Great work!

I was just about to do the same thing but while struggling with the math 
to express the pigment functions I luckily found your implementation ;)
But I'm wondering: while you cite the Preetham paper you do use 
different values for the A to C parameters and also for the Perez 
formulation.
Actually I do like the resulting renders better with the original values 
from the Preetham paper (might very well just be a matter of individual 
taste) but I'm curious where the other values did come from.

Also I'm wondering about the multiplication factor of 1000 for the 
zenith luminance. As far as I understand it this formula return the
luminance in cd/m2 therefore this additional multiplication just makes 
the exposure factor unnecessary small.

Here is what I do use (with the originalPreetham/Perez values):

// constant factors
#local AY = 0.1787*Turbidity- 1.4630;
#local BY =-0.3554*Turbidity+ 0.4275;
#local CY =-0.0227*Turbidity+ 5.3251;
#local DY = 0.1206*Turbidity- 2.5771;
#local EY =-0.0670*Turbidity+ 0.3703;

#local Ax =-0.0193*Turbidity- 0.2592;
#local Bx =-0.0665*Turbidity+ 0.0008;
#local Cx =-0.0004*Turbidity+ 0.2125;
#local Dx =-0.0641*Turbidity- 0.8989;
#local Ex =-0.0033*Turbidity+ 0.0452;

#local Ay =-0.0167*Turbidity- 0.2608;
#local By =-0.0950*Turbidity+ 0.0092;
#local Cy =-0.0079*Turbidity+ 0.2102;
#local Dy =-0.0441*Turbidity- 1.6537;
#local Ey =-0.0109*Turbidity+ 0.0529;


#macro computeZenithColor(T,thetas)
  // Calculate luminance

  #local zenithLuminance = ((4.0453*T - 4.9710) * tan( (4.0/9.0 - T/120) 
* (pi-2*thetas) ) - 0.2155*T + 2.4192);
  #if(zenithLuminance<=0)
   #local zenithLuminance=1e-11;
  #end

  // Calculate colour
  #local thetas2 = thetas*thetas;
  #local thetas3 = thetas*thetas2;
  #local T2 = T*T;

  #local zx= ( 0.00166*thetas3 - 0.00375*thetas2 + 0.00209*thetas) * T2 +
             (-0.02903*thetas3 + 0.06377*thetas2 - 0.03202*thetas + 
0.00394) * T +
             ( 0.11693*thetas3 - 0.21196*thetas2 + 0.06052*thetas + 
0.25886);

  #local zy= ( 0.00275*thetas3 - 0.00610*thetas2 + 0.00317*thetas) * T2 +
             (-0.04214*thetas3 + 0.08970*thetas2 - 0.04153*thetas + 
0.00516) * T +
             ( 0.15346*thetas3 - 0.26756*thetas2 + 0.06670*thetas + 
0.26688);

  <zx,zy,zenithLuminance>

#end


-Ive

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