Tool for evaluation of the BRDF/BTDF model 
for dirty glass (glass covered with a layer of scattering media) 
The illumination is collimated light (from top or from bottom)

Usage: evaluator.exe dust_tau dust_g dust_albedo dust_ior theta_i ngrids outputfile [bHK93=0]

dust_tau	 optical thickness of the contaminant. 
        	 typical in (0,0.4) for good approximation
dust_g		 scattering parameter of the contaminant. in (-1,1)
dust_albedo	 albedo of single scattering. in (0,1). 
           	 typical less than 0.5 for good approximation
dust_ior	 refraction index of the contaminant.
theta_i(wi)	 direction of the illumination. in (0,pi). 
           	 if less than pi/2, air2glass; otherwise glass2air 
ngrids		 sampling points. wo will be span in (0,pi/2) and (pi/2, pi) 
      		 into ngrids

Example: 
	evaluator.exe 0.4 0.7 0.5 1.0 0 200 result.txt

Format of the output: 

There are 4 parts in the output file.
The values are reflectance or transmittance
(i.e. physically corresponds to radiance (W/m2/sr)
So they are BRDF*cos(theta_i) or BTDF*cos(theta_i) 

  Specular Reflectance (angle, value)
  Diffuse Reflectance  (angles, values)
  Specular Transmission (angle, value)
  Diffuse Transmission (angles values)

Jinwei GU (jwgu@cs.columbia.edu). 2007/6/5