[Bf-committers] From Farsthary another anouncement

Yves Poissant ypoissant2 at videotron.ca
Fri Feb 6 14:57:29 CET 2009

From: "joe" <joeedh at gmail.com>
Sent: Friday, February 06, 2009 3:29 AM

> Out of curiosity, does it really look terrible in practice, if the
> legacy<->brdf conversions aren't always perfect?

Typically, users who will want to do that conversion will want to use their 
material inside a physically plausible renderer. They will end up 
redesigning the material anyway. Look on BlenderArtists for the threads 
where Blender users try to use external renderers such as Yaf(a)ray, indigo, 
luxrender and such. They all invariably end up redesigning all their 
materials because the conversion did look terrible enough to be 
unacceptable. Automatic conversion need to be based on a few assumptions and 
the result is that the automatically converted materials all end up looking 
like made out of the same material, usually some kind of plastic.

So it does not look "terrible" per se. It is just that it does not look like 
the originally intended look. That's all.

The situation is not so bad for most users. Those who will switch to 
physical render will quickly switch to physical material too and start 
designing their material in a physically manner anyway. They will face 
issues when they will attempt to convert previously designed materials 
though but I would guess that rather rare are the users who managed to build 
their own relatively large material library so that is a small issue too. 
Beside, those who have built such libraries always have the choice to keep 
using the non physically plausible render engine and keep hacking their 
scene setup to get good enough renders so there is a way. Large material 
libraries would be nearly impossible to import as BRDF I predict. So the 
Blender Material Repository would most probably be useless in a physically 
plausible rendering context.

> I mean if it looks
> "good enough" (as pixar's researchers put it :) ) then it'd probably
> be fine.

How "good enough" is "good enough"? When you read Pixar research papers, you 
quickly realize that their "good enough" is pretty high standards. When 
pixar publish algorithms they say is "good enough", it is just a label, for 
them, to point out that the result is not strictly physically correct. It is 
a bunch of approximations designed to efficiently aproach the physical 
correctness quality. They don't try to physically simulate lighting and 
materials the way Maxwell or Fryrender do. But they carefully base their 
approximations on physical laws, and design probabilistic and statistical 
approximations that are not exact but are "good enough". Compared to this 
level of "good enough" standard, I would never dare say that an automatic 
Legacy to BRDF conversion would be "good enough" except in very rare cases.

BTW, I think Pixar is one of the best CG research labs around today. Their 
researchers are big names and very high calibers. I like that they focus on 
production solutions. They are the only ones to do that that well. And I 
thank them for publishing their results in papers and distributing their 
papers freely on the web. Before a new physically plausible render engine 
would be designed for Blender, I suggest that a good reading of Pixar papers 
should be made in order to get a good sense of how such a renderer should be 
designed to allow those kind of approximations and optimizations. It would 
be a mistake to start designing a new renderer engine based on pure 
physically rendering approaches.

> I'm also wondering how a shading language would fit into
> such a system.  You couldn't really restrict people to writing pure
> BRDF's, so it wouldn't always be correct, necessarily.  It's kind of
> like how in DSM, node materials can't always be handled correctly,
> because they can output anything.

It is a free world. If someone somewhere wanted to design a pseudo BRDF, 
that looks like a BRDF and can be used like a BRDF but does not behave like 
a BRDF, or is not physically plausible, then where is the problem? If 
someone wanted to experiment with some form of alien materials, then why 
not? Why couldn't imagination be released free when designing BRDF? And if 
the final render looks cool or amazing, then so much the better. And if 
someone else find the BRDF cool enough to use it then so be it.

I think we need to start to put BRDF design out of the hands of researchers 
and into the hands of the artists. We need to push the envelope in term of 
BRDF design. And if a BRDF is really not physically plausible, I doubt the 
renderer will crash or melt down. In extreme cases, the resulting render may 
well look lika a renderer bug but the bug will be in the BRDF. Not in the 
renderer (at least, the renderer should be robust enough to elegantly handle 
weird BRDFs. But IMO, it is the intrinsic nature of a physically plausible 
render engine to be quite robust).


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