diff -u -r1.1 -r1.2
--- BilinearInterpolatorTest.java	4 Jun 2008 05:16:39 -0000	1.1
+++ BilinearInterpolatorTest.java	6 Jun 2008 07:16:32 -0000	1.2
@@ -4,7 +4,7 @@

  *
  * Created on June 3, 2008, 4:06 PM
  *

- * $Id: BilinearInterpolatorTest.java,v 1.1 2008/06/04 05:16:39 ngraf Exp $

+ * $Id: BilinearInterpolatorTest.java,v 1.2 2008/06/06 07:16:32 ngraf Exp $

  */
 
 package org.lcsim.math.interpolation;

@@ -45,6 +45,7 @@

             for(int j=0; j<b.length; ++j)
             {
 //                System.out.println("a= "+a[i]+", b= "+b[j]+" ,  interpolates to "+instance.interpolateValueAt(a[i],b[j]));

+//                System.out.println("should be "+vals[i][j]);

                 assertEquals(instance.interpolateValueAt(a[i],b[j]), vals[i][j]);
             }
         }

@@ -53,9 +54,12 @@

         {
             for(double j=b[0]; j<b[b.length-1]; j+=.2)
             {

-//               System.out.format("( %4.2f , %4.2f ) interpolates to %4.2f%n",i, j, instance.interpolateValueAt(i,j));
-                assertEquals(i*i+j*j, instance.interpolateValueAt(i,j),.0001);

+                double pred = instance.interpolateValueAt(i,j);
+                double real = i*i+j*j;
+//                System.out.format("( %4.2f , %4.2f ) interpolates to %4.2f%n",i, j, pred);
+//                System.out.println("should be "+real);
+                assertEquals(real, pred ,.5);

             }
         }

-    }   

+    }

 }

\ No newline at end of file

diff -u -r1.1 -r1.2
--- BilinearInterpolator.java	4 Jun 2008 05:16:09 -0000	1.1
+++ BilinearInterpolator.java	6 Jun 2008 07:16:32 -0000	1.2
@@ -3,7 +3,7 @@

  *
  * Created on June 3, 2008, 4:04 PM
  *

- * $Id: BilinearInterpolator.java,v 1.1 2008/06/04 05:16:09 ngraf Exp $

+ * $Id: BilinearInterpolator.java,v 1.2 2008/06/06 07:16:32 ngraf Exp $

  */
 //
 package org.lcsim.math.interpolation;

@@ -11,7 +11,7 @@

 import static java.lang.Math.abs;
 
 /**

- * A class to provide interpolated values for values determined at discrete points 

+ * A class to provide interpolated values for values determined at discrete points

  * on a 2D grid
  *
  * @author Norman Graf

@@ -19,7 +19,13 @@

 public class BilinearInterpolator
 {
     private double[] _x;

+    private double _xmin;
+    private double _xmax;
+    private int _xDim;

     private double[] _y;

+    private double _ymin;
+    private double _ymax;
+    private int _yDim;

     private double[][] _val;
     /**
      * Creates a new instance of BilinearInterpolator

@@ -29,9 +35,25 @@

      */
     public BilinearInterpolator(double[] x, double[] y, double[][] z)
     {

-        _x = x;
-        _y = y;
-        _val = z;

+        _x = new double[x.length];;
+        System.arraycopy(x,0,_x,0, x.length);
+        _xDim = _x.length;
+        _xmin = _x[0];
+        _xmax = _x[_xDim-1];
+        
+        _y = new double[y.length];
+        System.arraycopy(y,0,_y,0, y.length);
+        _yDim = _y.length;
+        _ymin = _y[0];
+        _ymax = _y[_yDim-1];
+        
+ 
+        
+        _val = new double[z.length][z[0].length];
+        for(int i=0; i< z.length; ++i)
+        {
+            System.arraycopy(z[i],0,_val[i],0,z[i].length);
+        }

     }
     
     //TODO add protection for values out of range

@@ -43,7 +65,8 @@

      */
     public double interpolateValueAt(double x, double y)
     {

-        return polin2(_x, _y, _val, x, y);

+        return trueBilinear( x, y );
+//        return polin2(_x, _y, _val, x, y);

     }
     
     /*

@@ -82,7 +105,7 @@

         double dy;
         double[] c = new double[n];
         double[] d = new double[n];

-        dif=abs(x-xa[1]);

+        dif=abs(x-xa[0]);

         
         for (int i=0;i<n;++i)
         {	//Here we find the index ns of the closest table entry,

@@ -95,8 +118,8 @@

             c[i]=ya[i];	// and initialize the table of c's and d's.
             d[i]=ya[i];
         }

-        double y = ya[ns];	// This is the initial approximation to y.
-        ns = ns-1;

+        double y = ya[ns--];	// This is the initial approximation to y.
+//        ns = ns-1;

         for (int m=1; m<n ; ++m)
         {
             //For each column of the table,

@@ -131,4 +154,42 @@

         }
         return y;
     }

+    
+    double trueBilinear(double x, double y)
+    {
+        // find bin for x
+        int ixlo = 0;
+        int ixhi = 0;
+        for(int i=0; i<_xDim-1; ++i)
+        {
+            if(x>=_x[i] && x<=_x[i+1])
+            {
+                ixlo = i;
+                ixhi = i+1;
+                break;
+            }
+        }
+        
+        // find bin for y
+        int iylo = 0;
+        int iyhi = 0;
+        for(int i=0; i<_yDim-1; ++i)
+        {
+            if(y>=_y[i] && y<=_y[i+1])
+            {
+                iylo = i;
+                iyhi = i+1;
+                break;
+            }
+        }
+        double v1 = _val[ixlo][iylo];
+        double v2 = _val[ixhi][iylo];
+        double v3 = _val[ixhi][iyhi];
+        double v4 = _val[ixlo][iyhi];
+        
+        double t = (x - _x[ixlo])/(_x[ixhi] - _x[ixlo]);
+        double u = (y - _y[iylo])/(_y[iyhi] - _y[iylo]);
+        
+        return (1-t)*(1-u)*v1 +t*(1-u)*v2+t*u*v3+(1-t)*u*v4;
+    }

 }

\ No newline at end of file