[Radiance-general] BSDF question

Mon Jul 7 08:39:29 PDT 2014

Reciprocity works for every BSDF type in Radiance except isotropic tensor trees, so specify +f with the -t3 option if you want transmission to go both ways.  Also, you need +f to get the reflected rays on the opposite side of the surface, so you may want it under some other circumstances as well.

The geometry (if present) is used for shadow calculation during the direct calculation and for viewing the system, while the BSDF is used for all other purposes.

Cheers,
-Greg

> From: CHI-German Molina <gmolina at hdlao.com>
> Subject: Re: [Radiance-general] BSDF question
> Date: July 7, 2014 7:39:33 AM PDT
> 
> Andy, 
> 
> How about using BSDFs for ray-tracing calculations? Does it use reciprocity as well? or should we calculate +b ?
> 
> Also, was I correct when I said that the BSDFd "Proxy Geometry" is only used for ambient calculations, and the geometry for Direct?
> 
> THANKS!
> 
> German
> 
> 
> 2014-07-07 10:32 GMT-04:00 Andrew McNeil <amcneil at lbl.gov>:
> Hi Giuseppe,
> 
> You only really need either +f or +b, not both.  genBSDF does +b by default so if you only wanted to do the forward direction you should use +f -b. Doing one direction takes about half as much time as doing both directions. However, if you want to import into Window you'll need to do both directions.
> 
> Andy
> 
> 
> On Thu, Jul 3, 2014 at 12:45 AM, Giuseppe De Michele <giudm.87 at gmail.com> wrote:
> Germán and Andy,
> 
> meanwhile thank you for the answers and sorry if my questions are a little bit confused. I am a newbie of the "lighting art" and I am studying this concepts almost alone. 
> So if I understood, when I create a BSDF matrix with genBSDF I add the +f +b options in order to obtain both transmission matrix and then dctimestep does the rest, doesn't it?
>  
> Thanks for yours great help,
> Giuseppe
> 
> 
> 2014-07-02 19:12 GMT+02:00 Andrew McNeil <amcneil at lbl.gov>:
> 
> Giuseppe,
> 
> The front/back inconsistency is mostly related to the BSDF material primitive. For the 3-phase method you don't have to worry about front/back of the fenestration. (unless you want to use a BSDF backwards, which might be impossible without manually editing the BSDF file).
> 
> The BSDF handling has matured quite a bit since the early days of the three-phase method. These days dctimestep can use either front transmission or back transmission applying reciprocity principles for the three phase method.
> 
> Andy
> 
> 
> 
> 
> On Wed, Jul 2, 2014 at 9:24 AM, CHI-German Molina <gmolina at hdlao.com> wrote:
> Guiseppe, 
> 
> The three-phase method simulation method does not use the geometry of the shading device to work. In fact, one advantage of it is that only the BSDF.xml file is needed. If you check Andy's Three-phase method tutorial, you will see that the calculation of the V and D matrices do not require neither the geometry of the shading device nor the BSDF of it (thus making it very easy to try multiple fenestration systems).
> 
> Adding Geometry information to the BSDF file is useful for using pkgBSDF; not really for the 3phase method. When rays have to go through the fenestration system (i.e. rendering a room with venetian blinds, or  when calculating the third matrix of the 5 phase method) a combination of two illums that "wrap" the geometry and the geometry itself seems to be more efficient. My understanding (I might be completely wrong) is that the illums are used to transfer the diffuse (indirect??) radiation; and the geometry is used for transfering direct radiation, casting shadows.
> 
> My answer to the first question:
> 
> 1.  in the equation i=VTDs the T matrix has to contain the window geometry or take knowledge of these from the other two matrix(V,D)? 
> 
> Nope. After the V and D matrices are calculated, the simulation is performed by simply multiplying matrices... no geometry.
> 
> 
> The second question is something that confuses me... it would be good to have clarification here.
> 
> Hope this helps!
> 
> Germán
> 
> 
> 2014-07-02 11:54 GMT-04:00 Giuseppe De Michele <giudm.87 at gmail.com>:
> Dear all,
> 
> I am working with the 3 phase method (only from a few weeks) and I am trying to understand the BSDF file format and how the 3 phase method read it.
> 
>  I have noticed that if I use the bsdf created with the command:
> genBSDF -n 4 +f +b -geom meter material.rad blinds.rad > blinds.xml
> the .xml file contain same geometrical dimension (height, width). In fact if I use the command: 
> pkgBSDF -s blinds.xml > blinds_pkg.rad
> the output file contain the original geometry.
> 
> Instead if I use the BSDF created or contained  in the database of WINDOW 7 these file do not contain a geometry.
> With pkgBSDF I got an error. 
>  
> My questions are: 
> 1.  in the equation i=VTDs the T matrix has to contain the window geometry or take knowledge of these from the other two matrix(V,D)? 
> 2.  I have understood that 3 phase method uses only the front transmission data [A. McNeil, Three-Phase Method tutorial]  but in
>  an other discussion [ http://www.radiance-online.org/pipermail/radiance-general/2012-October/008969.html ]  I have discovered that WINDOW and genBSDF use an opposite meaning of front and back, so the front transmission regards radiance or window meaning?And what part of the T matrix I have to use in the 3 phase method for the respective cases (WINDOW or genBSDF)?  
> 
> Kind regards,
> Giuseppe
> 
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> -- 
> Germán Molina L.
> Ingeniero Trainee
> Hunter Douglas Chile S.A.
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> Giuseppe De Michele
> 
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> -- 
> Germán Molina L.
> Ingeniero Trainee
> Hunter Douglas Chile S.A.
> Celular +569 89224445
> 
> Nota de Confidencialidad: Este mensaje incluído los archivos adjuntos son confidenciales y pueden contener informacion privilegiada protegida por ley. Si Ud. no es el destinatario, deberia abstenerse de copiarlo, distribuirlo, divulgarlo o usar la informacion contenida. Por favor, avise inmediatamente al emisor y borre este mensaje de su sistema. Los mensajes electronicos son susceptibles de ser cambiados, infectados o adulterados sin autorizacion. No asumimos responsabilidad alguna por ninguna clase de cambios o sus consecuencias. Usted debe estar informado que la compania puede hacer un seguimiento de sus mensajes electronicos y su contenido, gracias.
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