--- ray/src/rt/m_clip.c	2004/03/30 16:13:01	2.8
+++ ray/src/rt/m_clip.c	2017/04/08 00:09:35	2.12
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char	RCSid[] = "$Id: m_clip.c,v 2.8 2004/03/30 16:13:01 schorsch Exp $";
+static const char	RCSid[] = "$Id: m_clip.c,v 2.12 2017/04/08 00:09:35 greg Exp $";
 #endif
 /*
  *  m_clip.c - routine for clipped (cut) objects.
@@ -12,23 +12,35 @@ static const char	RCSid[] = "$Id: m_clip.c,v 2.8 2004/
 
 /*
  *  Clipping objects permit holes and sections to be taken out
- *  of other objects.  The method is simple:  
+ *  of other objects. 
  *
  *  The argument is the clipped materials;
  *  the first is used to shade upon exit.
+ *
+ *  In the simple case of the first argument being "void", we
+ *  just add or subtract (depending on whether we're coming or going)
+ *  the list of modifiers to the ray's "newcset", which will then
+ *  take over for "clipset" on penetration.  Any surface modifier
+ *  names found in "clipset" will be treated as invisible in raycont().
+ *
+ *  In the more complicated case of a non-void material as the
+ *  first argument, we have to backtrack up the ray tree to count
+ *  the number of times we've penetrated the front side of one of
+ *  the surfaces we care about.  This relies on outward-facing
+ *  surface normals and closed objects, so is somewhat error-prone.
  */
 
 
-extern int
+int
 m_clip(			/* clip objects from ray */
-	register OBJREC  *m,
-	register RAY  *r
+	OBJREC  *m,
+	RAY  *r
 )
 {
 	OBJECT  cset[MAXSET+1], *modset;
 	OBJECT  obj, mod;
 	int  entering;
-	register int  i;
+	int  i;
 
 	obj = objndx(m);
 	if ((modset = (OBJECT *)m->os) == NULL) {
@@ -55,30 +67,30 @@ m_clip(			/* clip objects from ray */
 		}
 		m->os = (char *)modset;
 	}
+	if (r == NULL)
+		return(0);			/* just initializing */
 	if (r->clipset != NULL)
 		setcopy(cset, r->clipset);
 	else
 		cset[0] = 0;
 
-	entering = r->rod > 0.0;		/* entering clipped region? */
+	entering = (r->rod > 0.0);		/* entering clipped region? */
 
-	for (i = modset[0]; i > 0; i--) {
+	for (i = modset[0]; i > 0; i--)
 		if (entering) {
 			if (!inset(cset, modset[i])) {
 				if (cset[0] >= MAXSET)
 					error(INTERNAL, "set overflow in m_clip");
 				insertelem(cset, modset[i]);
 			}
-		} else {
-			if (inset(cset, modset[i]))
-				deletelem(cset, modset[i]);
-		}
-	}
+		} else if (inset(cset, modset[i]))
+			deletelem(cset, modset[i]);
+
 					/* compute ray value */
 	r->newcset = cset;
 	if (strcmp(m->oargs.sarg[0], VOIDID)) {
 		int  inside = 0;
-		register RAY  *rp;
+		const RAY  *rp;
 					/* check for penetration */
 		for (rp = r; rp->parent != NULL; rp = rp->parent)
 			if (!(rp->rtype & RAYREFL) && rp->parent->ro != NULL