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{ SCCSid "$SunId$ LBL" } |
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{ RCSid: $Id$ } |
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{ |
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Sky brightness function for sunny and cloudy skies. |
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6/6/86 |
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Additional arguments required for calculation of skybright: |
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A1 - boolean, positive for cloudy sky |
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(A1>=2 for uniform sky) |
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A1 - 1 for CIE clear, 2 for CIE overcast, |
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3 for uniform, 4 for CIE intermediate |
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A2 - zenith brightness |
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A3 - ground plane brightness |
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A4 - normalization factor based on sun direction |
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A5,A6,A7 - sun direction |
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} |
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skybright = wmean((Dz+1.01)^10, if(A1, cloudysky, sunnysky), |
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(Dz+1.01)^-10, A3); |
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cosgamma = Dx*A5 + Dy*A6 + Dz*A7; |
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wmean(a, x, b, y) = (a*x + b*y) / (a + b); |
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gamma = Acos(cosgamma); { angle from sun to this point in sky } |
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cloudysky = A2 * if(A1-1.5, 1, (1 + 2*Dz)/3); |
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zt = Acos(A7); { angle from zenith to sun } |
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sunnysky = A2 * (.91 + 10*exp(-3*Acos(cosgamma)) + .45*cosgamma*cosgamma) |
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eta = Acos(Dz); { angle from zenith to this point in sky } |
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wmean(a, x, b, y) : (a*x + b*y) / (a + b); |
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skybr = wmean((Dz+1.01)^10, |
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select(A1, sunnysky, cloudysky, unifsky, intersky), |
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(Dz+1.01)^-10, A3); |
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sunnysky = A2 * (.91 + 10*exp(-3*gamma) + .45*cosgamma*cosgamma) |
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* if( Dz - .01, 1.0 - exp(-.32/Dz), 1.0) / A4; |
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cosgamma = Dx*A5 + Dy*A6 + Dz*A7; |
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cloudysky = A2 * (1 + 2*Dz)/3; |
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unifsky = A2; |
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intersky = A2 * ( (1.35*sin(5.631-3.59*eta)+3.12)*sin(4.396-2.6*zt) |
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+ 6.37 - eta ) / 2.326 * |
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exp(gamma*-.563*((2.629-eta)*(1.562-zt)+.812)) / A4; |
