[Radiance-general] Mesh Rendering Performance

[Marcus Jacobs]

Dear Group

I have been using obj2mesh to convert my 3D scene to the Radiance triangular 
mesh (rtm) format for the last 6-8 months. Although I do prefer using this 
method to convert my scenes, a recent test confirmed that the performance 
suffers greatly when using a mesh when compared to using a scene consisting 
of polygonal objects (i.e. void polygon triangle1 ......) . Case in point, 
one rendering took 8.7 for the scene consisting of a mesh primitive and 3.7 
hours to render for the scene consisting of triangular polygon primitives. 
This is with the -n option set to 6, which should yield the best performance 
(actually, its about the same whether I leave -n to the default or if I use 
a lower value). A slight performance decrease (5%-10%) would be acceptable 
but 135% is extreme. On top of that, I have some modified source code for 
o_face that can yield a 10%-20%+ performance increase without any additional 
memory cost (It's just an implementation of Moller-Trumbore ray/triangle 
intersection). What is surprising to me is that I would normally assume that 
the mesh primitive would be more efficient due to its caching of the 
ray/edge comparisons during ray tracing but this does not seem to be the 
case. Out of curiosity, has anyone here experienced the same? Does anyone 
here know if Radiance sees the rtm file as one huge mesh or as several  
smaller meshes? If this is the case, does anyone know if breaking each scene 
object as a separate mesh primitive would improve performance?

Thanks

Marcus