[Radiance-general] Trans material

[Greg Ward]

Hi Martin,

Your understanding of the -i option is correct in the basics, but there is a "hack" in the code to ignore glass and other typically transparent surfaces.  Otherwise, you would end up calculating illuminance on surfaces that generally don't care about illuminance.  And in the case of your diffuse trans material, it is unclear whether you would care more about illuminance on the inside (which would be the default reporting for interior views), or illuminance arriving at the other side.  Do you see the problem?

Lars' solution of using rtrace to compute the location then a second rtrace to compute irradiance (or illuminance) using -I+ is the best, because it gives you full control over what gets reported.  John Mardaljevic has used this to good effect in several specialized cases, which he has described here:

	http://radsite.lbl.gov/radiance/refer/Notes/jm_technote_02.pdf

As Lars mentions, Axel Jacobs also has some great tutorials:

	http://www.jaloxa.eu/resources/radiance/documentation/docs/radiance_tutorial.pdf

	http://www.jaloxa.eu/resources/radiance/documentation/docs/radiance_cookbook.pdf

Just generally speaking, you're looking for a command of the form:

	vwrays -ff -vf view.vf -x 1000 -y 1000 | rtrace -ff -h -opN scene.oct | rtrace -ffc -I+ `vwrays -d -vf view.vf -x 1000 -y 1000` [rendering options] scene.oct > result.hdr

If you want to speed up the second calculation on a multiprocessing machine, add a -n option to the second rtrace with the number of parallel processes. Be sure to specify an ambient file with -n if -ab > 0, or you won't see much speedup.

Cheers,
-Greg

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: January 25, 2013 2:58:04 AM PST
> 
> Hi Martine!
> 
>> You were right that it appears to be "perfectly" diffused when considering luminance. However, just for future references, isn't it possible to evaluate the illuminance incident on a surface with trans or glass material applied?
> 
> You can do so by piping "sensor locations/directions" into rtrace and use the -I+ switch. That way, you calculate irradiance at that location without having a visible surface in place. You can get one value for a given location by e.g. piping one location / direction vector, e.g. echo "0 0 0 0 0 1" | rtrace -I+ (all other parameters...). Or you can generate a grid of such values, eventuelly forming an "irradiance image", by using vwrays to generate the locations / directions for you. This is much more flexible then rpict -i, which always depends on a surface in your scene. Have a quick look at Axel Jacob's tutorial to see some examples.
> 
> Cheers, Lars.

----------------
> From: Martin Vraa Nielsen <martinvraa at gmail.com>
> Date: January 25, 2013 1:49:49 AM PST
> 
> Hi all,
> 
> Thank you all for your help - is was very beneficial.
> 
> You were right that it appears to be "perfectly" diffused when considering luminance. However, just for future references, isn't it possible to evaluate the illuminance incident on a surface with trans or glass material applied?
> Greg: with respect to the -i rendering option or not, I must admit I simply do not know enough to have a sufficient grasp on this. As I read it from the manual -i basically determined if you consider radiance or irradiance and therefore subsequently if you consider illuminance or luminance. Do I understand that correctly?
> 
> Sorry for all the potential trivial and basic questions and thank you for taking the time to answer them!
> 
> -- 
> Best regards,
> 
> Martin Vraa Nielsen
> 
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