--- ray/src/rt/ambcomp.c	2014/05/01 16:01:59	2.43
+++ ray/src/rt/ambcomp.c	2014/08/19 15:04:40	2.64
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char	RCSid[] = "$Id: ambcomp.c,v 2.43 2014/05/01 16:01:59 greg Exp $";
+static const char	RCSid[] = "$Id: ambcomp.c,v 2.64 2014/08/19 15:04:40 greg Exp $";
 #endif
 /*
  * Routines to compute "ambient" values using Monte Carlo
@@ -8,6 +8,10 @@ static const char	RCSid[] = "$Id: ambcomp.c,v 2.43 201
  *	for Irradiance Caching" by Schwarzhaupt, Wann Jensen, & Jarosz
  *	from ACM SIGGRAPH Asia 2012 conference proceedings.
  *
+ *  Added book-keeping optimization to avoid calculations that would
+ *	cancel due to traversal both directions on edges that are adjacent
+ *	to same-valued triangles.  This cuts about half of Hessian math.
+ *
  *  Declarations of external symbols in ambient.h
  */
 
@@ -17,151 +21,111 @@ static const char	RCSid[] = "$Id: ambcomp.c,v 2.43 201
 #include  "ambient.h"
 #include  "random.h"
 
-#ifdef NEWAMB
+#ifndef OLDAMB
 
 extern void		SDsquare2disk(double ds[2], double seedx, double seedy);
 
 typedef struct {
+	COLOR	v;		/* hemisphere sample value */
+	float	d;		/* reciprocal distance (1/rt) */
+	FVECT	p;		/* intersection point */
+} AMBSAMP;		/* sample value */
+
+typedef struct {
 	RAY	*rp;		/* originating ray sample */
-	FVECT	ux, uy;		/* tangent axis unit vectors */
 	int	ns;		/* number of samples per axis */
+	int	sampOK;		/* acquired full sample set? */
 	COLOR	acoef;		/* division contribution coefficient */
-	struct s_ambsamp {
-		COLOR	v;		/* hemisphere sample value */
-		FVECT	p;		/* intersection point */
-	} sa[1];		/* sample array (extends struct) */
+	double	acol[3];	/* accumulated color */
+	FVECT	ux, uy;		/* tangent axis unit vectors */
+	AMBSAMP	sa[1];		/* sample array (extends struct) */
 }  AMBHEMI;		/* ambient sample hemisphere */
 
-typedef struct s_ambsamp	AMBSAMP;
+#define AI(h,i,j)	((i)*(h)->ns + (j))
+#define ambsam(h,i,j)	(h)->sa[AI(h,i,j)]
 
-#define ambsam(h,i,j)	(h)->sa[(i)*(h)->ns + (j)]
-
 typedef struct {
 	FVECT	r_i, r_i1, e_i, rcp, rI2_eJ2;
 	double	I1, I2;
 } FFTRI;		/* vectors and coefficients for Hessian calculation */
 
