// 2002 May 23: derive color from normal.  Anton Sherwood bronto@pobox.com

#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 Rlayer = pigment {
	slope <-G1/Y1, R1, -B1*R1/Y1>
	color_map { [ 0 rgb <0,0,0> ] [ 1 rgb <3,0,0> ] }
	}

#declare Glayer = pigment {
	slope <R1/Y1, G1, -B1*G1/Y1>
	color_map { [ 0 rgb <0,0,0> ] [ 1 rgb <0,3,0> ] }
	}

#declare Blayer = pigment {
	slope <0,B1,Y1>
	color_map { [ 0 rgb <0,0,0> ] [ 1 rgb <0,0,3> ] }
	}

#declare Tufte = pigment
	{
	average
	pigment_map { [1, Rlayer] [1, Glayer] [1, Blayer] }
	}