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Date: Fri, 11 Jul 2014 18:52:05 +0000 (UTC)
From: "Adam Bedrossian (JIRA)" <jira@apache.org>
To: issues@commons.apache.org
Message-ID: <JIRA.12726859.1405104690134.26266.1405104725681@arcas>
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Subject: [jira] [Created] (MATH-1138) BicubicSplineInterpolator is returning
 incorrect interpolated values
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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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