[Radiance-general] using transdata for roller shade simulation

[Greg Ward]

Hi Anna,

Other than the source code(!), we just have the Radiance reference manual:

	http://radsite.lbl.gov/radiance/refer/ray.html#Materials

Here's what it says about BRTDfunc:

BRTDfunc
The material BRTDfunc gives the maximum flexibility over surface reflectance and transmittance, providing for spectrally-dependent specular rays and reflectance and transmittance distribution functions.
        mod BRTDfunc id
        10+  rrefl  grefl  brefl
             rtrns  gtrns  btrns
             rbrtd  gbrtd  bbrtd
             funcfile  transform
        0
        9+   rfdif gfdif bfdif
             rbdif gbdif bbdif
             rtdif gtdif btdif
             A10 ..
The variables rrefl, grefl and brefl specify the color coefficients for the ideal specular (mirror) reflection of the surface. The variables rtrns, gtrns and btrns specify the color coefficients for the ideal specular transmission. The functions rbrtd, gbrtd and bbrtd take the direction to the incident light (and its solid angle) and compute the color coefficients for the directional diffuse part of reflection and transmission. As a special case, three identical values of '0' may be given in place of these function names to indicate no directional diffuse component.
Unlike most other material types, the surface normal is not altered to face the incoming ray. Thus, functions and variables must pay attention to the orientation of the surface and make adjustments appropriately. However, the special variables for the perturbed dot product and surface normal, RdotP, NxP, NyP and NzP are reoriented as if the ray hit the front surface for convenience.

A diffuse reflection component may be given for the front side with rfdif, gfdif and bfdif for the front side of the surface or rbdif, gbdif and bbdif for the back side. The diffuse transmittance (must be the same for both sides by physical law) is given by rtdif, gtdif and btdif. A pattern will modify these diffuse scattering values, and will be available through the special variables CrP, CgP and CbP.

Care must be taken when using this material type to produce a physically valid reflection model. The reflectance functions should be bidirectional, and under no circumstances should the sum of reflected diffuse, transmitted diffuse, reflected specular, transmitted specular and the integrated directional diffuse component be greater than one.



> From: "Atzeri Anna Maria (Student NaTec SusEn12)" <AnnaMaria.Atzeri at natec.unibz.it>
> Subject: Re: [Radiance-general] using transdata for roller shade simulation
> Date: May 8, 2015 8:24:08 AM PDT
> 
> Hi Greg,
> and thank you for your super quick response!
> At the moment I can’t implement in my simulation the BSDF type since I’m using Radiance via DIVA, and the actual Daysim version implemented in DIVA doesn’t support BSDF type for the annual calculation.
> This was the reason for whose I thought to use transdata.
> Instead the BRTDfunc seems to work. I tried to run an annual simulation and it seems like nothing wrong happened.
>  
> So, I’ve only to understand how BRTDfunc exactly works. Any suggestion will be very appreciate!
>  
> Thank you,
>  
> Anna
>  
> From: Greg Ward [mailto:gregoryjward at gmail.com] 
> Sent: 07 May 2015 23:48
> To: Radiance general discussion
> Subject: Re: [Radiance-general] using transdata for roller shade simulation
>  
> Hi Anna,
>  
> The transdata (and transfunc) materials do not fully model material behavior as well as the newer BSDF type (or even BRTDfunc).
>  
> I have a few models of roller shades, which you might be able to modify to your purpose.  I do not (currently) have any measurements to offer.
>  
> Best,
> -Greg
>  
> -------
>  
> # Shade material s99745 by Mechoshade, white facing front (inside)
> # Measurements by Mehlika Inanici & Judy Lai 9/20/2004
> # Diffuse measurements by Mike Rubin 10/7/2004
> void BRTDfunc Mechoshade99745
> 10
>             0 0 0
>             .02*tspec .02*tspec .02*tspec
>             0 0 0
>             shade.cal
> 0
> 11
>             .628     .628     .628
>             .377     .377     .377
>             .0225   .0225   .0225
>             56        68
>  
> -------------
>  
> # Shade material #1 by Mechoshade
> # Measurements from Steve Hebeisen <steveh at mechoshade.com>
> void BRTDfunc Mechoshade6020
> 10
>             0 0 0
>             tspec tspec tspec
>             0 0 0
>             mechoshade.cal
> 0
> 9
>             .37       .37       .37
>             .56       .56       .56
>             .03       .03       .03
>  
> --------------
>  
> # Shade material #2 by Hexcelshade
> # Measured from sample 11/3/2003
> void BRTDfunc HexcelshadeS065
> 10
>             0 0 0
>             tspec tspec tspec
>             0 0 0
>             hexcelshade.cal
> 0
> 9
>             .26       .28       .30
>             .59       .61       .60
>             .02       .02       .02
>  
> -----------
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