[Radiance-general] Illums and 3 Sided Polygons

[Greg Ward]

Hi Marcus,

Didn't we have this conversation a few weeks ago?  Look at the recent 
postings from Mark de la Fuente and the various responses regarding 
illum's and illum shapes.

The simplistic source sampling code in Radiance assumes that a 
rectangular distribution of rays may be used.  This is done more in the 
interest of speed than anything else, but it does place some limits on 
the shapes of sources you can model.  The closer to a rectangle (or 
parallelepiped) your source is, the closer to -dj 1 you can go with 
your source sampling.  Triangles are problematic because Radiance fits 
them into a rectangle and distributes the rays about this rectangle's 
center.  In the case of a right triangle, this sends a ray exactly to 
the center of the hypoteneus, and floating point errors take over from 
there to produce a hit about 50% of the time.  Not a good situation, 
and other triangles guarantee we'll miss every time with this sampling 
strategy.

A better approach for illum's, as I mentioned in one of my earlier 
responses to Mark, is to create a larger rectangular illum to cover 
your opening, disregarding the actual shape of your window elements.  
For a large window with many elements, you're better off using a single 
illum to cover it, anyway, so you don't send multiple samples when it 
isn't really necessary.  Mkillum will compute the correct average for 
the larger illum surface.  So long as this surface doesn't intersect 
the walls or have similar problems with its fit to the window, the 
result should be a reasonable approximation.

The "aiming failure" warning message should be taken seriously, unless 
you happen to know from geometric arguments that it is a rare condition 
generating this error.  In general, it means that you are 
underestimating contributions from the associated source or illum.

-Greg