{ 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;