What kinds of reflection_flags and sampled_type I should use if I am creating my own BRDF?

[fzy28]

I am creating my own BRDF and I want it to be used in mitsuba. But I am not familiar with the reflection_flags and sampled_type these two attributes, I wonder how do they actually work in the mitsuba, what it will control?

bs = mi.BSDFSample3f()
bs.wo = mi.warp.square_to_cosine_hemisphere(sample2)
bs.pdf = mi.warp.square_to_cosine_hemisphere_pdf(bs.wo)
bs.eta = 1.0
bs.sampled_type = mi.UInt32(+self.m_flags)
bs.sampled_component = 0

For example, the above code has defined a sampling method, then why it still should add the sampled_type and sampled_component?

More specifically, if I have design my own BRDF, with my own way to generate samples as well as pdf, what kinds of reflection_flags and sampled_type I should use?

Thanks!

[rtabbara]

Hi @fzy28,

When you have a multi-lobed BSDF, the sampled_type and sampled_component can provide information regarding the category and index of the sampled lobe.

One primary motivation for this information is for debugging purposes. An additional use case is if you perhaps are also defining your own custom integrator, you could supply a filtered BSDFContext that could restrict the types of lobes sampled.

Finally, existing integrators leverage the sampled_type information. As an example, with the standard path integrator, if the BSDF sample is a delta type (BSDFFlags.Delta), no direct illumination sampling (NEE) is performed for the subsequent bounce.

You can see the full list of the types of BSDFFlags and their interpretations here. But in short, it's good practice to populate these correctly in your custom BSDF. If you don't have a multi-lobed BSDF, then you simply set sampled_component=0.