diff -u -r1.8 -r1.9
--- SiPixels.java	7 May 2009 22:33:44 -0000	1.8
+++ SiPixels.java	22 Sep 2009 18:26:53 -0000	1.9
@@ -112,12 +112,20 @@

 
     // Neigbor ncells away along each axis
     public int getNeighborCell(int cell_id, int ncells_row, int ncells_col) {

-        int neighbor = cell_id + ncells_row * _ncols + ncells_col;
-        if (isValidCell(neighbor)) {
-            return neighbor;
-        } else {
-            return -1;
-        }

+
+        //  Get the column and row numbers of the neighbor cell
+        int nbrcol = getColumnNumber(cell_id) + ncells_col;
+        int nbrrow = getRowNumber(cell_id) + ncells_row;
+
+        //  Get teh neighbor cell ID and check that it's valid
+        int nbrcell = getCellID(nbrrow, nbrcol);
+        if (nbrcell < 0) return nbrcell;
+
+        //  Check that the cell is valid (needed for non-rectangular geometries)
+        if (!isValidCell(cell_id)) return -1;
+
+        //  Valid neighbor - return the cell ID
+        return nbrcell;

     }
 
     // Get all nearest neighbor cells

@@ -186,6 +194,8 @@

 
     // ID of cell from row and column number
     public int getCellID(int row_number, int column_number) {

+        if (row_number < 0 || row_number >=_nrows) return -1;
+        if (column_number < 0 || column_number >= _ncols) return -1;

         return row_number * _ncols + column_number;
     }
 

@@ -348,7 +358,7 @@

                 int ipixel = getCellID(ixmin + ix, iymin + iy);
 
                 //  Make sure we have a valid pixel

-                if (isValidCell(ipixel)) {

+                if (ipixel >= 0) {

 
                     //  Calculate the charge in this pixel
                     int pixel_charge = (int) Math.round(normalization * prob[ix][iy]);