Fixed location of stencils for interpolation.

This closes PR CactusPUGH/745.


git-svn-id: http://svn.cactuscode.org/arrangements/CactusPUGH/PUGHInterp/trunk@16 1c20744c-e24a-42ec-9533-f5004cb800e5

1 files changed, 11 insertions, 2 deletions

diff --git a/src/Interpolate.c b/src/Interpolate.c
index b387b19..65e797a 100644
--- a/src/Interpolate.c
+++ b/src/Interpolate.c
@@ -17,6 +17,7 @@
    @version   $Id$
  @@*/
 
+#include <math.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -250,8 +251,16 @@ int PUGHInterp_Interpolate (int order,
     /* loop over all dimensions */
     for (i = 0; i < num_dims; i++)
     {
-      /* nearest grid point for stencil */
-      point[i] = (coord[num_dims*n + i] - origin[i]) / delta[i];
+      /* closest grid point for stencil */
+      point[i] = floor ((coord[num_dims*n + i] - origin[i]) / delta[i]
+                        - 0.5 * (order - 1));
+
+      /* if beyond lower bound shift the grid point to the right */
+      shift = point[i];
+      if (shift < 0)
+      {
+        point[i] -= shift;
+      }
 
       /* if beyond upper bound shift the grid point to the left */
       shift = point[i] + order - (dims[i] - 1);