# Something wrong with BRTDfunc material in a simulation

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
10
if(Rdot,0.0045,0.0038)
if(Rdot,0.0030,0.0032)
if(Rdot,0.0015,0.0017)
0.0116 0.0139 0.0155
0 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
0
0
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 )
A5=Sr
A4=Rs
A3=Cb / ( (1-Rs)(1-A6) )
A2=Cg / ( (1-Rs)
(1-A6) )
A1=Cr / ( (1-Rs)*(1-A6) )

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!

Regards,
Yao

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.

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
convention:

I always recommend that whenever you are constructing potential file
names it is best to avoid
(https://www.mtu.edu/umc/services/digital/writing/characters-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.