{
     skycolor.cal - Sky color function for Radiance skies, meant
                    to work in place of skybright.cal

     Portions Copyright Mark Stock, 1998
     All the rest, LBL, 1993

     Caveat: This routine does not work with an intermediate sky
             yet, I haven't separated the sky's influence from the
             sun's influence.

     These first 7 args are the same as for skybright.cal:

     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

     A8,A9,A10		- zenith color (rgb)
     A11,A12,A13	- horizon color (rgb)
     A14,A15,A16	- nadir color (rgb)
     A17,A18,A19	- sun color (rgb)

     An example usage for a late afternoon sky is:

     # gensky 4 30 16.2 -a 42 +s
     # Local solar time: 16.11
     # Solar altitude and azimuth: 30.6 81.9
     # Ground ambient level: 15.4

     void light solar
     0
     0
     3 6.19e+06 6.19e+06 6.19e+06

     solar source sun
     0
     0
     4 -0.852270 -0.121498 0.508797 0.5

     #void brightfunc skyfunc
     #2 skybr skybright.cal
     #0
     #7 1 7.62e+00 1.51e+01 4.03e-01 -0.852270 -0.121498 0.508797
     # notice how I stole this line for skycolor below?

     void colorfunc skyfunc
     4 skyr skyg skyb skycolor.cal
     0
     19 1 7.62e+00 1.51e+01 4.03e-01 -0.852270 -0.121498 0.508797
        0.5 0.6 2.0
        1.5 1.5 1.5
        1.2 0.9 0.3
        1.1 1.1 0.9
     # that's zenith, horizon, nadir, sun colors

     skyfunc glow sky_glow
     0
     0
     4 1.0 1.0 1.0 0

     sky_glow source sky
     0
     0
     4 0 0 1 360

}

{ some utilities }

wmean(a, x, b, y) = (a*x + b*y) / (a + b);
zfactor = (Dz+1.01)^10;

{ the brightness calculation }

skybr = wmean(zfactor,
	select(A1, sunnysky, cloudysky, unifsky, intersky),
	1.0/zfactor, A3);

{ a is the color/weight of the sun, b is the weight of the sky }
sunnymult(a, b) = (b*.91 + a*(10*exp(-3*gamma) + .45*cosgamma*cosgamma))
	    * if( Dz - .01, 1.0 - exp(-.32/Dz), 1.0) / A4;
sunnysky = A2 * sunnymult(1.0, 1.0);

cloudymult = (1 + 2*Dz)/3;
cloudysky = A2 * (1 + 2*Dz)/3;

unifsky = A2;

intermult = ( (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;
intersky = A2 * intermult;

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 }


{ the color calculation }

nohr = wmean(zfactor,
       select(A1, sunnymult(arg(17),A8)*A2, cloudymult*A2*A8, A2*A8, intermult*A2*A8),
       1.0/zfactor, A3*arg(14));
nohg = wmean(zfactor,
       select(A1, sunnymult(arg(18),A9)*A2, cloudymult*A2*A9, A2*A9, intermult*A2*A9),
       1.0/zfactor, A3*arg(15));
nohb = wmean(zfactor,
       select(A1, sunnymult(arg(19),A10)*A2, cloudymult*A2*A10, A2*A10, intermult*A2*A10),
       1.0/zfactor, A3*arg(16));

{ add the horizon color calculation }

{ the "2" is the sharpness of the horizon color band, higher is sharper;
  the "10.0" is the width of the band, higher is narrower }
hfactor = 1.0/(1.0+(Dz*10.0)^2);

skyr = wmean(hfactor, skybr*arg(11), 1-hfactor, nohr);
skyg = wmean(hfactor, skybr*arg(12), 1-hfactor, nohg);
skyb = wmean(hfactor, skybr*arg(13), 1-hfactor, nohb);