 
-static AMBHEMI *
-inithemi(			/* initialize sampling hemisphere */
-	COLOR	ac,
-	RAY	*r,
-	double	wt
+static int
+ambsample(				/* initial ambient division sample */
+	AMBHEMI	*hp,
+	int	i,
+	int	j,
+	int	n
 )
 {
-	AMBHEMI	*hp;
-	double	d;
-	int	n, i;
-					/* set number of divisions */
-	if (ambacc <= FTINY &&
-			wt > (d = 0.8*intens(ac)*r->rweight/(ambdiv*minweight)))
-		wt = d;			/* avoid ray termination */
-	n = sqrt(ambdiv * wt) + 0.5;
-	i = 1 + 5*(ambacc > FTINY);	/* minimum number of samples */
-	if (n < i)
-		n = i;
-					/* allocate sampling array */
-	hp = (AMBHEMI *)malloc(sizeof(AMBHEMI) + sizeof(AMBSAMP)*(n*n - 1));
-	if (hp == NULL)
-		return(NULL);
-	hp->rp = r;
-	hp->ns = n;
-					/* assign coefficient */
-	copycolor(hp->acoef, ac);
-	d = 1.0/(n*n);
-	scalecolor(hp->acoef, d);
-					/* make tangent plane axes */
-	hp->uy[0] = 0.5 - frandom();
-	hp->uy[1] = 0.5 - frandom();
-	hp->uy[2] = 0.5 - frandom();
-	for (i = 3; i--; )
-		if ((-0.6 < r->ron[i]) & (r->ron[i] < 0.6))
-			break;
-	if (i < 0)
-		error(CONSISTENCY, "bad ray direction in inithemi");
-	hp->uy[i] = 1.0;
-	VCROSS(hp->ux, hp->uy, r->ron);
-	normalize(hp->ux);
-	VCROSS(hp->uy, r->ron, hp->ux);
-					/* we're ready to sample */
-	return(hp);
-}
-
-
-/* Sample ambient division and apply weighting coefficient */
-static int
-getambsamp(RAY *arp, AMBHEMI *hp, int i, int j, int n)
-{
+	AMBSAMP	*ap = &ambsam(hp,i,j);
+	RAY	ar;
 	int	hlist[3], ii;
 	double	spt[2], zd;
+					/* generate hemispherical sample */
 					/* ambient coefficient for weight */
 	if (ambacc > FTINY)
-		setcolor(arp->rcoef, AVGREFL, AVGREFL, AVGREFL);
+		setcolor(ar.rcoef, AVGREFL, AVGREFL, AVGREFL);
 	else
-		copycolor(arp->rcoef, hp->acoef);
-	if (rayorigin(arp, AMBIENT, hp->rp, arp->rcoef) < 0)
+		copycolor(ar.rcoef, hp->acoef);
+	if (rayorigin(&ar, AMBIENT, hp->rp, ar.rcoef) < 0)
 		return(0);
 	if (ambacc > FTINY) {
-		multcolor(arp->rcoef, hp->acoef);
-		scalecolor(arp->rcoef, 1./AVGREFL);
+		multcolor(ar.rcoef, hp->acoef);
+		scalecolor(ar.rcoef, 1./AVGREFL);
 	}
 	hlist[0] = hp->rp->rno;
-	hlist[1] = i;
-	hlist[2] = j;
+	hlist[1] = j;
+	hlist[2] = i;
 	multisamp(spt, 2, urand(ilhash(hlist,3)+n));
-	if (!n) {			/* avoid border samples for n==0 */
-		if ((spt[0] < 0.1) | (spt[0] > 0.9))
+					/* avoid coincident samples */
+	if (!n && (0 < i) & (i < hp->ns-1) &&
+			(0 < j) & (j < hp->ns-1)) {
+		if ((spt[0] < 0.1) | (spt[0] >= 0.9))
 			spt[0] = 0.1 + 0.8*frandom();
-		if ((spt[1] < 0.1) | (spt[1] > 0.9))
+		if ((spt[1] < 0.1) | (spt[1] >= 0.9))
 			spt[1] = 0.1 + 0.8*frandom();
 	}
-	SDsquare2disk(spt, (i+spt[0])/hp->ns, (j+spt[1])/hp->ns);
+	SDsquare2disk(spt, (j+spt[1])/hp->ns, (i+spt[0])/hp->ns);
 	zd = sqrt(1. - spt[0]*spt[0] - spt[1]*spt[1]);
 	for (ii = 3; ii--; )
-		arp->rdir[ii] =	spt[0]*hp->ux[ii] +
+		ar.rdir[ii] =	spt[0]*hp->ux[ii] +
 				spt[1]*hp->uy[ii] +
 				zd*hp->rp->ron[ii];
-	checknorm(arp->rdir);
-	dimlist[ndims++] = i*hp->ns + j + 90171;
-	rayvalue(arp);			/* evaluate ray */
-	ndims--;			/* apply coefficient */
-	multcolor(arp->rcol, arp->rcoef);
+	checknorm(ar.rdir);
+	dimlist[ndims++] = AI(hp,i,j) + 90171;
+	rayvalue(&ar);			/* evaluate ray */
+	ndims--;
+	if (ar.rt <= FTINY)
+		return(0);		/* should never happen */
+	multcolor(ar.rcol, ar.rcoef);	/* apply coefficient */
+	if (ar.rt*ap->d < 1.0)		/* new/closer distance? */
+		ap->d = 1.0/ar.rt;
+	if (!n) {			/* record first vertex & value */
+		if (ar.rt > 10.0*thescene.cusize)
+			ar.rt = 10.0*thescene.cusize;
+		VSUM(ap->p, ar.rorg, ar.rdir, ar.rt);
+		copycolor(ap->v, ar.rcol);
+	} else {			/* else update recorded value */
+		hp->acol[RED] -= colval(ap->v,RED);
+		hp->acol[GRN] -= colval(ap->v,GRN);
+		hp->acol[BLU] -= colval(ap->v,BLU);
+		zd = 1.0/(double)(n+1);
+		scalecolor(ar.rcol, zd);
+		zd *= (double)n;
+		scalecolor(ap->v, zd);
+		addcolor(ap->v, ar.rcol);
+	}
+	addcolor(hp->acol, ap->v);	/* add to our sum */
 	return(1);
 }
 
