--- ray/src/cv/bsdfrbf.c	2016/02/03 18:42:13	2.28
+++ ray/src/cv/bsdfrbf.c	2018/08/01 17:00:22	2.31
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: bsdfrbf.c,v 2.28 2016/02/03 18:42:13 greg Exp $";
+static const char RCSid[] = "$Id: bsdfrbf.c,v 2.31 2018/08/01 17:00:22 greg Exp $";
 #endif
 /*
  * Radial basis function representation for BSDF data.
@@ -31,11 +31,14 @@ static const char RCSid[] = "$Id: bsdfrbf.c,v 2.28 201
 #ifndef RSCA
 #define RSCA		2.0		/* radius scaling factor (empirical) */
 #endif
+#ifndef MAXSLOPE
+#define MAXSLOPE	200.0		/* maximum slope for smooth region */
+#endif
 #ifndef SMOOTH_MSE
 #define SMOOTH_MSE	5e-5		/* acceptable mean squared error */
 #endif
 #ifndef SMOOTH_MSER
-#define SMOOTH_MSER	0.03		/* acceptable relative MSE */
+#define SMOOTH_MSER	0.01		/* acceptable relative MSE */
 #endif
 #define MAX_RAD		(GRIDRES/8)	/* maximum lobe radius */
 
@@ -209,6 +212,8 @@ smooth_region(int x0, int x1, int y0, int y1)
 		return(1);		/* colinear values */
 	A = DOT(rMtx[0], xvec);
 	B = DOT(rMtx[1], xvec);
+	if (A*A + B*B > MAXSLOPE*MAXSLOPE)	/* too steep? */
+		return(0);
 	C = DOT(rMtx[2], xvec);
 	sqerr = 0.0;			/* compute mean squared error */
 	for (x = x0; x < x1; x++)