--- ray/src/rt/m_clip.c	1991/11/12 17:08:41	2.1
+++ ray/src/rt/m_clip.c	2017/04/08 00:09:35	2.12
@@ -1,37 +1,49 @@
-/* Copyright (c) 1986 Regents of the University of California */
-
 #ifndef lint
-static char SCCSid[] = "$SunId$ LBL";
+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.
- *
- *     3/17/86
  */
 
+#include "copyright.h"
+
 #include  "ray.h"
+#include  "rtotypes.h"
 
 /*
  *  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.
  */
 
 
-m_clip(m, r)			/* clip objects from ray */
-register OBJREC  *m;
-register RAY  *r;
+int
+m_clip(			/* clip objects from ray */
+	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) {
-		register OBJECT  mod;
-
 		if (m->oargs.nsargs < 1 || m->oargs.nsargs > MAXSET)
 			objerror(m, USER, "bad # arguments");
 		modset = (OBJECT *)malloc((m->oargs.nsargs+1)*sizeof(OBJECT));
@@ -41,7 +53,7 @@ register RAY  *r;
 		for (i = 0; i < m->oargs.nsargs; i++) {
 			if (!strcmp(m->oargs.sarg[i], VOIDID))
 				continue;
-			if ((mod = modifier(m->oargs.sarg[i])) == OVOID) {
+			if ((mod = lastmod(obj, m->oargs.sarg[i])) == OVOID) {
 				sprintf(errmsg, "unknown modifier \"%s\"",
 						m->oargs.sarg[i]);
 				objerror(m, WARNING, errmsg);
@@ -55,43 +67,44 @@ register RAY  *r;
 		}
 		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
-					&& inset(modset, rp->parent->ro->omod))
+					&& inset(modset, rp->parent->ro->omod)) {
 				if (rp->parent->rod > 0.0)
 					inside++;
 				else
 					inside--;
+			}
 		if (inside > 0) {	/* we just hit the object */
 			flipsurface(r);
-			rayshade(r, modifier(m->oargs.sarg[0]));
-			return;
+			return(rayshade(r, lastmod(obj, m->oargs.sarg[0])));
 		}
 	}
 	raytrans(r);			/* else transfer ray */
+	return(1);
 }