 
-static AMBSAMP *
-ambsample(				/* initial ambient division sample */
-	AMBHEMI	*hp,
-	int	i,
-	int	j
-)
-{
-	AMBSAMP	*ap = &ambsam(hp,i,j);
-	RAY	ar;
-					/* generate hemispherical sample */
-	if (!getambsamp(&ar, hp, i, j, 0))
-		goto badsample;
-					/* limit vertex distance */
-	if (ar.rt > 10.0*thescene.cusize)
-		ar.rt = 10.0*thescene.cusize;
-	else if (ar.rt <= FTINY)	/* should never happen! */
-		goto badsample;
-	VSUM(ap->p, ar.rorg, ar.rdir, ar.rt);
-	copycolor(ap->v, ar.rcol);
-	return(ap);
-badsample:
-	setcolor(ap->v, 0., 0., 0.);
-	VCOPY(ap->p, hp->rp->rop);
-	return(NULL);
-}
-
-
 /* Estimate errors based on ambient division differences */
 static float *
 getambdiffs(AMBHEMI *hp)
 {
-	float	*earr = calloc(hp->ns*hp->ns, sizeof(float));
+	float	*earr = (float *)calloc(hp->ns*hp->ns, sizeof(float));
 	float	*ep;
 	AMBSAMP	*ap;
 	double	b, d2;
@@ -179,25 +143,31 @@ 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);
 }
@@ -205,56 +175,126 @@ getambdiffs(AMBHEMI *hp)
 
 /* Perform super-sampling on hemisphere (introduces bias) */
 static void
-ambsupersamp(double acol[3], AMBHEMI *hp, int cnt)
+ambsupersamp(AMBHEMI *hp, int cnt)
 {
 	float	*earr = getambdiffs(hp);
-	double	e2sum = 0;
+	double	e2rem = 0;
 	AMBSAMP	*ap;
 	RAY	ar;
-	COLOR	asum;
 	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();
-		setcolor(asum, 0., 0., 0.);
-		for (n = 1; n <= nss; n++) {
-			if (!getambsamp(&ar, hp, i, j, n)) {
-				nss = n-1;
-				break;
-			}
-			addcolor(asum, ar.rcol);
-		}
-		if (nss) {		/* update returned ambient value */
-			const double	ssf = 1./(nss + 1);
-			for (n = 3; n--; )
-				acol[n] += ssf*colval(asum,n) +
-						(ssf - 1.)*colval(ap->v,n);
-		}
-		e2sum -= *ep++;		/* update remainders */
-		cnt -= nss;
+		if (e2rem <= FTINY)
+			goto done;	/* nothing left to do */
+		nss = *ep/e2rem*cnt + frandom();
+		for (n = 1; n <= nss && ambsample(hp,i,j,n); n++)
+			--cnt;
+		e2rem -= *ep++;		/* update remainder */
 	}
+done:
 	free(earr);
 }
 
