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From "Hank Grabowski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MATH-1138) BicubicSplineInterpolator is returning incorrect interpolated values
Date Mon, 06 Oct 2014 22:35:34 GMT

    [ https://issues.apache.org/jira/browse/MATH-1138?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14161138#comment-14161138
] 

Hank Grabowski commented on MATH-1138:
--------------------------------------

I've finished coding up the Akima spline interpolator, and while it works better than the
natural spline it is still nowhere near as close as the b-splines in Octave.  It nails it
for linear and quadratic curves, but even a cubic function throws it (and the natural spline)
for a loop.  I've confirmed the uncertainties of both spline methods by comparing to Math.NET
output.  I therefore want to implement a B-Spline.  I found this BSD-licensed B-spline library.
 I intend to use for that implementation.

http://www.eol.ucar.edu/homes/granger/bspline/doc/index.html



> 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
>         Attachments: Interpolated Values from CM and MatLab.docx
>
>
> 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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