[Radiance-general] Cutting a cylinder, or modeling smooth geometry...

[Greg Ward]

Hi Lars,

Your idea of applying a mixfunc is clever, and I believe it should  
work, at least in Radiance "classic".  I have no idea if material  
mixing is supported in the photon mapping extension, however.

Best,
-Greg

> From: "Lars O. Grobe" <akilog at nus.edu.sg>
> Date: February 5, 2009 11:21:26 PM PST
>
> Hi,
>
> I am running into problems due to the lack of csg in radiance. I  
> need to accurately model a tube with a very specular material,  
> which is bent. The real tube is produced by cutting a cylinder  
> under 22.5 degree and assembling it, leading to a nice 45degree  
> angle with one ellyptical edge. As I am not able to cut a cylinder  
> geometry in radiance, I have modeled it as a mesh, but the  
> specularity leads to visible edges.
>
> I have been sonsidering several solutions so far:
>
> 1) Using antimatter to cut away parts of the cylinder would  
> probably not work, as the second cylinder, which also has to be  
> cut, would lead to two overlapping antimatter volumes.
>
> 2) Refining the mesh would result in huge geometry, as the segments  
> would become thinner while keeping their length, this would lead to  
> a real worst case for oconv. Also to make the edges disappear, I  
> would have to get into dimensions where one facet should not take  
> much more then one pixel in the final rendering.
>
> 3) Using "smooth" surface normals would lead to an visual  
> improvement. But I have my doubts if these would be considered for  
> photon mapping and especially caustics? In my setup, the pipe ends  
> with a diffusor (currently modeled as a box modified by a rather  
> generic trans material). I would expect caustic effects to become  
> visible caused by a round tube geometry with very high specularity.
>
> 4) The last option I see is using a cylinder object with a mixfunc  
> modifier and a simple cal file containing just the line
>
> condition=if(Pz-Px,1,0);
>
> to cut my geometry in 45 degrees, using void to get parts of the  
> geometry invisible. Would that work with overlapping geometries  
> (which would still exist, even if invisible), and would it be  
> relieable with the pmap extension? The main disadvantage here is  
> that I would have to maintain an extra set of geometry for  
> radiance, while I could use converters to import from other  
> programs so far.
>
> CU Lars.