 
+static AMBHEMI *
+samp_hemi(				/* sample indirect hemisphere */
+	COLOR	rcol,
+	RAY	*r,
+	double	wt
+)
+{
+	AMBHEMI	*hp;
+	double	d;
+	int	n, i, j;
+					/* set number of divisions */
+	if (ambacc <= FTINY &&
+			wt > (d = 0.8*intens(rcol)*r->rweight/(ambdiv*minweight)))
+		wt = d;			/* avoid ray termination */
+	n = sqrt(ambdiv * wt) + 0.5;
+	i = 1 + 5*(ambacc > FTINY);	/* minimum number of samples */
+	if (n < i)
+		n = i;
+					/* allocate sampling array */
+	hp = (AMBHEMI *)malloc(sizeof(AMBHEMI) + sizeof(AMBSAMP)*(n*n - 1));
+	if (hp == NULL)
+		error(SYSTEM, "out of memory in samp_hemi");
+	hp->rp = r;
+	hp->ns = n;
+	hp->acol[RED] = hp->acol[GRN] = hp->acol[BLU] = 0.0;
+	memset(hp->sa, 0, sizeof(AMBSAMP)*n*n);
+	hp->sampOK = 0;
+					/* assign coefficient */
+	copycolor(hp->acoef, rcol);
+	d = 1.0/(n*n);
+	scalecolor(hp->acoef, d);
+					/* make tangent plane axes */
+	hp->uy[0] = 0.5 - frandom();
+	hp->uy[1] = 0.5 - frandom();
+	hp->uy[2] = 0.5 - frandom();
+	for (i = 3; i--; )
+		if ((-0.6 < r->ron[i]) & (r->ron[i] < 0.6))
+			break;
+	if (i < 0)
+		error(CONSISTENCY, "bad ray direction in samp_hemi");
+	hp->uy[i] = 1.0;
+	VCROSS(hp->ux, hp->uy, r->ron);
+	normalize(hp->ux);
+	VCROSS(hp->uy, r->ron, hp->ux);
+					/* sample divisions */
+	for (i = hp->ns; i--; )
+	    for (j = hp->ns; j--; )
+		hp->sampOK += ambsample(hp, i, j, 0);
+	copycolor(rcol, hp->acol);
+	if (!hp->sampOK) {		/* utter failure? */
+		free(hp);
+		return(NULL);
+	}
+	if (hp->sampOK < hp->ns*hp->ns) {
+		hp->sampOK *= -1;	/* soft failure */
+		return(hp);
+	}
+	n = ambssamp*wt + 0.5;
+	if (n > 8) {			/* perform super-sampling? */
+		ambsupersamp(hp, n);
+		copycolor(rcol, hp->acol);
+	}
+	return(hp);			/* all is well */
+}
+
+
+/* Return brightness of farthest ambient sample */
+static double
+back_ambval(AMBHEMI *hp, const int n1, const int n2, const int n3)
+{
+	if (hp->sa[n1].d <= hp->sa[n2].d) {
+		if (hp->sa[n1].d <= hp->sa[n3].d)
+			return(colval(hp->sa[n1].v,CIEY));
+		return(colval(hp->sa[n3].v,CIEY));
+	}
+	if (hp->sa[n2].d <= hp->sa[n3].d)
+		return(colval(hp->sa[n2].v,CIEY));
+	return(colval(hp->sa[n3].v,CIEY));
+}
+
+
 /* Compute vectors and coefficients for Hessian/gradient calcs */
 static void
-comp_fftri(FFTRI *ftp, FVECT ap0, FVECT ap1, FVECT rop)
+comp_fftri(FFTRI *ftp, AMBHEMI *hp, const int n0, const int n1)
 {
 	double	rdot_cp, dot_e, dot_er, rdot_r, rdot_r1, J2;
-	int	i;
+	int	ii;
 
-	VSUB(ftp->r_i, ap0, rop);
-	VSUB(ftp->r_i1, ap1, rop);
-	VSUB(ftp->e_i, ap1, ap0);
+	VSUB(ftp->r_i, hp->sa[n0].p, hp->rp->rop);
+	VSUB(ftp->r_i1, hp->sa[n1].p, hp->rp->rop);
+	VSUB(ftp->e_i, hp->sa[n1].p, hp->sa[n0].p);
 	VCROSS(ftp->rcp, ftp->r_i, ftp->r_i1);
 	rdot_cp = 1.0/DOT(ftp->rcp,ftp->rcp);
 	dot_e = DOT(ftp->e_i,ftp->e_i);
@@ -266,8 +306,8 @@ comp_fftri(FFTRI *ftp, FVECT ap0, FVECT ap1, FVECT rop
 	ftp->I2 = ( DOT(ftp->e_i, ftp->r_i1)*rdot_r1 - dot_er*rdot_r +
 			dot_e*ftp->I1 )*0.5*rdot_cp;
 	J2 =  ( 0.5*(rdot_r - rdot_r1) - dot_er*ftp->I2 ) / dot_e;
-	for (i = 3; i--; )
-		ftp->rI2_eJ2[i] = ftp->I2*ftp->r_i[i] + J2*ftp->e_i[i];
+	for (ii = 3; ii--; )
+		ftp->rI2_eJ2[ii] = ftp->I2*ftp->r_i[ii] + J2*ftp->e_i[ii];
 }
 
