as indentations on the other side

[donmccurdy]

To test this thoroughly, I believe we'll need to consider all permutations of the options below:

• material.flatShaded (true / false)
• material.side (front / back / double)
• material.normalMap.flipY (true / false)
• material.anisotropy (on /off)
• vertex tangents (present / not present)

@WestLangley @elalish have I missed anything? Or are there any of these we can ignore?

[donmccurdy]

I'll try to create a scene to reproduce the issue soon, but I don't expect to have a fix for this before the r151 release. If this blocks other work (#25693?) we should discuss options.

[elalish]

SGTM. The only of those that are available in glTF (so far as I know) are:

• material.flatShaded (true / false)
• material.side (front / double)
• material.anisotropy (on /off)
• vertex tangents (present / not present)

If we could also export a scene with that subset as GLB, it would be a great model to add to the gltf-sample-models repo. That would also make it easier to catch regressions using MV's fidelity tests.

[elalish]

FYI @emackey