--- ray/src/cal/cal/WalterBTDF.cal	2014/03/18 16:16:37	1.1
+++ ray/src/cal/cal/WalterBTDF.cal	2014/03/19 19:06:20	1.2
@@ -1,6 +1,6 @@
-{ RCSid $Id: WalterBTDF.cal,v 1.1 2014/03/18 16:16:37 greg Exp $ }
+{ RCSid $Id: WalterBTDF.cal,v 1.2 2014/03/19 19:06:20 greg Exp $ }
 {
-	BTDF model by Walter et al.
+	Isotropic BTDF model by Walter et al.
 }
 			{ Model parameters (change these) }
 ksint : 0.126221381019119;
@@ -12,12 +12,14 @@ alpha : 0.149990535073450;
 			{ Useful functions }
 s(x) : x*x;
 abs(x) : if(x, x, -x);
+noneg(x) : if(x, x, 0);
+
+			{ Lobe calculation }
 G(x) : 2/(1 + sqrt(1 + s(alpha)*(1-s(x))/s(x)));
 F(w) : f0 + (1-f0)*(1-w)^5;
 D(htn) : s(alpha)/PI/s( s(htn)*(s(alpha)-1) + 1 );
 
-			{ Lobe calculation }
-lobe3(wiht,woht,win,won,htn) : abs(wiht)*abs(woht)/(abs(win)*abs(won)) *
+lobe3(wiht,woht,win,won,htn) : abs(wiht*woht/(win*won)) *
 		s(sigma_o)*(1-F(wiht))*D(htn)*G(win)*G(won) /
 			s(sigma_i*wiht + sigma_o*woht);
 
@@ -30,7 +32,7 @@ lobe1(ix,iy,iz,ox,oy,oz,htx,hty,htz) : lobe2(ix,iy,iz,
 			{ Final function call - transmission only! }
 			{ vectors normalized, pointing away from surface }
 walter(ix,iy,iz,ox,oy,oz) : if(iz*oz, 0,
-				ksint * lobe1(ix,iy,iz,ox,oy,oz,
+				ksint*noneg(lobe1(ix,iy,iz,ox,oy,oz,
 				-sigma_i*ix-sigma_o*ox,
 				-sigma_i*iy-sigma_o*oy,
-				-sigma_i*iz-sigma_o*oz) );
+				-sigma_i*iz-sigma_o*oz)) );