/usr/share/radiance/skybright.cal is in radiance-materials 4R0+20110410-1build1.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | { RCSid: $Id: skybright.cal,v 2.7 2003/02/22 02:07:24 greg Exp $ }
{
Sky brightness function for sunny and cloudy skies.
Additional arguments required for calculation of skybright:
A1 - 1 for CIE clear, 2 for CIE overcast,
3 for uniform, 4 for CIE intermediate
A2 - zenith brightness
A3 - ground plane brightness
A4 - normalization factor based on sun direction
A5,A6,A7 - sun direction
}
cosgamma = Dx*A5 + Dy*A6 + Dz*A7;
gamma = Acos(cosgamma); { angle from sun to this point in sky }
zt = Acos(A7); { angle from zenith to sun }
eta = Acos(Dz); { angle from zenith to this point in sky }
wmean(a, x, b, y) : (a*x + b*y) / (a + b);
skybr = wmean((Dz+1.01)^10,
select(A1, sunnysky, cloudysky, unifsky, intersky),
(Dz+1.01)^-10, A3);
sunnysky = A2 * (.91 + 10*exp(-3*gamma) + .45*cosgamma*cosgamma)
* if( Dz - .01, 1.0 - exp(-.32/Dz), 1.0) / A4;
cloudysky = A2 * (1 + 2*Dz)/3;
unifsky = A2;
intersky = A2 * ( (1.35*sin(5.631-3.59*eta)+3.12)*sin(4.396-2.6*zt)
+ 6.37 - eta ) / 2.326 *
exp(gamma*-.563*((2.629-eta)*(1.562-zt)+.812)) / A4;
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