Problem with Trans material

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?

<img src=’/uploads/default/original/1X/65cd71737adebb3e0d646d9d141b8514ea51e9a9.png’ width=‘690’ height=‘293’>

Trans material
void trans Fabric
0
0
7 0.8 0.8 0.8 0.01 0 0.45 0

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@polimi.it

*@: *andrea89.zani@gmail.com

www.polimi. <http://www.polimi.it/>it

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
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@polimi.it <mailto:andrea.zani@polimi.it>

*@: *andrea89.zani@gmail.com <mailto:andrea89.zani@gmail.com>

www.polimi. <http://www.polimi.it/>it

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--
Dr.-Ing. Jan Wienold
Ecole Polytechnique F�d�rale de Lausanne (EPFL)
EPFL ENAC IA LIPID

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

Hi Andrea,
I just ran your BSDF command and got a BSDF with 35.6% direct-hemispherical
transmission for all incident angles.
How are you calculating the transmission from the BSDF file? Can you send
me your BSDF file in a separate email?
Andy

···

On Thu, May 18, 2017 at 7:03 AM, Andrea Zani <andrea89.zani@gmail.com> 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?

[image: Immagine incorporata 1]

Trans material
void trans Fabric
0
0
7 0.8 0.8 0.8 0.01 0 0.45 0

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 <+39%2002%202399%206015>

*@: *andrea.zani@polimi.it

*@: *andrea89.zani@gmail.com

www.polimi. <http://www.polimi.it/>it

_______________________________________________
Radiance-general mailing list
Radiance-general@radiance-online.org
https://www.radiance-online.org/mailman/listinfo/radiance-general

Hi Andrea,

It is important to use the latest CVS head when generating Klems distributions with genBSDF. David Geisler-Moroder and Christian Knoflach discovered a bug in the corrections for Klems solid angles built into wrapBSDF, which is called by genBSDF. These errors are on the order of what you are seeing, so if you are using a distribution built before November of 2016, this could well be your problem.

Best,
-Greg

···

From: Andy McNeil <mcneil.andrew@gmail.com>
Date: May 18, 2017 7:52:15 AM PDT

Hi Andrea,
I just ran your BSDF command and got a BSDF with 35.6% direct-hemispherical transmission for all incident angles.
How are you calculating the transmission from the BSDF file? Can you send me your BSDF file in a separate email?
Andy

On Thu, May 18, 2017 at 7:03 AM, Andrea Zani <andrea89.zani@gmail.com> 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?

Trans material
void trans Fabric
0
0
7 0.8 0.8 0.8 0.01 0 0.45 0

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