 
@@ -316,7 +356,7 @@ comp_hessian(FVECT hess[3], FFTRI *ftp, FVECT nrm)
 		hess[i][j] = m1[i][j] + d1*( I3*m2[i][j] + K3*m3[i][j] +
 						2.0*J3*m4[i][j] );
 		hess[i][j] += d2*(i==j);
-		hess[i][j] *= 1.0/PI;
+		hess[i][j] *= -1.0/PI;
 	    }
 }
 
@@ -338,7 +378,7 @@ rev_hessian(FVECT hess[3])
 /* Add to radiometric Hessian from the given triangle */
 static void
 add2hessian(FVECT hess[3], FVECT ehess1[3],
-		FVECT ehess2[3], FVECT ehess3[3], COLORV v)
+		FVECT ehess2[3], FVECT ehess3[3], double v)
 {
 	int	i, j;
 
@@ -359,7 +399,7 @@ comp_gradient(FVECT grad, FFTRI *ftp, FVECT nrm)
 	f1 = 2.0*DOT(nrm, ftp->rcp);
 	VCROSS(ncp, nrm, ftp->e_i);
 	for (i = 3; i--; )
-		grad[i] = (-0.5/PI)*( ftp->I1*ncp[i] + f1*ftp->rI2_eJ2[i] );
+		grad[i] = (0.5/PI)*( ftp->I1*ncp[i] + f1*ftp->rI2_eJ2[i] );
 }
 
 
@@ -375,7 +415,7 @@ rev_gradient(FVECT grad)
 
 /* Add to displacement gradient from the given triangle */
 static void
-add2gradient(FVECT grad, FVECT egrad1, FVECT egrad2, FVECT egrad3, COLORV v)
+add2gradient(FVECT grad, FVECT egrad1, FVECT egrad2, FVECT egrad3, double v)
 {
 	int	i;
 
