[Radiance-general] experiences with the photon map

[Jan Wienold]

Hi Lars,


Lars O. Grobe wrote:
> Hi Jan!
>> 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  ;-)  ).
>>   
>
> Well the pipe is closed by the 45 degree mirror, but I guess that I
> must not have geometry extend from the end (so that a part of it is
> inside the system and a part is outside). This is really important for
> modelling and was not clear to me so far! So the viewer in the sketch
> below will see the surfaces A and B lid even though light is not able
> to reach those, because the photons hitting the surface are counted
> even though not visible from the viewer position, right?
>
> Surface A
> ___________________________
>                          |
> light source      |    viewer
> ________________|___________
>
> Surface B
>
Yes! And the higher the bandwidth is the more will be transferred in the
the "dark" space. But its only a problem, if the "outside" is very dark
and no photons are stored there. Then, the density estimate searches for
nearby photons and collects the "wrong" ones.



>> 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.
>>   
> This is good news for all Radiance users! Will it be possible to use
> BRTDfunc modifiers with the photon map than? At the moment we have a
> strange situation - most Radiance users most probably use the software
> for advanced daylighting simulation. But they cannot model the
> advanced systems with the core distribution (pmap is not integrated),
> and even if they patch Radiance to support photon mapping, they cannot
> use the galzing definitions as given e.g. by the glaze script because
> of a lack of support for BRTDfunc in the pmap extension. I wonder how
> one can model a daylight set-up with light redirection systems in
> Radiance at all at the moment.
We will probably include BRTDfunc into pmap, but this is not clear now.

A remark from me: I never use BRTF in RADIANCE now, since all the
angular information you put into your model is lost for the glow
material. That means, if you model a specific sky luminance distribution
and you are using BTDF-func, no angular information of your BTDF-model
is used! It is treated lambertian ! And this is especially hard, if you
want to model a system, which is intended to redirect the bright zenith sky.
 If I model advanced glazings, I use either standard glass and modify it
by brightfunc or for high reflective materials I use a mixfunc of glass
(+brightfunc) and metal. I always check the angular transmission and
reflection of the model by a virtual measuement - and I also test, if it
still works for the sky.

But Greg  presented at the workshop the BTDF implementation into mkillum
- this seems to me a very good approach to calculate the advanced window
systems! So this can probably solve your problem now. But I don't know,
how it treats the sky and if the angular information is also "lost".
This is also new to me - I haven't tested it yet.
>
> How are experiences with the pmap for Radiance 3.8? I could use that
> one for now (I cannot really wait for half a year, so I must use what
> is available) if it is found to have been relieable. Or should I stay
> with 3.7?
I have to admit, that we work here still with the 3.7 version (RADIANCE
and pmap). No experience with 3.8.
If you are using pmap, don't use the -I option for rtrace - it is giving
you wrong values. This is one of the bugs we will solve in the near
future. If you want to calculate the illuminance, you should calculate a
180 degree fish-eye-picture at  your calculation points and integrate
the picture  (you can use findglare or evalglare to calc an illuminance
from a picture)
>
> Again thanks to you and the other Fraunhofer folks making this
> important extension available to us - is there any hope that pmap
> could become part of the Radiance distribution itself one day?

Well, this is not so easy to answer. It is also a problem of maintenance
- we can maintain only, if we have funding. And we are still looking for
a volunteer  for that.....

Cheers,

Jan


>
> CU, Lars.
>
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-- 
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: 
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