[Radiance-general] Surface normal and Rdot in mix functions

[radiance-general@radiance-online.org]

At 14:02 2002-05-03 -0400, you wrote:
>You can assign different materials/patterns/textures to the front
>and back side of a surface simply by using "if(Rdot,1,0)" as the
>coefficient for a mixture.

So respectively "if(Rdot,Rdot, -Rdot)" returns cos(theta) if both sides are 
similar. - By the way, I never realised before that modifiers "foreground" 
and "background" could refer to the concrete front and back of a surface...

Thanks very much,
Markku N.