[Radiance-general] Lightpipes and rtcontrib

[Axel Jacobs]

Greg,

thanks for your reply.

>> e) then there's the new rtcontrib program
>> (http://luminance.londonmet.ac.uk/radiance_mailinglists/general/
>> 2005-May/002710.html):
>> "Rtcontrib can also be used by the more adventurous among you to
>> compute input/output relations for devices such as light pipes and
>> shading systems, although I have yet to test such an approach,
>> myself." --Greg
>> There is a directory ray/src/rtcontutor, but the example explains
>> rtcontrib's use for daylight coefficients, not light pipes. I
>> personally can't quite work out how it would possibly replace a
>> forward raytracer, so has anybody been adventurous yet used it for
>> this?

> It's the talk entitled, "Applying the Radiance rtcontrib program,"
> but heed the warning and download the QT movie rather than hitting
> the link on your browser, because most browsers (including Safari)
> barf on it.  The key part you want to check out is "Step 2" in the
> mirrored light pipe example.  I'm using flat surfaces, but only for
> comparison purposes.  The rtcontrib approach should work for any
> geometry.

Found your talk. I also looked at the examples in the rtcontutor
directory (which seems to have disappeared in the 3R9 tar ball). The
geometry you used for the light ducts is relatively simple. I am
particularly interested in round light pipes, not square light ducts.

I noticed that in model.mat, you define the material for the duct as
metal, only to immediately re-define it as mirror:

----------------8<-----------------
void metal mirror_mat
0
0
5 .9 .9 .9 1 0

void mirror mirror_mat
0
0
3 .9 .9 .9
----------------8<-----------------

I understand why you would prefer mirror instead of metal: it creates
virtual sources of the sun, dealing with the direct component. So
would the only option with a round pipe be to break it up into
polygons?

It strikes me that this particular example of rtcontrib is useful for
demonstrating its use with sky components, so a large number of sky
distributions and sun positions may be computed rather quickly once
the coefficients (individual images) are generated. Guess you used the
light ducts simply to add some interest to the scene. The same scene
complexity could as well have been simulated without rtcontrib if an
animation ("input/output relations", as you called it) was not sought.

Many thanks

Axel