[Radiance-general] BSDF xml into Radiance

[Greg Ward]

Hi Lars,

I'll answer your questions inline...

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: July 30, 2012 3:10:43 AM PDT
> 
> Hi Greg,
> 
> thank you for the reply. I tried a bit since, so it is almost one week
> later that I can report on what I experienced so far.
> 
>>> 2) rtcontrib and patch-based model. ...
>> 
>> The accuracy of non-planar, specular reflectors is actually better than
>> #1, but the results are somewhat noisy.  A new -c option to vwrays
>> (coupled with the rtcontrib -c option) is a good way to reduce noise
>> that is available in the latest HEAD.  This is a better way to reduce
>> noise than increasing -ad, and less costly.
> 
> Classical Radiance scenes with both "light" and "glow" sources used to
> require high settings of -ad to generate "noiseless" images. This led to
> something like, say, -ab 4 -ad 1024 -as 512. This was typically
> impossible to render, so mkillum was used to smooth out the ambient
> transmission through fenestration and keep the sharp transmission
> distribution only in the direct calculation. The ambient settings would
> be released to e.g. -ab 2 -ad 256 -as 128 - fast and noise-reduced.
> 
> With rtcontrib, we typically have no "light" sources any more,
> everything is "glow". At the same time, mkillum is not available as a
> tool any more.

Actually, "light" still does something in rtcontrib by attracting sample rays.  It isn't always an improvement, however.  The "light" type is best reserved for actual luminaires and the like -- this is what Andy found in his experiments if I remember right.

> It is possible to reduce the amount of rays spawned at any ambient
> bounce by not increasing -ad but -c and having the jitter smooth out the
> distributions from different sky patches instead. This means that
> additional rays are spawned only at the first level (direct) but not for
> daughter rays. However it reduces only the direct noise - noise in e.g.
> shadow patterns for the patches representing the direct sun. Noise for
> areas lit only by indirect, diffuse reflected light e.g. in the back of
> the room is not reduced.

Actually, all noise should be reduced somewhat.

> My guess is that the -c option aims at the same strategy as what Mark
> Stock found to be promising for scenes with a lot of geometry and many
> ambient bounces:
> 
> http://markjstock.org/radmisc/aa0_ps1_test/final.html
> 
> What is the actual advantage of using -c 4 instead of doubling x- and
> y-resolution?

Yes, I think so.  The main advantage of increasing -c as opposed to -x and -y is efficiency -- the output picture files are smaller, so you don't have as much disk i/o (which can be a bottleneck) and don't have to filter the results, either.

Cheers,
-Greg