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I am currently working on BRDF and their associated probability density functions (PDF) in order to importance-sample them. It is really hard for me to find the correct way to compute the PDF of a BRDF. I know some characteristics that the PDF should verify (the integral must be equal to 1...) but I have really no idea about how to do it.

I am now working on the Oren-Nayar BRDF.

If someone knows a generic way to do it or has any good resources to propose, I will be happy to read that!

Thank you very much by advance,

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  • $\begingroup$ What do you mean by the PDF associated to a BRDF? Do you mean the distribution of normals (NDF), or something else? $\endgroup$
    – Nathan Reed
    Commented Jun 1, 2021 at 18:09
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    $\begingroup$ @NathanReed - I think he is talking about the PDF used to sample rays from when trying to sample w.r.t brdfs $\endgroup$
    – gallickgunner
    Commented Jun 2, 2021 at 6:55
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    $\begingroup$ @NathanReed you are right, thanks. This is the PDF I am talking about. I will update my post. $\endgroup$
    – Balfar
    Commented Jun 2, 2021 at 7:34

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PBRT3 is a really good resource and has a lot of brdfs in it. The book and source code are available for free online.

Regarding oren-nayar, from what I can gather they are just using cosine weighted hemisphere sampling.

https://pbr-book.org/3ed-2018/Light_Transport_I_Surface_Reflection/Sampling_Reflection_Functions

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