[Radiance-general] masking openings

[Christopher Kings-Lynne]

> The best would be to render the 
> mask as a separated image, such that they can be combined using pcomb.

Using a mask would seem sensible to me.  There's a very small HOWTO on 
the radiance wiki that gives some basic info:

http://www.radiance-wiki.org/index.php?title=Mask_an_image

You'd probably want to generate your mask as a black and white image 
based on the polygon at that point.  eg. if the polygon is 'sky', then 
output a 0, else output a 1.

Chris