--- ray/src/rt/ambcomp.c	2014/05/07 20:20:24	2.51
+++ ray/src/rt/ambcomp.c	2014/05/09 16:05:09	2.55
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char	RCSid[] = "$Id: ambcomp.c,v 2.51 2014/05/07 20:20:24 greg Exp $";
+static const char	RCSid[] = "$Id: ambcomp.c,v 2.55 2014/05/09 16:05:09 greg Exp $";
 #endif
 /*
  * Routines to compute "ambient" values using Monte Carlo
@@ -105,7 +105,7 @@ vdb_edge(int db1, int db2)
 	case VDB_xY:	return(db2==VDB_x ? VDB_y : VDB_X);
 	case VDB_Xy:	return(db2==VDB_y ? VDB_x : VDB_Y);
 	}
-	error(INTERNAL, "forbidden diagonal in vdb_edge()");
+	error(CONSISTENCY, "forbidden diagonal in vdb_edge()");
 	return(-1);
 }
 
@@ -243,25 +243,31 @@ getambdiffs(AMBHEMI *hp)
 			ep[0] += d2;
 			ep[-hp->ns] += d2;
 		}
-		if (j) {		/* from behind */
-			d2 = b - bright(ap[-1].v);
-			d2 *= d2;
-			ep[0] += d2;
-			ep[-1] += d2;
-		}
+		if (!j) continue;
+					/* from behind */
+		d2 = b - bright(ap[-1].v);
+		d2 *= d2;
+		ep[0] += d2;
+		ep[-1] += d2;
+		if (!i) continue;
+					/* diagonal */
+		d2 = b - bright(ap[-hp->ns-1].v);
+		d2 *= d2;
+		ep[0] += d2;
+		ep[-hp->ns-1] += d2;
 	    }
 					/* correct for number of neighbors */
-	earr[0] *= 2.f;
-	earr[hp->ns-1] *= 2.f;
-	earr[(hp->ns-1)*hp->ns] *= 2.f;
-	earr[(hp->ns-1)*hp->ns + hp->ns-1] *= 2.f;
+	earr[0] *= 8./3.;
+	earr[hp->ns-1] *= 8./3.;
+	earr[(hp->ns-1)*hp->ns] *= 8./3.;
+	earr[(hp->ns-1)*hp->ns + hp->ns-1] *= 8./3.;
 	for (i = 1; i < hp->ns-1; i++) {
-		earr[i*hp->ns] *= 4./3.;
-		earr[i*hp->ns + hp->ns-1] *= 4./3.;
+		earr[i*hp->ns] *= 8./5.;
+		earr[i*hp->ns + hp->ns-1] *= 8./5.;
 	}
 	for (j = 1; j < hp->ns-1; j++) {
-		earr[j] *= 4./3.;
-		earr[(hp->ns-1)*hp->ns + j] *= 4./3.;
+		earr[j] *= 8./5.;
+		earr[(hp->ns-1)*hp->ns + j] *= 8./5.;
 	}
 	return(earr);
 }
@@ -272,22 +278,24 @@ static void
 ambsupersamp(double acol[3], AMBHEMI *hp, int cnt)
 {
 	float	*earr = getambdiffs(hp);
-	double	e2sum = 0.0;
+	double	e2rem = 0;
 	AMBSAMP	*ap;
 	RAY	ar;
 	double	asum[3];
 	float	*ep;
-	int	i, j, n;
+	int	i, j, n, nss;
 
