--- ray/src/common/interp2d.h	2013/02/09 20:56:40	2.3
+++ ray/src/common/interp2d.h	2013/02/11 22:56:22	2.4
@@ -1,6 +1,6 @@
-/* RCSid $Id: interp2d.h,v 2.3 2013/02/09 20:56:40 greg Exp $ */
+/* RCSid $Id: interp2d.h,v 2.4 2013/02/11 22:56:22 greg Exp $ */
 /*
- * Header for interpolation of anisotropic values on 2-D plane.
+ * Header for interpolation of anisotropic samples on 2-D plane.
  *
  *	G.Ward Feb 2013
  */
@@ -12,16 +12,16 @@
 extern "C" {
 #endif
 
-#define	NI2DSMF	0.42f			/* minimal smoothing radius */
+#define	NI2DSMF	0.42f			/* minimal smoothing factor */
 
 #define NI2DIR	(2*4)			/* # interpolation directions */
 
 /* Data structure for interpolant */
 typedef struct {
 	int		ns;		/* number of sample positions */
-	float		rmin;		/* minimum valid radius (default=.5) */
+	float		dmin;		/* minimum diameter (default=1) */
 	float		smf;		/* smoothing factor (def=NI2DSMF) */
-	unsigned short	(*ra)[NI2DIR];	/* anisotropic radii (private) */
+	unsigned short	(*da)[NI2DIR];	/* anisotropic distances (private) */
 	float		spt[1][2];	/* sample positions (extends struct) */
 } INTERP2;
 
@@ -43,6 +43,30 @@ extern void	interp2_free(INTERP2 *ip);
 /* (Re)compute anisotropic basis function interpolant (normally automatic) */
 extern int	interp2_analyze(INTERP2 *ip);
 
+/***************************************************************
+ * Typical use is to allocate an INTERP2 struct and assign the
+ * spt[] array with the ordered sample locations in x & y.  (The
+ * actual orientation of the axes is not important so long as
+ * the application is consistent.)  During interpolation, either
+ * interp2_weights() is called to obtain a normalized weighting
+ * vector for all the samples, or interp2_topsamp() is called to
+ * get the most important N samples for the specified location.
+ * The weights (and indexes in the case of interp2_topsamp)
+ * are then used as coefficients for corresponding sample values
+ * in a vector sum together that interpolates the function at that
+ * location.  Between the initial allocation call and the first
+ * weight evaluation, the dmin member may be adjusted to
+ * the distance under which samples will start to merge.  If this
+ * parameter is later changed, interp2_analyze() should be called
+ * to recompute the interpolant.
+ * The smf member sets the smoothing factor for interpolation.
+ * The default setting of NI2DSMF produces near-optimal
+ * interpolation when well-separated sample values are known
+ * precisely.  If a greater degree of mixing is desired, this
+ * paremter may be increased and it will affect the next call
+ * to interp2_weights() or interp2_topsamp().
+ **************************************************************/
+ 
 #ifdef __cplusplus
 }
 #endif