Normals: Tangent space to world space transforms errors about pbrt-v4 HOT 4 CLOSED

Interestingly, this fixes a number of errors in other scenes that use normal mapping; compare the curb, bistro glass doors, and menu to the left of the doors.

from pbrt-v4.

Very interesting. Thanks for reporting this!

The underlying issue comes from pbrt being a little weird in how it handles shading normals: it wants to have surface partial derivatives that, when you take their cross product, you get the desired shading normal. Then the way it handles normal maps is a bit unusual as well: by the time the normal map is evaluated, we no longer have access to the full object space->rendering space transformation, which would be nice to have around to bring the normal map's object-space normal into rendering space...

For this case, the problem comes from this line in src/pbrt/materials.h:

Sooooo..... Replacing that line with:

    Frame frame = Frame::FromXZ(Normalize(ctx.shading.dpdu), Vector3f(ctx.shading.n));

gives this image:

which is at least consistent regardless of the rotations, though it doesn't match blender.

I'm not sure what the discrepancy is (pbrt being left handed and blender right-handed?) and would like to figure that out, but will at least check in that fix after testing it with more scenes.

from pbrt-v4.

I can tested on Blender since I have a full operative plugin. BTW.. in Blender, Z is up

from pbrt-v4.

Hello, thank you for your reply, the updated line works just fine now :)
No need to look deeper into the light orientation, the blender cycles render was a way to highlight the normal map bug.
I'm closing the issue, thanks !

from pbrt-v4.