[Radiance-general] ambient accuracy parameter

[Mark Stock]

On Thu, 28 Dec 2006, Mark de la Fuente wrote:
> Mark,
>
> Awesome pictures.  What are the limitations of your low -ad and 
> high over sampling approach?  When is appropriate and when is 
> it not appropriate?

Mark,

One disadvantage is hard drive space: how many 3 GB images can I 
keep around? Another problem that occurs more with some pieces 
then others is speckles caused by particularly bright pixels. 
Since pfilt works on the full dynamic range, a single 
super-bright pixel in the full-res image can cause a 
significant bright spot in the final reduced-res image. Note the 
speckles in this image:

http://mark.technolope.org/image/p54.html
http://mark.technolope.org/image/img54_detail_900.png

That second image is at print resolution, and was filtered down 
from a 4x or 6x resolution original. That scene is very 
contrasty. Some of my more recent work mitigates the speckling by 
rendering in a closed room: allowing enough bounces to provide a 
more even background.

I find that "-aa 0" is appropriate when the octree for a scene 
with little overlapping geometry takes more than a few hundred MB 
to create. Or for scenes in which I want detailed interreflection 
among a great number of very tiny surfaces (probably the same 
thing). It seems to me to be an easy way to get rid of the 
splotches that plagued some of my earlier renderings.

> Probably not something you are concerned about in your work, 
> but when you use such a low -ad setting and such a high over 
> sampling rate, I'd like to know what effect there is on overall 
> light level accuracy in a typical interior space with 
> daylighting and/or electric lighting?

I have not studied this at all. I could posit that the low -ad 
with high oversampling rate allows me to bias the interreflection 
toward the first (and most important) bounce. Shooting 36 rays 
into a final image pixel (what 6x oversampling does) but letting 
each of those only spawn 8 child rays (-ad 8) at the second and 
deeper levels effectively means that I'm doing a more accurate 
1st bounce.

> I guess there is no over sampling on an rtrace calculation, but 
> for an irradiance calculation image, my -ad setting is usually 
> in the thousands and my ambient bounces are at 4 or so 
> (depending on the results of a parameter test)  I'm guessing 
> the -ab and -ad parameters can't be reduced if light level 
> accuracy is important.  Maybe I'm wrong?  Sure would be nice to 
> slash some of those crazy calculation times.

My impetus was to slash the computation times and fit the problem 
in memory. Of course, back then I had 2 GB of RAM *at most* and 
no access to anything better than a dual-proc machine. Now I am 
blessed with access to a 32 GB monster 8-core intel box and a 
number of medium-sized clusters.

One other advantage of "-aa 0" is that one pixel trace does not 
depend on another, so parallelization is trivial. I use rpiece 
almost exclusively for my big jobs now. OpenMP or pthreads 
anyone?

Mark