[Radiance-general] Lightpipes and rtcontrib

[Greg Ward]

Hi Axel,

> 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.

As I said, I used planar geometry so I could compare to the virtual  
source result.  Rtcontrib will work with any geometry.

> 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?

I don't recall why I left the original definition in there, but the  
later one is all that matters.  There's not a lot of sense in  
breaking your cylinder into long polygons, as it would end up  
creating an insane number of virtual sources.  Better to leave it as  
a cylinder and use the metal type, since mirror doesn't do anything  
for curved geometries.

> 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.

Actually, I was trying to come up with a scene that would be very  
difficult to simulate w/o rtcontrib -- if the light pipe had curved  
surfaces, for example.  In such case, you wouldn't have been able to  
render even a single time point if it included the sun without  
rtcontrib.

-Greg