[Radiance-general] Re: rtcontrib parameter settings

[Gregory J. Ward]

Hi Andy,

Looks like this one slipped through the cracks during my absence as  
well.  Not too surprising, considering the subject...

> From: "Andy Stone" <andy.stone at bdsp.com>
> Date: July 17, 2007 12:13:21 PM GMT+02:00
>
> Hi all,
>
> I've been using rtcontrib recently and I'm having problems finding  
> a set
> of rendering parameters (ad and lw) that don't give "out of memory"
> errors but do give sufficiently accurate results.
>
> About the highest settings that I can use before an out of memory  
> error
> occurs is:
>
> cat luxmeters_test_office.txt | rtcontrib -V+ -o  
> solar_coefficients.bin
> -M sun_modifiers.txt -fad -ab 5 -ad 4096 -lw 1e-8 -I+ -h
> octrees/scene_with_suns.oct
>
> ...but when I use these settings the results contain a lot of noise.

Individual source values/images from rtcontrib often do contain a lot  
of noise, which is reduced when you sum several sources together as  
is normally done in a daylight coefficient method, but more  
problematic for sun sources.  (By the way, using the -V+ option  
shouldn't be necessary in this case.)

A funny trick you can try is using multiple identical calculations  
then averaging the result into your output file.  I recommend doing  
this in two steps, so you can recover the run if you have to.  The  
following would take 16 samples at each illuminance point and average  
them together to reduce noise by a factor of 4:

sed -n 'p;p;p;p;p;p;p;p;p;p;p;p;p;p;p;p' pluxmeters_test_office.txt \
	| rtcontrib -V+ -o solar_coefficients16.bin \
	-M sun_modifiers.txt -fad -ab 5 -ad 4096 -lw 1e-8 -I+ -h \
	octrees/scene_with_suns.oct
total -m -id -16 -od solar_coefficients16.bin > solar_coefficients.bin
rm solar_coefficients16.bin

Or, you can put everything in one command if you're feeling brave and  
want to save time & file space:

sed -n 'p;p;p;p;p;p;p;p;p;p;p;p;p;p;p;p' pluxmeters_test_office.txt \
	| rtcontrib -V+ -o '!total -m -id -16 -od > solar_coefficients.bin' \
	-M sun_modifiers.txt -fad -ab 5 -ad 4096 -lw 1e-8 -I+ -h \
	octrees/scene_with_suns.oct

Note that C-shell users will have to add a backslash ('\') before the  
exclamation mark ('!') in the -o option of rtcontrib.

I should probably build this trick straight into rtcontrib, as I've  
used it myself to good effect.

> The scene consists of a fairly complex building and 72 light sources
> (representative suns for the year), each of which has a unique  
> modifier
> so that I get a coefficient for each light source at each measurement
> point.
>
> So far I've come up with two possible explanations for my problems:
>
> 1. I'm getting my settings all wrong.  I'm using -ad and -lw to  
> control
> the accuracy, but I haven't got a good feel for how to vary them -
> should I set -ad as I would for a normal rtrace-type calculation and
> then reduce lw until I get what I want, or does ad need to be  
> higher for
> an rtcontrib type calculation?  Or is there another setting I need to
> look at?

By default, rtcontrib uses Russian roulette to avoid the usual bias  
in Monte Carlo ray-tracing, but it does come at the cost of higher  
noise levels.  I wouldn't know what other parameters to suggest you  
fool with, either.

> 2. I'm using cygwin.  Is anyone aware of any memory limitations with
> cygwin? I've seen the web page about increasing the memory  
> available to
> cygwin (http://www.cygwin.com/cygwin-ug-net/setup-maxmem.html) and  
> that
> didn't help at all - apparently cygwin on my machine has a maximum  
> heap
> size of about 1.5Gb.

I can't really help you there, but perhaps someone else has a  
suggestion for the cygwin memory issue.

-Greg