Re: Question about BSDFClosure functions


Alex Conty <aco...@...>
 

Apparently I can't post here from my work address, so resending. Sorry
if somebody gets a duplicate.

Hi Brecht, I'm Alejandro Conty (jandro in the old blender days), we
met
several times at the Blender conference. Nice hearing from you about
this.


I have a question that I couldn't find an answer to in the
documentation or source code. The BSDFClosure has eval_reflect,
eval_transmit and sample functions. Some rendering engines have an
additional function to compute the average reflected or transmitted
color independent of the incoming light direction, by the integrating
the bsdf over the incoming light direction without any lights. In PBRT
for example this function is called rho.

I understand the main use for this is importance sampling. I'm afraid
I'm not allowed to say how we do things here at imageworks, but there
are other ways to do importance sampling. That function would make
things easier, of course, the problem is that not all BSDF's can be
easily integrated. The expensive method is not an option, it would be
overkill. So you need an analytical solution which is not always
available.

There is also a practical issue too. We have this sample function
that
has to return a normalized PDF and so. It turns out that people find
it
very hard to implement it in some cases. Even when the PDF doesn't
have
to match the BSDF exactly. So adding that extra function could be
asking
for too much. But the discussion is open and you are wellcome in.

Let's see what the other guys think about this ...

Alex

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