@@ -384,33 +424,8 @@ add2gradient(FVECT grad, FVECT egrad1, FVECT egrad2, F
 }
 
 
-/* Return brightness of furthest ambient sample */
-static COLORV
-back_ambval(AMBSAMP *ap1, AMBSAMP *ap2, AMBSAMP *ap3, FVECT orig)
-{
-	COLORV	vback;
-	FVECT	vec;
-	double	d2, d2best;
-
-	VSUB(vec, ap1->p, orig);
-	d2best = DOT(vec,vec);
-	vback = colval(ap1->v,CIEY);
-	VSUB(vec, ap2->p, orig);
-	d2 = DOT(vec,vec);
-	if (d2 > d2best) {
-		d2best = d2;
-		vback = colval(ap2->v,CIEY);
-	}
-	VSUB(vec, ap3->p, orig);
-	d2 = DOT(vec,vec);
-	if (d2 > d2best)
-		return(colval(ap3->v,CIEY));
-	return(vback);
-}
-
-
 /* Compute anisotropic radii and eigenvector directions */
-static int
+static void
 eigenvectors(FVECT uv[2], float ra[2], FVECT hessian[3])
 {
 	double	hess2[2][2];
@@ -432,9 +447,10 @@ 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]));
@@ -492,8 +508,7 @@ ambHessian(				/* anisotropic radii & pos. gradient */
 	}
 					/* compute first row of edges */
 	for (j = 0; j < hp->ns-1; j++) {
-		comp_fftri(&fftr, ambsam(hp,0,j).p,
-				ambsam(hp,0,j+1).p, hp->rp->rop);
+		comp_fftri(&fftr, hp, AI(hp,0,j), AI(hp,0,j+1));
 		if (hessrow != NULL)
 			comp_hessian(hessrow[j], &fftr, hp->rp->ron);
 		if (gradrow != NULL)
@@ -503,8 +518,7 @@ ambHessian(				/* anisotropic radii & pos. gradient */
 	for (i = 0; i < hp->ns-1; i++) {
 	    FVECT	hesscol[3];	/* compute first vertical edge */
 	    FVECT	gradcol;
-	    comp_fftri(&fftr, ambsam(hp,i,0).p,
-			ambsam(hp,i+1,0).p, hp->rp->rop);
+	    comp_fftri(&fftr, hp, AI(hp,i,0), AI(hp,i+1,0));
 	    if (hessrow != NULL)
 		comp_hessian(hesscol, &fftr, hp->rp->ron);
 	    if (gradrow != NULL)
@@ -512,12 +526,11 @@ ambHessian(				/* anisotropic radii & pos. gradient */
 	    for (j = 0; j < hp->ns-1; j++) {
 		FVECT	hessdia[3];	/* compute triangle contributions */
 		FVECT	graddia;
-		COLORV	backg;
-		backg = back_ambval(&ambsam(hp,i,j), &ambsam(hp,i,j+1),
-					&ambsam(hp,i+1,j), hp->rp->rop);
+		double	backg;
+		backg = back_ambval(hp, AI(hp,i,j),
+					AI(hp,i,j+1), AI(hp,i+1,j));
 					/* diagonal (inner) edge */
-		comp_fftri(&fftr, ambsam(hp,i,j+1).p,
-				ambsam(hp,i+1,j).p, hp->rp->rop);
+		comp_fftri(&fftr, hp, AI(hp,i,j+1), AI(hp,i+1,j));
 		if (hessrow != NULL) {
 		    comp_hessian(hessdia, &fftr, hp->rp->ron);
 		    rev_hessian(hesscol);
@@ -529,17 +542,15 @@ ambHessian(				/* anisotropic radii & pos. gradient */
 		    add2gradient(gradient, gradrow[j], graddia, gradcol, backg);
 		}
 					/* initialize edge in next row */
-		comp_fftri(&fftr, ambsam(hp,i+1,j+1).p,
-				ambsam(hp,i+1,j).p, hp->rp->rop);
+		comp_fftri(&fftr, hp, AI(hp,i+1,j+1), AI(hp,i+1,j));
 		if (hessrow != NULL)
 		    comp_hessian(hessrow[j], &fftr, hp->rp->ron);
 		if (gradrow != NULL)
 		    comp_gradient(gradrow[j], &fftr, hp->rp->ron);
 					/* new column edge & paired triangle */
-		backg = back_ambval(&ambsam(hp,i,j+1), &ambsam(hp,i+1,j+1),
-					&ambsam(hp,i+1,j), hp->rp->rop);
-		comp_fftri(&fftr, ambsam(hp,i,j+1).p, ambsam(hp,i+1,j+1).p,
-				hp->rp->rop);
+		backg = back_ambval(hp, AI(hp,i+1,j+1),
+					AI(hp,i+1,j), AI(hp,i,j+1));
+		comp_fftri(&fftr, hp, AI(hp,i,j+1), AI(hp,i+1,j+1));
 		if (hessrow != NULL) {
 		    comp_hessian(hesscol, &fftr, hp->rp->ron);
 		    rev_hessian(hessdia);
@@ -594,6 +605,64 @@ ambdirgrad(AMBHEMI *hp, FVECT uv[2], float dg[2])
 }
 
