[Radiance-general] Illegal Power warning

[G.Harding1 at Bradford.ac.uk]

Hi,

I am trying to implement a measured BRDF using the BRTDfunc material type.

In my .cal file, I fit the LaFortune cosine lobe BRDF model for my data to the
diffuse and specular variables.

This seems to work ok in principal (although the results don't look quite
right), but when calculating the specular componets I get the following
warning:

rpict: warning - compute error for BRTDfunc "measured_surface"
rpict: Illegal power


As far as I am aware all the raised powers in my functions are reasonable
values, so I'm not sure why I'm getting this error.
Does any one know what the conditions are that would give rise to this error
(ie, do powers have to be integers for instance)?

Many thanks,
Glen Harding.


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