Index: trunk/Ohana/src/opihi/cmd.data/Makefile
===================================================================
--- trunk/Ohana/src/opihi/cmd.data/Makefile	(revision 40006)
+++ trunk/Ohana/src/opihi/cmd.data/Makefile	(revision 40007)
@@ -74,4 +74,5 @@
 $(SRC)/integrate.$(ARCH).o	\
 $(SRC)/interpolate.$(ARCH).o	\
+$(SRC)/interpolate_presort.$(ARCH).o	\
 $(SRC)/join.$(ARCH).o		\
 $(SRC)/jpeg.$(ARCH).o		\
Index: trunk/Ohana/src/opihi/cmd.data/init.c
===================================================================
--- trunk/Ohana/src/opihi/cmd.data/init.c	(revision 40006)
+++ trunk/Ohana/src/opihi/cmd.data/init.c	(revision 40007)
@@ -63,4 +63,5 @@
 int integrate        PROTO((int, char **));
 int interpolate      PROTO((int, char **));
+int interpolate_presort PROTO((int, char **));
 int join             PROTO((int, char **));
 int jpeg             PROTO((int, char **));
@@ -241,5 +242,5 @@
   {1, "imstats",      imstats,          "statistics on a portion of an image"},
   {1, "integrate",    integrate,        "integrate a vector"},
-  {1, "interpolate",  interpolate,      "interpolate between vector pairs"},
+  {1, "interpolate_presort",  interpolate_presort,      "interpolate between vector pairs"},
   {1, "join",         join,             "find the join of two ID vectors"},
   {1, "jpeg",         jpeg,             "convert display image to JPEG"},
Index: trunk/Ohana/src/opihi/cmd.data/interpolate_presort.c
===================================================================
--- trunk/Ohana/src/opihi/cmd.data/interpolate_presort.c	(revision 40007)
+++ trunk/Ohana/src/opihi/cmd.data/interpolate_presort.c	(revision 40007)
@@ -0,0 +1,75 @@
+# include "data.h"
+
+// XXX use 'threshold' to interpolate to a value
+int interpolate_presort (int argc, char **argv) {
+
+  int  i, j;
+  double x0, dx, dy, y0;
+  Vector *xout, *yout, *xin, *yin;
+
+  /** check basic syntax **/
+  if (argc != 5) {
+    gprint (GP_ERR, "USAGE: interpolate Xi Yi Xo Yo\n");
+    gprint (GP_ERR, "  Xi Yi - sorted reference vectors\n");
+    gprint (GP_ERR, "  Xo    - output positions (vector)\n");
+    gprint (GP_ERR, "  Yo    - output values (vector)\n");
+    gprint (GP_ERR, "  (vectors must be pre-sorted)\n");
+    gprint (GP_ERR, "  (use 'threshold' to interpolate to a value)\n");
+    return (FALSE);
+  }
+
+  if ((xin  = SelectVector (argv[1],  OLDVECTOR, TRUE)) == NULL) return (FALSE);
+  if ((yin  = SelectVector (argv[2],  OLDVECTOR, TRUE)) == NULL) return (FALSE);
+
+  // target positions are a vector
+  if ((xout = SelectVector (argv[3],  OLDVECTOR, TRUE)) == NULL) return (FALSE);
+  if ((yout = SelectVector (argv[4],  ANYVECTOR, TRUE)) == NULL) return (FALSE);
+
+  REQUIRE_VECTOR_FLT (xin, FALSE); 
+  REQUIRE_VECTOR_FLT (yin, FALSE); 
+  REQUIRE_VECTOR_FLT (xout, FALSE); 
+  ResetVector (yout, OPIHI_FLT, xout[0].Nelements);
+
+  dx = xin[0].elements.Flt[1] - xin[0].elements.Flt[0];
+  dy = yin[0].elements.Flt[1] - yin[0].elements.Flt[0];
+  x0 = xin[0].elements.Flt[0];
+  y0 = yin[0].elements.Flt[0];
+  
+  /* in and out vectors are sorted */
+  j = 0;
+
+  // every output pixel should get a value unless 
+  // we are below the lowest in or above the highest in
+  for (i = 0, j = 0; (i < xout[0].Nelements) && (j < xin[0].Nelements - 1); ) {
+
+    yout[0].elements.Flt[i] = NAN;
+
+    // are we with range of the current input pixel? (xin[j] <= xout[i] < xin[j+1]) ?
+    if (xout[0].elements.Flt[i] >= xin[0].elements.Flt[j+1]) { 
+      j ++; 
+      continue;
+    }
+    if (xout[0].elements.Flt[i] < xin[0].elements.Flt[j]) { 
+      i ++; 
+      continue;
+    }
+
+    dx = xin[0].elements.Flt[j+1] - xin[0].elements.Flt[j];
+    dy = yin[0].elements.Flt[j+1] - yin[0].elements.Flt[j];
+    x0 = xin[0].elements.Flt[j];
+    y0 = yin[0].elements.Flt[j];
+    // x1 = xin[0].elements.Flt[j+1];
+
+    yout[0].elements.Flt[i] = (dy/dx)*(xout[0].elements.Flt[i] - x0) + y0;
+    i ++;
+  }
+
+  // fill in the rest with NANs
+  while (i < yout[0].Nelements) {
+    yout[0].elements.Flt[i] = NAN;
+    i++;
+  }
+
+  return (TRUE);
+    
+}