[Radiance-general] experiences with the photon map

[Jan Wienold]

Hi Lars,

I'm pretty sure its a question how you modeled the end of your "light-pipe".
Photon mapping is doing an density estimate to calculate the radiance
and is searching for photons in the nearby.
Unfortunately the algorithm is doing this also for surfaces , which are
"leaving" a closed space. For example, if you have a closed box and you
add a surface to the scene, which is half inside and half outside the
box, you will get light into the box.
You can avoid this phenomena, if you really "close" your pipe (I assume
so  ;-)  ).

The question of photons and how many of them should be collected within
the density estimate cannot be answered without knowing more about the
scene. Its similar to "pure Radiance", the parameter setting is depending
on the scene and materials.....  ;-)  .  Anyhow, 2000 for the "collecting"= bandwidth seems to be high for me.
Have you also tested the bias compensation (-apcb) in order to reduce the splotches? 

By the way, we will probably update photon-mapping for the 3.9 version
(or the 4.0) version within the next half year, fixing also some bugs.
This depends on a new project probably launched in January.

Cheers,

Jan

Lars O. Grobe wrote:

> > Hi list!
> >
> > While I am trying this on Radzilla now (as there is no photon-mapping
> > available in Radiance-3.9 at the time of this writing), I am pretty sure
> > that the effects I get are not related to Radzilla but on the way I use
> > the pmap-System.
> >
> > I am trying to model a simple, highly specular horizontal light duct,
> > with a rectangular cross-section (yes I know that in THIS case I could
> > use the mirror material and Radiance, but I will go for non-planar
> > surfaces later).
> >
> > Now either I get very splotchy luminance images in the inside, or,
> > increasing the `bandwith` to rather high values, have a blurry result
> > with - and this is the problem - light leaving the geometry even though
> > it is closed (the duct has a 45 degree mirror at its end that should
> > actually close it, but the sides extend besides the mirror and appear
> > illuminated even behind it).
> >
> > I am working with an initial photon count for both global and caustic
> > photons of 2.000.000 and a bandwith value of 2000 at the moment.
> >
> > Is it possible to get such systems rendered in a way that I have both a
> > smooth luminance and no light escaping the geometry?
> >
> > Is there any source for a starting point in what way rtcontrib could be
> > used for modeling such systems? I know the qt-movie from the workshop,
> > but I did not get it - and it seams that I know noone except Greg who
> > got it so far  ;-) 
> >
> > Thank You & CU, Lars.
> >
> >
> > _______________________________________________
> > Radiance-general mailing list
> > Radiance-general at radiance-online.org
> > http://www.radiance-online.org/mailman/listinfo/radiance-general
>   


-- 
Dipl.-Ing. Jan Wienold
Project Manager
Fraunhofer-Institut für Solare Energiesysteme
Thermal Systems and Buildings, Lighting and Daylighting
Heidenhofstr. 2, 79110 Freiburg, Germany
Phone: +49(0)761 4588 5133 Fax:+49(0)761 4588 9133
jan.wienold at ise.fraunhofer.de
http://www.ise.fraunhofer.de

In office: 
Mo,Tue: 8:30-18:00
We,Thu: 8:30-16:00
Fr:  8:30-15:30

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://radiance-online.org/pipermail/radiance-general/attachments/20081205/65a1c294/attachment.html