Cycles : Correct tagging of tangent primitive variables to allow normal maps to render properly.

[boberfly]

Generally describe what this PR will do, and why it is needed

• Tangents need to exist and be tagged correctly for Cycles to use these with normal maps
• For these to function properly the user needs to do a few steps:

1. Create a MeshTangents node for a given UV, and name it with the name of the UV with .tangent on the end eg. uv.tangent
2. Create a primitive variable of type float + FaceVarying and name it based on the UV name with .tangent_sign on the end eg. uv.tangent_sign (I use PrimitiveVariables + ResamplePrimitiveVariables nodes)
3. Plug a tangent-space cyclesImageTexture into a cyclesNormalMap node, and plug that into a cyclesPrincipledShader's normal socket.

I am not sure exactly on the specifics of tangent_sign but it is needed for it to render.

Related issues

• It came up again in the Discord from a user

Dependencies

• NA

Breaking changes

• NA

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

Todo: Get the proper MikkTSpace code into place again, now that it is bundled with Cycles 3.5 and have it implicitly be created in the absence of tangent and tangent_sign primitive variables. This will be targetting for Gaffer 1.3.x probably.

[boberfly]

Ok a quick study, the biTangent is calculated in-kernel via the signage:
vector B = tangent_sign * cross(ninterp, tangent);

I'm not sure if we could just scrape this from the existing biTangent somehow or we should extend MeshAlgoTangents to also output tangent_sign as an option?

We also have MikkTSpace now in third_party: https://github.com/blender/cycles/tree/main/third_party/mikktspace perhaps we should add this too for other renderers or workflows to benefit?