_tau_I’m guessing it’s probably because of Unity recalculating tangents instead of using imported ones? Being completely honest, I have known of tangents but have never understood their function or relation to normals for years

_tau_ Okay quick update. I did change it to imported tangents but that created some more weird lighting. First is with imported tangents and second is with calculated Mikktspace

Edit: Even further digging reveals there isn't really a way to modify tangents within blender. The calculation is mostly done in Unity, which is not good at decoding custom normal tangents.

Davilos: I believe tangents are calculated from a UV channel somehow?

_tau_ They rely on a myriad of things from what I’ve learned. UV orientation is a factor, yes, but not all modeling programs generate tangent data, such as Blender. For reference, none of the UVs on this model are stacked and everything is properly unwrapped (see attached file). However, Unity is trying to generate tangent data based on the custom normals which is throwing off lighting angles greatly. I think a good fix for this would simply (if possible) to make it so that reflectance can be masked out or just doesn’t render on parts of the material that aren’t smooth. As is though, I think this could lead to issues for anyone who uses custom normals since Blender, the most commonly used program to make VRC avatars, does not support tangent editing.

Davilos:

> just doesn’t render on parts of the material that aren’t smooth

That is, unfortunately, not easy or even possible in a shader. As far as I am aware, most existing community shaders would break down with invalid tangent data as well. I'm not sure this is something that we can fix on our end from a technical perspective.

_tau_ I wonder what exactly causes it then because my models with the exact same custom normal setup have worked fine with Poiyomi, XSToon, Silent Shader, Standard (+ Lite), and others. This is the first example of reflections appearing on a mesh with a smoothness value of 0 for me.

_tau_ Did yet a little more digging and it has something to do with realtime lights it seems. I've provided a couple of pictures, but suffice to say it seems to be adding specular highlights when it shouldn't. I've modified the custom normals of my eye mesh and while it's better, it's definitely not fixed.

The first image shows the inside of the head mesh (namely focusing on the eye cavity and the tongue mesh) with one realtime directional light. There's a specular highlight on the tongue and an erroneous highlight in the eye, where the gloss map specifies it should be completely rough.

The second image shows the exact same scene, this time with the realtime light disabled.

The third image shows how it looks with a gloss of 0 across the whole mesh and the light is re-enabled.

Something about the reflectance adds glossy highlights to the mesh based on the realtime light even when no gloss should be present at all. Does that help narrow anything down in terms of the issue's source?