--- ray/src/util/rttree_reduce.c	2011/05/31 20:50:26	2.2
+++ ray/src/util/rttree_reduce.c	2011/06/01 16:51:03	2.3
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: rttree_reduce.c,v 2.2 2011/05/31 20:50:26 greg Exp $";
+static const char RCSid[] = "$Id: rttree_reduce.c,v 2.3 2011/06/01 16:51:03 greg Exp $";
 #endif
 /*
  *  A utility called by genBSDF.pl to reduce tensor tree samples and output
@@ -11,6 +11,7 @@ static const char RCSid[] = "$Id: rttree_reduce.c,v 2.
 #include "rterror.h"
 #include "platform.h"
 #include <stdlib.h>
+#include <math.h>
 
 float	*datarr;		/* our loaded BSDF data array */
 int	ttrank = 4;		/* tensor tree rank */
@@ -18,18 +19,9 @@ int	log2g = 4;		/* log2 of grid resolution */
 int	infmt = 'a';		/* input format ('a','f','d') */
 double	pctcull = 99.;		/* target culling percentile */
 
-#define HISTLEN		300	/* histogram resolution */
-#define HISTMAX		10.	/* maximum recorded value in histogram */
-
-int	histo[HISTLEN];		/* histogram freq. of max-min BSDF */
-
-double	tthresh;		/* acceptance threshold (TBD) */
-
 #define dval3(ix,ox,oy)		datarr[((((ix)<<log2g)+(ox))<<log2g)+(oy)]
 #define dval4(ix,iy,ox,oy)	datarr[((((((ix)<<log2g)+(iy))<<log2g)+(ox))<<log2g)+(oy)]
 
-#define above_threshold(tp)	((tp)->vmax - (tp)->vmin > tthresh)
-
 /* Tensor tree node */
 typedef struct ttree_s {
 	float		vmin, vmax;	/* value extrema */
@@ -37,6 +29,17 @@ typedef struct ttree_s {
 	struct ttree_s	*kid;		/* 2^ttrank children */
 } TNODE;
 
+#define HISTLEN		300	/* histogram resolution */
+#define HISTMAX		10.	/* maximum recorded measure in histogram */
+
+int	histo[HISTLEN];		/* histogram freq. of variance measure */
+
+double	tthresh;		/* acceptance threshold (TBD) */
+
+#define var_measure(tp)		( ((tp)->vmax - (tp)->vmin) / \
+					(sqrt((tp)->vavg) + .03) )
+#define above_threshold(tp)	(var_measure(tp) > tthresh)
+
 /* Allocate a new set of children for the given node (no checks) */
 static void
 new_kids(TNODE *pn)
@@ -85,7 +88,7 @@ build_tree(TNODE *tp, const int bmin[], int l2s)
 		}
 		tp->vavg /= (float)(1<<ttrank);
 					/* record stats */
-		i = (HISTLEN/HISTMAX) * (tp->vmax - tp->vmin);
+		i = (HISTLEN/HISTMAX) * var_measure(tp);
 		if (i >= HISTLEN) i = HISTLEN-1;
 		++histo[i];
 		return;