[Radiance-general] What does "ray coefficient" mean?

[Greg Ward]

Rays you specify on the input are traced by rcontrib until they strike a surface whose modifier matches one of the names you've specified with a -m or -M option. At that point, the fraction of that surface's color that contributes to the final ray value is reported (in RGB coefficients) if operating with the default -V- option.  If -V+ is specified, then the RGB contribution to the final ray color is reported, instead.

In other words, the difference between -V- and -V+ is that only the latter considers the actual color returned by the surface itself, whether it be a light source, a window, or another material.

The 3 values reported in your example are the RGB values for each ray that hits the sky_glow material.  Going from the name of the file and nothing else, I'm guessing this is an overcast sky distribution, so you are computing partial sky contributions rather than daylight coefficients in this case.

If you want to compute daylight coefficients, you would need to divide your sky hemisphere into bins according to the Tregenza or Reinhart sky subdivisions and compute coefficients (-V-) rather than contributions (-V+).

There are methods and tutorials for doing so elsewhere -- I'm not sure where at this point as I'm bad at tracking such things, but I'm sure there are others who can help you to find them.  Check "Goog's" latest posting for starters.

Cheers,
-Greg

> From: Ji Zhang <hope.zh at gmail.com>
> Subject: Re: [Radiance-general] What does "ray coefficient" mean?
> 
> Dear list, advices regarding the following questions are greatly appreciated! - Ji
> 
> 
> On Sun, Dec 29, 2013 at 12:04 AM, Ji Zhang <hope.zh at gmail.com> wrote:
> Dear List, 
> 
> May I ask what the "ray coefficient" means in the RCONTRIB man page? 
> 
> I'm going back to Axel Jacobs' tutorial on "rtcontrib(rcontrib now)":
> http://www.jaloxa.eu/resources/radiance/documentation/docs/rtcontrib_lesson.pdf
> 
> and I tried the following with the -ffa option for rcontrib to see what is the output in ascii format:
> 
> vwrays -ff -x 600 -y 600 -vf ./views/back.vf \
> | rcontrib -ab 3 -ad 1024 -ar 256 -aa .1 -ffa \
> `vwrays -d -x 600 -y 600 -vf ./views/back.vf` -m sky_glow -n 2 -V+ \
> ./testroom_overcast.oct > ./tmp/rcontrib-ffa.txt
> 
> The output contains the following:
> 
> #?RADIANCE
> oconv materials/testroom.mat objects/testroom.rad skies/sky_overcast.mat skies/sky.rad
> rcontrib -ab 3 -ad 1024 -ar 256 -aa .1 -ffa -x 600 -y 430 -ld- -m sky_glow -n 2 -V+
> SOFTWARE= RADIANCE 4.2a lastmod Sat Jun 23 22:34:17 SGT 2012 by root on oats-iMac.local
> CAPDATE= 2013:12:28 23:54:10
> GMT= 2013:12:28 15:54:10
> FORMAT=ascii
> 
> -Y 430 +X 600
> 1.359378e-02    1.359378e-02    1.359378e-02    
> 1.359378e-02    1.359378e-02    1.359378e-02   
> ...
> (this continues for 258000 rows in total)
> 
> May I ask:
> 1. what the 3 values in each row represents for each pixel? 
> 2. How is this function of rcontrib related to the calculation of daylight coefficient? 
> 
> 
> Thanks you all for your kind help and wish you all a happy new year!
> 
> - Ji
> 
> _______________________________________________
> Radiance-general mailing list
> Radiance-general at radiance-online.org
> http://www.radiance-online.org/mailman/listinfo/radiance-general
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20140310/b007ba3e/attachment-0001.html>