# transdata formula & sky component

Hi!

1) I am using the transdata material for incorporating measured
BTDF's [1/sr] in Radiance. With the help of 'rtrace', I will verify the
results. Is it possible to compute luminance values with 'rtrace'?

2) In the Radiance Digest I have found a formula for the metdata
material:

radiance (watts/sr/m^2) Lo = spec * cos(theta_i) * omega_i * Li * f

where:
f = BTDF in 1/sr
mega_i = solid angle of source in sr
Li = avg. radiance of source in watts/sr/m^2
theta_i = angle between surface normal and source direction
spec = multiplier for specular component from material arguments

I would like to calculate the luminance values back to the
measured BTDF's using the transdata formula. How can I get the
transdata formula?

3) Another question concerns the diffuse sky component. How will
Radiance compute this component with the transdata material?

Could anybody answer my questions or give me reference how I
can solve these problems?

Best regards,

Frank

Hi Frank,

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

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