Something wrong with BRTDfunc material in a simulation

Hi Radiance experts,

I was simulating on a translucent material based on Radiance BRTDfunc material primitive. And the model I used was from CIE test case (CIE 171:2006), a simple 4m×4m×3m box with a roof opening of 1m×1m in the middle. But I found the results confusing because the BRTDfunc result was almost the same as that of an opening without any cover, as illustrated in the following picture. I also used a Trans material primitive to compare with BRTDfunc, and it seemed more correct.

Here’s my BRTDfunc material definition:

void BRTDfunc Sample4_front&back
0.0116 0.0139 0.0155
0 0 0
9 0.5875 0.5892 0.6238 0.5892 0.5897 0.6243 0.4291 0.3959 0.3836

And here’s my Trans material definition, using the same measured optical data as in BRTDfunc:

void trans Sample4_Trans
7 1 1 1 0 0 0.4 0.03

And the formulas I use for calculating Trans parameters are:

A7=Ts / ( Td+Ts )
A6=( Td+Ts ) / ( Rd+Td+Ts )
A3=Cb / ( (1-Rs)(1-A6) )
A2=Cg / ( (1-Rs)
(1-A6) )
A1=Cr / ( (1-Rs)*(1-A6) )

from Accurate definition of trans material

And now I don’t know what the problem is. Maybe it’s the way I using BRTDfunc that goes wrong.
Can you kindly help me out with this issue and give some advice? Thank you in advance!


I suspect there may be something else wrong with your model. Did you use the exact same skylight geometry in both cases? When I compare these materials as you’ve given them, they look very nearly identical in my renderings. I can only see slight differences in color.

Perhaps if you uploaded the rest of your model and your calculation parameters, we can figure out where things are going wrong.

Thanks for your quick reply Greg!
Well I think I have found where the problem is but I need your support for evidence.

I’m using Radiance through DIVA. Since I used the symbol “&” for naming the material, and after running the command obj2rad, I found no information about BRTDfunc in the converted .rad file. But when I changed the name to “Sample4_BRTDf” with “&” deleted, there came the information of BRTDfunc in the .rad file. And the result turned out pretty nice.
So I was wondering if it’s a bug of DIVA (or even Radiance but in my opinion less possibly).
Hope this helps!

I checked, and it seems that obj2rad does not care any more than the rest of Radiance if a material name has an ampersand (’&’) in it. However, if this information gets passed carelessly through XML or any related file format, it would likely cause problems, so your surmise of the issue is probably correct. I do not know enough about DIVA to speculate further.

Following Greg’s argument, if the material name becomes a file name, or
is manipulated like a file name in DIVA then assume the file naming

I always recommend that whenever you are constructing potential file
names it is best to avoid
( #
pound, % percent, & ampersand, { left curly bracket, } right curly
bracket, \ back slash, < left angle bracket, > right angle bracket, *
asterisk, ? question mark, / forward slash, blank spaces, $ dollar sign,
! exclamation point, ’ single quotes, " double quotes, : colon, @ at sign

Your operating system, scripts and other file name manipulation routines
may use any of the above characters by default!

Re: Request for a change
Kind Regards
Terrance McMinn

Thanks Greg and Terrance!!! I’ll pay more attention to that.:sweat_smile: