--- ray/src/rt/ambient.c	2014/04/11 22:54:34	2.72
+++ ray/src/rt/ambient.c	2014/04/19 02:39:44	2.74
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char	RCSid[] = "$Id: ambient.c,v 2.72 2014/04/11 22:54:34 greg Exp $";
+static const char	RCSid[] = "$Id: ambient.c,v 2.74 2014/04/19 02:39:44 greg Exp $";
 #endif
 /*
  *  ambient.c - routines dealing with ambient (inter-reflected) component.
@@ -51,6 +51,8 @@ static int  nunflshed = 0;	/* number of unflushed ambi
 #define MAX_SORT_INTVL	(SORT_INTVL<<6)
 #endif
 
+
+static double  qambacc = 0.;		/* ambient accuracy to the 1/4 power */
 static double  avsum = 0.;		/* computed ambient value sum (log) */
 static unsigned int  navsum = 0;	/* number of values in avsum */
 static unsigned int  nambvals = 0;	/* total number of indirect values */
@@ -132,8 +134,11 @@ setambacc(				/* set ambient accuracy */
 	if (newa < 0.0)
 		newa = 0.0;
 	ambdiff = fabs(newa - ambacc);
-	if (ambdiff >= .01 && (ambacc = newa) > FTINY && nambvals > 0)
-		sortambvals(1);			/* rebuild tree */
+	if (ambdiff >= .01 && (ambacc = newa) > FTINY) {
+		qambacc = sqrt(sqrt(ambacc));
+		if (nambvals > 0)
+			sortambvals(1);		/* rebuild tree */
+	}
 }
 
 
@@ -320,7 +325,7 @@ multambient(		/* compute ambient component & multiply 
 	ok = makeambient(acol, r, nrm, rdepth-1);
 	rdepth--;
 	if (ok) {
-		multcolor(aval, acol);		/* got new value */
+		multcolor(aval, acol);		/* computed new value */
 		return;
 	}
 dumbamb:					/* return global value */
@@ -454,11 +459,10 @@ makeambient(		/* make a new ambient value for storage 
 		amb.weight = 1.25*r->rweight;
 	setcolor(acol, AVGREFL, AVGREFL, AVGREFL);
 						/* compute ambient */
-	if (!doambient(acol, r, amb.weight, uvw, amb.rad, amb.gpos, amb.gdir)) {
-		setcolor(acol, 0.0, 0.0, 0.0);
-		return(0);
-	}
+	i = doambient(acol, r, amb.weight, uvw, amb.rad, amb.gpos, amb.gdir);
 	scalecolor(acol, 1./AVGREFL);		/* undo assumed reflectance */
+	if (i <= 0)				/* no Hessian => no storage */
+		return(i);
 						/* store value */
 	VCOPY(amb.pos, r->rop);
 	amb.ndir = encodedir(r->ron);
@@ -530,7 +534,7 @@ avinsert(				/* insert ambient value in our tree */
 	at = &atrunk;
 	VCOPY(ck0, thescene.cuorg);
 	s = thescene.cusize;
-	while (s*(OCTSCALE/2) > av->rad[1]*ambacc) {
+	while (s*(OCTSCALE/2) > av->rad[1]*qambacc) {
 		if (at->kid == NULL)
 			if ((at->kid = newambtree()) == NULL)
 				error(SYSTEM, "out of memory in avinsert");