{
	Compute fractal clouds over image

	Greg Ward	11/1/95

	Needs also noise2.cal

	Usage:
		pcomb -f noise2.cal -f clouds.cal planet.pic > cloudy.pic

	Typically, this will be used to add clouds to Earth.pic or
	bigEarth.pic in the standard library distribution.
}

{ Modify the following two constants to your liking: }

cover_frac : 0.65;	{ average cloud cover (non-clear fraction) }

cloud_refl : 0.7;	{ average cloud reflectance (always white) }

{ Leave the rest alone. }

PI : 3.14159265358979323846;

clouds(u,v,s) = if(s-.5, noise2(sin(2*PI/xmax*x)+13.5,sin(2*PI/ymax*y)-10.3),
		noise2(u/s,v/s)*s + clouds(u,v,2*s));

minres : if(xmax-ymax, ymax, xmax);

clamp(x) : if(x, if(x-1, 1, x), 0);

coef = clamp( 1 +
	(.5/sqrt(cover_frac)) * (clouds(x/minres, y/minres, 10/minres) - 1)
		) ;

cv = coef * cloud_refl;
pv = 1 - coef;

ro = cv + pv*ri(1);
go = cv + pv*gi(1);
bo = cv + pv*bi(1);