[Radiance-general] transfunc with measured data (poor man's BSDF)

[Stephen Wasilewski]

I'm trying to set up a transfunc material to approximate some rough
measured data I have.  Assume my material is isotropic and I have a
unscaled transmission profile from 0-90 degrees.

here is my material:

void transfunc pane
2 tdis distribution.cal
0
7 .9 .9 .9 0 .8 1 12

.8 is my measured total VLT, and 12 is a magical factor that fits the
simulation with a physical model (this number is different for every
material, which is why I know I'm missing something, mostly likely math
skills)

and here is the contents of distribution.cal:

tcoef = .5*90*A7;

ang(x,y,z) = acos(x*Dx+y*Dy+z*Dz)/DEGREE;

bound2(x,y,z) = if(ang(x,y,z)-90,90,ang(x,y,z));

tdis(ix,iy,iz) = tcoef*tdat(91-floor(bound2(ix,iy,iz)));

tdat(i) : select(i,
0.000310656,
0.000312042,
0.000315021,
0.00032472,
0.000326868,
0.000330262,
0.000332341,
0.000343634,
0.00034689,
0.000354026,
0.00035749,
0.000359083,
0.000361647,
0.00036906,
0.000371554,
0.000377512,
0.000380076,
0.000389082,
0.000392061,
0.000394902,
0.000402315,
0.000408758,
0.000407857,
0.000420743,
0.000421228,
0.000429542,
0.0004346,
0.000436401,
0.000447971,
0.000449356,
0.000454414,
0.000458086,
0.000478385,
0.000477,
0.00048947,
0.000499585,
0.000507414,
0.000521339,
0.000536512,
0.000545726,
0.000556811,
0.000570598,
0.000586741,
0.000594084,
0.000607941,
0.000625399,
0.000634129,
0.000651241,
0.000673411,
0.000703202,
0.000728143,
0.000735071,
0.000774561,
0.000804352,
0.000824444,
0.000845921,
0.000895803,
0.000942914,
0.000982404,
0.001031594,
0.001085633,
0.001198561,
0.001237358,
0.00129694,
0.00136345,
0.001465293,
0.001581685,
0.001673136,
0.001792299,
0.001928783,
0.002159488,
0.002343776,
0.002594573,
0.002836364,
0.003092703,
0.00343218,
0.003813226,
0.004396572,
0.005160741,
0.006019825,
0.006495786,
0.008431496,
0.009456855,
0.011265089,
0.013821557,
0.017729005,
0.022952792,
0.052383356,
0.11064174,
0.238534447,
0.426770895)

Im getting the expected distribution for direct light sources (at least for
near normal incidence, which is all I am worried about for now), but glows
and indirect light seem to revert to a lambertian.   I have no idea how to
scale these values to insure the right transmittance or as the reference
manual puts it "The function brtd should integrate to 1 over each projected
hemisphere".

Help with integration, what is going on with diffuse transmission, anything
else I may be missing and/or that I am barking up the wrong tree entirely
would be greatly appreciated.

Thanks,

Stephen Wasilewski
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