Accessing Ray Differentials in BSDF's sample() function

[AndrewYiBC]

Greetings,

I am creating a custom BSDF which requires ray differentials to determine the ray's corresponding pixel footprint in texture space. However, currently I do not see a way of accessing the ray differentials' (or even the ray's) information in the BSDF's sample() function.

Therefore, may I know if it is possible to do so with a basic BSDF class? If not, may I have some suggestions on how to achieve it, possibly by implementing a custom integrator / renderer?

Any insights would be greatly appreciated!

[njroussel]

Hi @AndrewYiBC

There's a chain of methods to handle this, when fetching the bsdf of an interaction you should use: SurfaceInteraction3f.bsdf(). That method will call SurfaceInteraction3f.compute_uv_partials() which uses the ray's partials (ray.o_x and ray.o_y). Finally, that last method call will actually fill in the duv_dx, duv_dy fields in your si object. This only works well for the first bounce, after which the ray partials don't correctly account for the warping introduced by the first bounce's BSDF.

[AndrewYiBC]

Hello @njroussel ,

Thank you so much for your reply. I tried calling si.bsdf() but it doesn't seem to fill the duv_dx and duv_dy field of my si object (they remain as [0.0, 0.0]). I am also unable to call si.compute_uv_partials() on my own as it requires the parameter ray (mitsuba.core.RayDifferential3f) which I don't have access to in bsdf's sample() function (same for calling si.bsdf(ray)). Are there any ways to solve this issue or to access the ray reference?

Your assistance is truly appreciated!

[ziyi-zhang]

Hi,
You need to call si.bsdf(ray). The one without ray argument simply returns the BSDF without compute_uv_partials().

Overall, differentials are not well handled in Mitsuba3 (whose focus is mainly on inverse rendering). E.g., a common use case of differentials is to select a Mipmap level, but a mipmap is still not supported in Mitsuba3. It would be easier to write your own code for such features using Mitsuba 0.6 / PBRT as reference.

Best

[AndrewYiBC]

Hello @ziyi-zhang ,

Thank you for the clarifications and suggestions. However, I couldn't seem to find a way to reference ray in bsdf's sample() function, thus I am unable to use it as the argument for si.bsdf(ray). May I know how this could be resolved? Much appreciated!

[njroussel]

This is usually done at the intergrator level, not inside the BSDF.

Have a look at our pathtracer to see how this is usually done: https://github.com/mitsuba-renderer/mitsuba3/blob/master/src/integrators/path.cpp#L203

[AndrewYiBC]

Hello @njroussel ,

Many thanks for your time and clarifications! I will take a close look at the integrator implementation.