[Radiance-general] rewrite BRDF as .cal file for plasfunc

[Roland William Fleming]

Hi Katja ---

Here's the hack I use.  Pass the viewpoint as arguments to the .cal 
function.  Then view direction is just:

Vx = -(arg(1) - Px);
Vy = -(arg(2) - Py);
Vz = -(arg(3) - Pz);

where Px, Py and Pz are the Radiance predefined variable names for the 
ray intersection points.

Hope this helps,
Roland


On Jul 18, 2005, at 5:26 PM, Katja Doerschner wrote:

> I sent this message earlier - but I think it may have gotten lost. I
> apologize if you receive it twice!
>
> Hello,
>
> Has anyone an example of how to write a .cal function for plasfunc,
> for a given BRDF?
> Specifically, I am not sure which predefined variables correspond to
> the view direction vector.
>
> This is the BRDF I want to use in Radiance:
> V(i,j,n,a)=(1/PI)*(a/(a+(i dot n)(j dot n))
> :i is unit vector from the source direction, j is unit vector of view
> direction, n is surface normal, a some other parameter.
>
> I know the Radiance-predefined variable names for the source direction
> (x,y,z) and surface normal (Nx,Ny,Nz), but I am not sure what to use
> for the view direction.
>
> Can anyone help?
> Thank you!
> katja
>
>
>
>
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