--- ray/src/rt/glass.c	2013/08/07 05:10:09	2.20
+++ ray/src/rt/glass.c	2014/01/25 18:27:39	2.21
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: glass.c,v 2.20 2013/08/07 05:10:09 greg Exp $";
+static const char RCSid[] = "$Id: glass.c,v 2.21 2014/01/25 18:27:39 greg Exp $";
 #endif
 /*
  *  glass.c - simpler shading function for thin glass surfaces.
@@ -65,6 +65,11 @@ m_glass(		/* color a ray which hit a thin glass surfac
 		rindex = m->oargs.farg[3];	/* use their value */
 	else
 		objerror(m, USER, "bad arguments");
+						/* check back face visibility */
+	if (!backvis && r->rod <= 0.0) {
+		raytrans(r);
+		return(1);
+	}
 						/* check transmission */
 	setcolor(mcolor, m->oargs.farg[0], m->oargs.farg[1], m->oargs.farg[2]);
 	if ((hastrans = (intens(mcolor) > 1e-15))) {