Redirecting+scattering

Federico_Giovannetti · May 19, 2005, 1:24pm

Hallo Greg Ward, hallo Radiance Community!

I've been learning Radiance for a couple of weeks and i have to work intensively with it in the next weeks, as I´m investigating the energy saving potential and visual comfort of systems combining angular selective and diffusing panes.

Now I have to simulate a simple test-room with a complex fenestration, which includes an exterior prismatic structure as shading device and a diffusing pane for glare control. I try to find out the best way to model the system so as to get quantitative accurate results (RwR Chapters 10-13 and mails archive) but i´m still a little bit confused....

- As to the prismatic structure, i will use the PRISM2 material as virtual source. No redirection of diffuse incoming light and no dispersion are the only limitations of this material. And it would make no sense to use the PhotoMap port, because these materials are not implemented in this tool. Is it right?

- As to the diffuser, i would intuitively treat it as an ILLUM with a BRTDF function as alternate material (I´m measuring, modelling and comparing different kind of diffusers) and run MKILLUM. If i understood correctly, the BRTDF materials are only considered in the direct calculation and indirect specular, but not in the indirect diffuse, where they are approssimate to lambertian diffusers. Should i try another material?

- How are PRISM materials computed in the MKILLUM calculation of the window light output? Should I better try to run the simulation without MKILLUM and high ambient parameters (-ab, -ad, -as)?

- What if i measure the BRTDF of the complete system and i model it as a normal light source? It is really difficult (or impossible) to get sharp peaks in light output with high subsampling and jittering of the source?

Any other suggestion to simulate this system correctly (quantitative)?

Sorry for so many questions and THANKS A LOT for any contribution.

Regards,
Federico Giovannetti

···

---------------------------------------------------------
Federico Giovannetti
Institut für Solarenergieforschung GmbH
Am Ohrberg 1
D-31860 Emmerthal

tel: +49(0)5151-999-501
fax: +49(0)5151-999-500
e-mail: [email protected]
internet: www.isfh.de

Greg_Ward · May 19, 2005, 3:08pm

Hi Federico,

It sounds to me like you are "diving into the deep end" as they say about new swimmers who are very ambitious.

- As to the prismatic structure, i will use the PRISM2 material as virtual source. No redirection of diffuse incoming light and no dispersion are the only limitations of this material. And it would make no sense to use the PhotoMap port, because these materials are not implemented in this tool. Is it right?

I don't know about support in the photon map add-on, but the prism material types do redirect diffuse rays as well. They just don't have a diffuse component.

- As to the diffuser, i would intuitively treat it as an ILLUM with a BRTDF function as alternate material (I´m measuring, modelling and comparing different kind of diffusers) and run MKILLUM. If i understood correctly, the BRTDF materials are only considered in the direct calculation and indirect specular, but not in the indirect diffuse, where they are approssimate to lambertian diffusers. Should i try another material?

Your information is correct as far as I understand what you are saying. If you can fit your measurements to a Gaussian lobe distribution, you are better off using one of the native models (trans or trans2), as all ray paths will be followed appropriately. The general BRDF types, transfunc, transdata, and BRTDfunc, only use the directional-diffuse lobe specified for interactions with light sources. General ray sampling from an arbitrary distribution is quite expensive, and not done in Radiance for that reason.

- How are PRISM materials computed in the MKILLUM calculation of the window light output? Should I better try to run the simulation without MKILLUM and high ambient parameters (-ab, -ad, -as)?

The diffuse portion of the window distribution through a prismatic glazing will be computed by mkillum, leaving the "specular" (directional) portion to be done during the final rendering. This is the correct behavior, and you don't have to treat it specially with mkillum, which is helpful for considering contributions from the sky, ground, and neighboring structures.

- What if i measure the BRTDF of the complete system and i model it as a normal light source? It is really difficult (or impossible) to get sharp peaks in light output with high subsampling and jittering of the source?

I'm not sure what you mean, here. If you have some way to simulate your exterior and measure the actual output distribution of the window system as a light source, then you wish to represent that as a brightdata modifier on a window polygon, this would work. It's a very challenging measurement problem, though, which is why no one has done it to my knowledge. (Anyone out there have information on this?)

