Detail map normals and material normals are combined wrong when using different UV channels

[animech-sijo]

Detail map normals and material normals are combined wrong when using different UV channels

[aidinabedi]

Just wanted to let you (PlayCanvas devs) know, that we've isolated the issue and developed a workaround.

The root cause is the algorithm used to compute tangents in the standard material shader (i.e. TBNderivative.js). Since the computation depends on which UV channel the normalMap uses, the tangent can change handedness if the normalMapUVs are say mirrored. This may be fine for the regular normalMap but is problematic for a normalDetailMap that can use an independent UV channel. I've also noted that there is a TODO in the engine that seems to highlight this problem.

The best solution is, of course, to have pre-computed tangents, but unfortunately, the editor doesn't seem to keep tangents when importing models (at least not for fbx or glb).