 	if (earr == NULL)		/* just skip calc. if no memory */
 		return;
-					/* add up estimated variances */
-	for (ep = earr + hp->ns*hp->ns; ep-- > earr; )
-		e2sum += *ep;
+					/* accumulate estimated variances */
+	for (ep = earr + hp->ns*hp->ns; ep > earr; )
+		e2rem += *--ep;
 	ep = earr;			/* perform super-sampling */
 	for (ap = hp->sa, i = 0; i < hp->ns; i++)
 	    for (j = 0; j < hp->ns; j++, ap++) {
-		int	nss = *ep/e2sum*cnt + frandom();
+		if (e2rem <= FTINY)
+			goto done;	/* nothing left to do */
+		nss = *ep/e2rem*cnt + frandom();
 		asum[0] = asum[1] = asum[2] = 0.0;
 		for (n = 1; n <= nss; n++) {
 			if (!getambsamp(&ar, hp, i, j, n)) {
@@ -297,14 +305,15 @@ ambsupersamp(double acol[3], AMBHEMI *hp, int cnt)
 			addcolor(asum, ar.rcol);
 		}
 		if (nss) {		/* update returned ambient value */
-			const double	ssf = 1./(nss + 1);
+			const double	ssf = 1./(nss + 1.);
 			for (n = 3; n--; )
 				acol[n] += ssf*asum[n] +
 						(ssf - 1.)*colval(ap->v,n);
 		}
-		e2sum -= *ep++;		/* update remainders */
+		e2rem -= *ep++;		/* update remainders */
 		cnt -= nss;
 	}
+done:
 	free(earr);
 }
 
@@ -537,7 +546,7 @@ add2gradient(FVECT grad, FVECT egrad1, FVECT egrad2, F
 
 
 /* Compute anisotropic radii and eigenvector directions */
-static int
+static void
 eigenvectors(FVECT uv[2], float ra[2], FVECT hessian[3])
 {
 	double	hess2[2][2];
@@ -559,9 +568,10 @@ eigenvectors(FVECT uv[2], float ra[2], FVECT hessian[3
 	if (i == 1)			/* double-root (circle) */
 		evalue[1] = evalue[0];
 	if (!i || ((evalue[0] = fabs(evalue[0])) <= FTINY*FTINY) |
-			((evalue[1] = fabs(evalue[1])) <= FTINY*FTINY) )
-		error(INTERNAL, "bad eigenvalue calculation");
-
+			((evalue[1] = fabs(evalue[1])) <= FTINY*FTINY) ) {
+		ra[0] = ra[1] = maxarad;
+		return;
+	}
 	if (evalue[0] > evalue[1]) {
 		ra[0] = sqrt(sqrt(4.0/evalue[0]));
 		ra[1] = sqrt(sqrt(4.0/evalue[1]));
@@ -728,13 +738,18 @@ ambcorral(AMBHEMI *hp, FVECT uv[2], const double r0, c
 	double		avg_d = 0;
 	uint32		flgs = 0;
 	int		i, j;
-					/* check distances above us */
+					/* don't bother for a few samples */
+	if (hp->ns < 12)
+		return(0);
+					/* check distances overhead */
 	for (i = hp->ns*3/4; i-- > hp->ns>>2; )
 	    for (j = hp->ns*3/4; j-- > hp->ns>>2; )
 		avg_d += ambsam(hp,i,j).d;
 	avg_d *= 4.0/(hp->ns*hp->ns);
-	if (avg_d >= max_d)		/* too close to corral? */
+	if (avg_d*r0 >= 1.0)		/* ceiling too low for corral? */
 		return(0);
+	if (avg_d >= max_d)		/* insurance */
+		return(0);
 					/* else circle around perimeter */
 	for (i = 0; i < hp->ns; i++)
 	    for (j = 0; j < hp->ns; j += !i|(i==hp->ns-1) ? 1 : hp->ns-1) {
@@ -823,6 +838,7 @@ doambient(				/* compute ambient component */
 		K = 1.0;
 		pg = NULL;
 		dg = NULL;
+		crlp = NULL;
 	}
 	ap = hp->sa;			/* relative Y channel from here on... */
 	for (i = hp->ns*hp->ns; i--; ap++)
@@ -858,7 +874,8 @@ doambient(				/* compute ambient component */
 			if (ra[0] > maxarad)
 				ra[0] = maxarad;
 		}
-		if (crlp != NULL)	/* flag encroached directions */
+					/* flag encroached directions */
+		if ((wt >= 0.5-FTINY) & (crlp != NULL))
 			*crlp = ambcorral(hp, uv, ra[0]*ambacc, ra[1]*ambacc);
 		if (pg != NULL) {	/* cap gradient if necessary */
 			d = pg[0]*pg[0]*ra[0]*ra[0] + pg[1]*pg[1]*ra[1]*ra[1];