[jira] [Created] (MATH-1138) BicubicSplineInterpolator is returning incorrect interpolated values

[Adam Bedrossian (JIRA)]

Adam Bedrossian created MATH-1138:
-------------------------------------

             Summary: BicubicSplineInterpolator is returning incorrect 
interpolated values
                 Key: MATH-1138
                 URL: https://issues.apache.org/jira/browse/MATH-1138
             Project: Commons Math
          Issue Type: Bug
    Affects Versions: 3.3
            Reporter: Adam Bedrossian


I have encountered a use case with the BicubicSplineInterpolator where the 
interpolated values that are being returned seem incorrect.  Furthermore, the 
values do not match those generated by MatLab using the interp2 'cubic' method.

Here is a snippet of code that uses the interpolator:

double[] xValues = new double[] {36, 36.001, 36.002};
        double[] yValues = new double[] {-108.00, -107.999, -107.998};

        double[][] fValues = new double[][] {{1915, 1906, 1931},
                                        {1877, 1889, 1894},
                                        {1878, 1873, 1888}};

        BicubicSplineInterpolator interpolator = new
BicubicSplineInterpolator();
        BicubicSplineInterpolatingFunction interpolatorFunction =
interpolator.interpolate(xValues, yValues, fValues);

        double[][] results = new double[9][9];
        double x = 36;
        int arrayIndexX = 0, arrayIndexY = 0;

        while(x <= 36.002) {
            double y = -108;
            arrayIndexY = 0;
            while (y <= -107.998) {
                results[arrayIndexX][arrayIndexY] =
interpolatorFunction.value(x,  y);
                System.out.println(results[arrayIndexX][arrayIndexY]);
                y = y + 0.00025;
                arrayIndexY++;
            }

            x = x + 0.00025;
            arrayIndexX++;
        }

Attached is a grid showing x and y values and the corresponding interpolated 
value from both commons math and MatLab.

The values produced by commons math are far off from those created by MatLab.



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