[Radiance-general] Re: transdata formula & sky component

[Greg Ward]

Hi Frank,

There is a very helpful document that describes the material formulas on
the Radiance website:

	http://radsite.lbl.gov/radiance/refer/materials.pdf

You should note that the function you give to the transdata material for
the specular component should integrate to 1 over the sphere, not 4pi as
would be the case for a true BTDF.  This is because you are supplying
only the specular distribution, not the actual reflectance function, as
the formula for transdata includes some other factors (such as a diffuse
component) to build a complete BTDF.

Regarding the integration of the sky distribution, all of the BRTDF
types in Radiance approximate non-source contributions as diffuse,
because I never built in the machinery to properly integrate arbitrary
distribution functions.  So, the answer will not be exactly correct if
you use the custom reflectance materials.  Only light sources, such as
the sun, will utilize the distribution functions you specify.

I hope this helps.
-Greg