Default Normal Shader: transform tangents with modelViewMatrix

[WestLangley]

resolves #18005.

As mentioned in #18005, an alternate approach is:

vec3 transformedTangent = transformDirection( objectTangent, modelViewMatrix ); // normalized

which we can implement later if needed.

[mrdoob]

Thanks!

[WestLangley]

@mrdoob It appears from your earlier commits that you are aware that this is not correct, either:

#ifdef USE_INSTANCING

	transformedNormal = mat3( instanceMatrix ) * transformedNormal;

#endif

I am not sure of a performant fix to this. One option is to say instancing does not support non-uniform scale in the instance matrices. That is not very appealing to me, though...

[mrdoob]

@mrdoob It appears from your earlier commits that you are aware that this is not correct

Hmm, maybe I was aware at some point, but I sure forgot 😅

[WestLangley]

You need a normal matrix for each instance.

[WestLangley]

What you are doing is OK as long as there is no non-uniform scaling. But if one were to instance a grove of trees at varying heights (for example), non-uniform scaling support would be required.