Direction toward light source for BRTDfunc's specular reflection calculation

Direction toward light source for BRTDfunc’s specular reflection calculation
Hi all,

This is Xiaoming from Singapore. I am involved in a project to model various types of materials from measured data with Radiance. To transfer a functional description of the model, I use the BRTDfunc material type. I started with a purely reflective material.

I figured out how to set the directional diffuse part of the reflection for different incident light directions using rbrtd, gbrtd, bbrtd in the 4th line of the brtdfunc definition.

Then I stuck when I try to model the specular reflection because I can not get the direction of outgoing peak. For example, I tried to model the specular reflection with a Gaussian function, but then I need to know the direction of specular reflection which is the center of the Gaussian function. As the direction of incident light is not available (e.g. in or as function parameter) like in the case of diffuse reflection, how could I calculate the direction of the specular reflection?