[Radiance-general] Problem with Trans material

[Jan Wienold]

Hi Andrea,

I don't think trans material is the right way to model fabrics, since 
fabrics change their behavior depending on the angle of incidence. And 
since you don't have an option to model an angular behavior with trans 
it is better to use another material type. One option doing it is using 
BRTDfunc and "control" the direct transmitted part via a cal file. 
Typical fabrics have a very constant total transmission until 75° and 
then a drop similar as a glass. This can be modelled with an additional 
glass layer with a very low refraction index. The direct transmission 
depends on the fabric type and weaving process - typically you have 
cut-off angles for the direct transmission between 60-75°. This behavior 
you can model with a cal file. I've done that recently for a big 
simulation study as basis for the new European daylight standard and you 
can come close to the real behavior of the fabrics with this modeling 
approach. We compared BTDF measurements with the model.

good luck

Jan





On 18/05/17 16:03, Andrea Zani wrote:
> Hi everyone,
>
> I am trying to generate a BSDF of a complex translucent facade component.
> One of the materials in the model is a fabric and I'm trying to 
> simulate its optical behavior using a Trans material, however I have 
> some problems.
> Creating a BSDF of the trans material (you can see the material 
> definition below), I have obtained a particular angular transmission 
> behavior as showed in the pictures below. There is a particular 
> increment between 0° and 30° of the incidence angle, instead i was 
> expected something similar to the dashed lines in the graphs. Is it 
> normal to have this angular transmission trend? If not, how can I fix 
> this problem?
>
> Immagine incorporata 1
>
> Trans material
> voidtransFabric
> 0
> 0
> 70.80.80.80.0100.450
>
> BSDF:
> genBSDF -n 4 -c 4000 -dim 0.299 0.338 0.373 0.429 -0.018 0 -r -ab 10 
> trans_test_tr3.rad
> For the test the material geometry is a rectangle.
>
> Thanks in advance.
> Best
>
> -- 
>
> *Andrea Zani*
>
> Meng  |  Phd Student & Researcher
>
> *Politecnico di Milano*
>
> via Ponzio, 31 | 20133 Milano | Italia
>
> *t* +39 02 2399 6015
>
> *@: *andrea.zani at polimi.it <mailto:andrea.zani at polimi.it>
>
> *@: *andrea89.zani at gmail.com <mailto:andrea89.zani at gmail.com>
>
> www.polimi. <http://www.polimi.it/>it
>
>
>
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-- 
Dr.-Ing.  Jan Wienold
Ecole Polytechnique Fédérale de Lausanne (EPFL)
EPFL ENAC IA LIPID

http://people.epfl.ch/jan.wienold
LE 1 111 (Office)
Phone    +41 21 69 30849

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