[Radiance-general] mixfunc for dichroic film?

[Greg Ward]

Hi John,

Another argument for using the BRTDfunc or transfunc types is that they 
don't require new ray evaluations for each new material type, which is 
a penalty you pay when you blend materials with a mixfunc.  Although it 
is tempting, you really should avoid using mixfunc with materials 
unless there really is no other way.  Originally, mixfunc would only 
blend patterns and textures, and this is still its primary purpose.  
However, using mixfunc as a this-or-that switch with materials doesn't 
incur the same penalty as smoothly blending them, because only one or 
the other material is ever evaluated.  If one of the materials is 
"void," you can get a kind of selective absence of a surface, which can 
be useful for screen doors, gratings, etc.

I'm not familiar with dichroic materials in an architectural context.  
Can you be more specific as to the behavior you wish to simulate?

-Greg