[Radiance-general] irradiance calc that ignores incident angle

[Stephen Wasilewski]

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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