#declare R2 = 0.299;	// luminance values (`Y') taken from page 589
#declare G2 = 0.587;	// Foley, Van Dam, Feiner & Hughes
#declare B2 = 0.114;	// Computer Graphics, Principles & Practice (2d ed.)

#declare R1 = sqrt(R2);
#declare G1 = sqrt(G2);
#declare B1 = sqrt(B2);
#declare Y1 = sqrt(R2+G2);

#declare Rvec = <-G1/Y1, R1, -B1*R1/Y1>;
#declare Gvec = vnormalize(<R1/Y1, G1, -B1*G1/Y1>);
#declare Bvec = <0,B1,Y1>;

#declare Glayer = pigment {
	gradient Gvec
	color_map { [ 0 rgb 0 ] [ 1 rgb <0,3,0> ] }
	scale 2 translate -Gvec
	}

sky_sphere {
	pigment {
		Glayer
		}
	}