[Radiance-general] irradiance calc that ignores incident angle

[Greg Ward]

Hi Stephen,

The simplest method is to use rsensor to generate a set of uniformly distributed rays over the hemisphere, which you can re-use as often as you like.  The sensor file below specifies a uniform distribution out to 90°, and even samples a few rays below the horizon  Let's call it "uniform.dat":

degrees	0	90	180	270
0	1	1	1	1
5	1	1	1	1
85	1	1	1	1
90	1	1	1	1
95	0	0	0	0

You can run rsensor on the above file and either use it to call rtrace, or since you seem to want to call rtrace yourself, create instead a set of sample rays:

	rsensor -h -rd 100000 -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 uniform.dat > uniform_znorm_samp.txt

You can then give this as input to rtrace with whatever model you like:

	rtrace -h [options] octree < uniform_znorm_samp.txt | total -m

I've passed the output to total with the -m option, assuming you just want the average result.  If you're willing to call rsensor in place of rtrace, you can shorten the calculation time with:

	rsensor -rd 100000 -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 uniform.dat [rtrace options] octree

This has the added advantage of sampling light sources directly (unless you set -dn 0), and it even supports the -n multiprocessing option on Unix.  You will need to divide the result by the hemispherical solid angle (2π), or else change all your sensor file values by 1/(2π) to get a uniform average.

I did spent quite a bit of time writing and debugging rsensor for just this sort of thing, so you may as well use it.  (Thanks to Zack Rogers for specifying the problem and funding the initial work, and to David Geisler-Moroder for doggedly testing the code & helping me sort out its many issues.)

Cheers,
-Greg

> From: Stephen Wasilewski <stephen at coolshadow.com>
> Date: June 30, 2017 4:18:34 AM GMT+06:30
> 
> I'm curious if anyone knows of a way to simulate a sensor point that has a constant response (not cosine corrected) for all incident angles (at least over a hemisphere).  I know I can do this with an angular fisheye image and I assume I could use rsensor, but i would like a way to do this with a single rtrace or rpict calculation.  Applications include:
> 
> 1. measuring direct normal irradiance (when I know the source is small (cos(x) ~ 1), but not the incident angle.
> 2. calculating percent of a sky/source description that is visible from a point.
> 
> The one though I have is to place an analysis plane with a BRTDfunc material assigned and then do a radiance calc looking at the plane, but this only works for the 1st case, where it is part of the direct calculation.  For the second case I tried making a bsdf with bsdf2ttree (or bsdf2klems) but it seems to introduce a lot of sampling error and doesn't appear to be working quite right.
> 
> BRTDfunc approach:
> 
> #cal file:
> r(x,y,z) = 1/(x*NxP+y*NyP+z*NzP)/PI;
> 
> #material:
> void BRTDfunc test
> 10 0 0 0 0 0 0 r r r test.cal
> 0
> 9 0 0 0 0 0 0 0 0 0
> 
> BSDF approach:
> #cal file:
> rdif(ix,iy,iz,ox,oy,oz) = if(oz*iz,1/(ix*ox+iy*oy+iz*oz)/PI,0);
> 
> #bsdf2ttree:
> bsdf2ttree -t3 -g 7 -forward -f test.cal rdif > test.xml
> 
> #material:
> void BSDF test
> 6 0 test.xml 0 1 0 .
> 0
> 0
> 
> Either way, this feels like an inelegant approach, so anyone have a better idea or a way to get this working for a diffuse calculation?
> 
> Stephen Wasilewski
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