 
+/* Compute potential light leak direction flags for cache value */
+static uint32
+ambcorral(AMBHEMI *hp, FVECT uv[2], const double r0, const double r1)
+{
+	const double	max_d = 1.0/(minarad*ambacc + 0.001);
+	const double	ang_res = 0.5*PI/(hp->ns-1);
+	const double	ang_step = ang_res/((int)(16/PI*ang_res) + (1+FTINY));
+	double		avg_d = 0;
+	uint32		flgs = 0;
+	FVECT		vec;
+	double		u, v;
+	double		ang, a1;
+	int		i, j;
+					/* 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*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) {
+		AMBSAMP	*ap = &ambsam(hp,i,j);
+		if ((ap->d <= FTINY) | (ap->d >= max_d))
+			continue;	/* too far or too near */
+		VSUB(vec, ap->p, hp->rp->rop);
+		u = DOT(vec, uv[0]);
+		v = DOT(vec, uv[1]);
+		if ((r0*r0*u*u + r1*r1*v*v) * ap->d*ap->d <= u*u + v*v)
+			continue;	/* occluder outside ellipse */
+		ang = atan2a(v, u);	/* else set direction flags */
+		for (a1 = ang-.5*ang_res; a1 <= ang+.5*ang_res; a1 += ang_step)
+			flgs |= 1L<<(int)(16/PI*(a1 + 2.*PI*(a1 < 0)));
+	    }
+					/* add low-angle incident (< 20deg) */
+	if (fabs(hp->rp->rod) <= 0.342) {
+		u = -DOT(hp->rp->rdir, uv[0]);
+		v = -DOT(hp->rp->rdir, uv[1]);
+		if ((r0*r0*u*u + r1*r1*v*v) > hp->rp->rot*hp->rp->rot) {
+			ang = atan2a(v, u);
+			ang += 2.*PI*(ang < 0);
+			ang *= 16/PI;
+			if ((ang < .5) | (ang >= 31.5))
+				flgs |= 0x80000001;
+			else
+				flgs |= 3L<<(int)(ang-.5);
+		}
+	}
+	return(flgs);
+}
+
+
 int
 doambient(				/* compute ambient component */
 	COLOR	rcol,			/* input/output color */
@@ -602,18 +671,16 @@ doambient(				/* compute ambient component */
 	FVECT	uv[2],			/* returned (optional) */
 	float	ra[2],			/* returned (optional) */
 	float	pg[2],			/* returned (optional) */
-	float	dg[2]			/* returned (optional) */
+	float	dg[2],			/* returned (optional) */
+	uint32	*crlp			/* returned (optional) */
 )
 {
-	AMBHEMI	*hp = inithemi(rcol, r, wt);
-	int	cnt = 0;
+	AMBHEMI	*hp = samp_hemi(rcol, r, wt);
 	FVECT	my_uv[2];
-	double	d, K, acol[3];
+	double	d, K;
 	AMBSAMP	*ap;
-	int	i, j;
-					/* check/initialize */
-	if (hp == NULL)
-		return(0);
+	int	i;
+					/* clear return values */
 	if (uv != NULL)
 		memset(uv, 0, sizeof(FVECT)*2);
 	if (ra != NULL)
@@ -622,40 +689,24 @@ doambient(				/* compute ambient component */
 		pg[0] = pg[1] = 0.0;
 	if (dg != NULL)
 		dg[0] = dg[1] = 0.0;
-					/* sample the hemisphere */
-	acol[0] = acol[1] = acol[2] = 0.0;
-	for (i = hp->ns; i--; )
-		for (j = hp->ns; j--; )
-			if ((ap = ambsample(hp, i, j)) != NULL) {
-				addcolor(acol, ap->v);
-				++cnt;
-			}
-	if (!cnt) {
-		setcolor(rcol, 0.0, 0.0, 0.0);
-		free(hp);
-		return(0);		/* no valid samples */
+	if (crlp != NULL)
+		*crlp = 0;
+	if (hp == NULL)			/* sampling falure? */
+		return(0);
+
+	if ((ra == NULL) & (pg == NULL) & (dg == NULL) ||
+			(hp->sampOK < 0) | (hp->ns < 4)) {
+		free(hp);		/* Hessian not requested/possible */
+		return(-1);		/* value-only return value */
 	}
-	if (cnt < hp->ns*hp->ns) {	/* incomplete sampling? */
-		copycolor(rcol, acol);
-		free(hp);
-		return(-1);		/* return value w/o Hessian */
-	}
-	cnt = ambssamp*wt + 0.5;	/* perform super-sampling? */
-	if (cnt > 0)
-		ambsupersamp(acol, hp, cnt);
-	copycolor(rcol, acol);		/* final indirect irradiance/PI */
-	if ((ra == NULL) & (pg == NULL) & (dg == NULL)) {
-		free(hp);
-		return(-1);		/* no radius or gradient calc. */
-	}
-	if (bright(acol) > FTINY) {	/* normalize Y values */
-		d = 0.99*cnt/bright(acol);
+	if ((d = bright(rcol)) > FTINY) {	/* normalize Y values */
+		d = 0.99*(hp->ns*hp->ns)/d;
 		K = 0.01;
-	} else {			/* geometric Hessian fall-back */
-		d = 0.0;
+	} else {			/* or fall back on geometric Hessian */
 		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++)
@@ -683,7 +734,7 @@ doambient(				/* compute ambient component */
 			if (ra[1] < minarad)
 				ra[1] = minarad;
 		}
-		ra[0] *= d = 1.0/sqrt(sqrt(wt));
+		ra[0] *= d = 1.0/sqrt(wt);
 		if ((ra[1] *= d) > 2.0*ra[0])
 			ra[1] = 2.0*ra[0];
 		if (ra[1] > maxarad) {
@@ -691,6 +742,9 @@ doambient(				/* compute ambient component */
 			if (ra[0] > maxarad)
 				ra[0] = maxarad;
 		}
+					/* flag encroached directions */
+		if ((wt >= 0.89*AVGREFL) & (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];
 			if (d > 1.0) {