--- ray/src/rt/m_brdf.c	1993/11/18 09:43:04	2.9
+++ ray/src/rt/m_brdf.c	1994/12/21 09:51:46	2.11
@@ -16,6 +16,8 @@ static char SCCSid[] = "$SunId$ LBL";
 
 #include  "func.h"
 
+extern int  backvis;			/* back faces visible? */
+
 /*
  *	Arguments to this material include the color and specularity.
  *  String arguments include the reflection function and files.
@@ -268,7 +270,7 @@ register RAY  *r;
 		transdist = r->rot + sr.rt;
 	}
 	if (r->crtype & SHADOW)			/* the rest is shadow */
-		return;
+		return(1);
 						/* compute reflected ray */
 	setbrdfunc(&nd);
 	errno = 0;
@@ -308,6 +310,8 @@ register RAY  *r;
 						/* check distance */
 	if (transtest > bright(r->rcol))
 		r->rt = transdist;
+
+	return(1);
 }
 
 
@@ -321,7 +325,7 @@ register RAY  *r;
 	double  dtmp;
 						/* always a shadow */
 	if (r->crtype & SHADOW)
-		return;
+		return(1);
 						/* check arguments */
 	if (m->oargs.nsargs < (hasdata(m->otype)?4:2) | m->oargs.nfargs <
 			(m->otype==MAT_TFUNC|m->otype==MAT_TDATA?6:4))
@@ -347,9 +351,15 @@ register RAY  *r;
 						/* compute reflectance */
 	dtmp = 1.0 - nd.trans - nd.rspec;
 	setcolor(nd.rdiff, dtmp, dtmp, dtmp);
-						/* fix orientation */
-	if (r->rod < 0.0)
-		flipsurface(r);
+						/* check for back side */
+	if (r->rod < 0.0) {
+		if (!backvis && m->otype != MAT_TFUNC
+				&& m->otype != MAT_TDATA) {
+			raytrans(r);
+			return(1);
+		}
+		flipsurface(r);			/* reorient if backvis */
+	}
 						/* get modifiers */
 	raytexture(r, m->omod);
 	nd.pdot = raynormal(nd.pnorm, r);	/* perturb normal */
@@ -381,6 +391,8 @@ register RAY  *r;
 	}
 						/* add direct component */
 	direct(r, dirbrdf, &nd);
+
+	return(1);
 }