The purpose of mkillum of course is to create a light source from your surface(s) by simulating an outdoor situation. It doesn't always work, though. Specifically, mkillum has trouble resolving the contribution from the sun through curved, specular reflectors and devices. It can handle most everything else.

Any other suggestion to simulate this system correctly (quantitative)?

Measure and validate. Repeat.

-Greg

Jan_Wienold1 · May 19, 2005, 2:07pm

Hi Federico,

your task cries for using photon-mapping...

You should model your shading system geometrically and use dielectric as material (be careful about the surface orientation!). The diffusing material can be modeled by trans material. If you have a complex scattering material, you can adapt the trans material by an cal function. (Don't use BRTF-func or data with the current photon-mapping version).

To be more efficient using the diffuse sky, you should define a photon-port, where all the photons are emitted to. Usually the photon-port material is the window pane.

Then you can simply run the simulations, direct and diffuse contributions should be calculated correctly.

Last but not least, there exist also a version of daysim containing photon-mapping, so you can run these simulations also hourly (or other timesteps) if you want. Unfortunately, the latest daysim version didn't compile with the photon-mapping extension - but if you need it, you can use also the older version, Christoph could provide it.

Good luck!

Jan

Federico Giovannetti wrote:

···

Hallo Greg Ward, hallo Radiance Community!
I've been learning Radiance for a couple of weeks and i have to work intensively with it in the next weeks, as I�m investigating the energy saving potential and visual comfort of systems combining angular selective and diffusing panes.
Now I have to simulate a simple test-room with a complex fenestration, which includes an exterior prismatic structure as shading device and a diffusing pane for glare control. I try to find out the best way to model the system so as to get quantitative accurate results (RwR Chapters 10-13 and mails archive) but i�m still a little bit confused....
- As to the prismatic structure, i will use the PRISM2 material as virtual source. No redirection of diffuse incoming light and no dispersion are the only limitations of this material. And it would make no sense to use the PhotoMap port, because these materials are not implemented in this tool. Is it right?
- As to the diffuser, i would intuitively treat it as an ILLUM with a BRTDF function as alternate material (I�m measuring, modelling and comparing different kind of diffusers) and run MKILLUM. If i understood correctly, the BRTDF materials are only considered in the direct calculation and indirect specular, but not in the indirect diffuse, where they are approssimate to lambertian diffusers. Should i try another material?
- How are PRISM materials computed in the MKILLUM calculation of the window light output? Should I better try to run the simulation without MKILLUM and high ambient parameters (-ab, -ad, -as)?
- What if i measure the BRTDF of the complete system and i model it as a normal light source? It is really difficult (or impossible) to get sharp peaks in light output with high subsampling and jittering of the source?
Any other suggestion to simulate this system correctly (quantitative)?
Sorry for so many questions and THANKS A LOT for any contribution.
Regards,
Federico Giovannetti
---------------------------------------------------------
Federico Giovannetti
Institut f�r Solarenergieforschung GmbH
Am Ohrberg 1
D-31860 Emmerthal
tel: +49(0)5151-999-501
fax: +49(0)5151-999-500
e-mail: [email protected] <mailto:[email protected]>
internet: www.isfh.de <http://www.isfh.de>

------------------------------------------------------------------------

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

Giulio_Antonutto1 · May 19, 2005, 3:31pm

you may also consider to have a look to radzilla

This is a not official, not bullet proof, version of radiance that incorporates all new additions + some really crazy Carsten optimisations/changes/addons...

It seems really consistent and stable and I already modelled some prisms with pmap within it (like a 500 modular prisms wall).

It works well!

I believe it's worthy to try and I have been really impressed by the latest development.

The advantage over traditional radiance for me is that photon mapping is performed through the rpict command without having to use mkpmap... but they might be some bugs here of there.... watch out!

ciao,

giulio

···

________________________________

From: [email protected] [mailto:[email protected]] On Behalf Of Federico Giovannetti
Sent: 19 May 2005 14:24
To: [email protected]
Subject: [Radiance-general] Redirecting+scattering