[Radiance-general] Questions about local coordinate systems

[radiance-general@radiance-online.org]

Hi! 

I am using the transdata material for incorporating BTDF's in 
Radiance. In this context, I have to convert cartesian coordinates in 
polar coordinates.
For a surface (i.e. rectangle) the normal is determined by the right-
hand rule. So far - so good. Unfortunately, I am not sure how the 
local coordinate system for the rectangle is defined?! How are the x-
axis, y-axis and z-axis are oriented? The source ray direction 
(x,y,z) and the transmitted ray direction (Dx,Dy,Dz) are referring to 
this local coordinate system, aren't they?

Could anybody give a hint to solve my problems?

Best regards,

Frank