[Radiance-general] Illums and 3 Sided Polygons

[Marcus Jacobs]

Dear Radiance Gurus,

I have noticed that Radiance does not like 3 sided polygons for illums. My 
understanding is that a minimum of 4 vertices are needed (Out of curiosity, 
I understand that shadow rays are traced to the centroid of an object. 
Unless the triangle is obtuse, it seem that it should be simple to find its 
centroid.)

I would like to know whether or not the rendering time and accuracy would be 
adversely affected by using 3 sided polygons as illums and living with the 
"aiming failure" error messages. The reason why I ask is because sometimes I 
use windows that have some curved profiles (such as gothic windows) that can 
not be modeled as 4 sided polygons (my modeling software creates them as a 
mesh of 3 sided polygons by default). I may be able to export some of the 
polygons as 4 sided but for some windows, there is no getting around having 
some polygons 3-sided. Even when I do export some polygons as 4 sided, there 
still are some instances when I get the "aiming failure" error message. As 
far as I can tell, the polygons are non-concave and have no opening. I have 
looked at creating an illum object outside of the window but unfortunately 
this approach leads to some goofy reflections on the inside. Any 
suggestions?

Marcus

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