No matter what I try RPR 2.2.1 seems to be unable to correctly render some normal maps in tangent space (using either Principled BSDF or Uber Shader Node). No issue at all in Cycles & Eevee but RPR render it as if it was in World Space instead of Tangent space. Happens with both OBJ & FBX meshes (FBX stores the tangents so the renderer shouldn't have to mess with them ). See screenshots below. Eevee is perfect while RPR is messed up.
I finally got it to work. Had to re-export an FBX from the baker (Marmoset in this case) and this one worked. OBJ don't though as RPR seems to interpret tangents differently from EEVEE/Cycles (similarly to the Sketchfab webGL renderer which had exactly the same issue with this model so I had to do exactly the same thing there: African woman wood sculpture - 3D model by MAK11 (@MAK11) [1d24e8c] - Sketchfab ).
RPR uses Open GL setting for Tangent Space Normals, so in the future will try to Open GL setting for Normal map if Substance Painter/Designer, that's will make much correct normal maps results in RPR and Sketchfab. Thank you
I know that..but this isn't really the issue. The same normal map works perfectly fine with the OBJ mesh in eevee/cycles (which are also using OpenGL) & several other renderers. While it is only working on the FBX mesh in RPR (& also Sketchfab ) because the FBX stores the tangents which insure that they are the same tangents used for rendering (in the the case of the OBJ, RPR/Sketchfab are rendering tangents which are not aligned with the baked normal map). This is actually the only mesh which I've had an issue with. all my other OBJ meshes rendered correctly.
Also, try to change Image texture node Color space settings and to Non-color for Normal map texture, that's could fix an issue with Normal map. Thank you