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https://issues.apache.org/jira/browse/MATH-1138?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14132695#comment-14132695
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Hank Grabowski commented on MATH-1138:
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I have been investigating this much further. The good news is that from what I
can tell the implementation of this function is in line with the Wikipedia
article it is based on. The bad news is that this is just not working. The
unit tests associated with this aren't even passing a basic 2D linear function
test. It should be getting identical results to the expected values. These
aren't even close.
I re-implemented the WIkipedia implementation in Octave and I get the same
results the Apache Math function does. I rederived the equations from the
Wikipedia article and confirmed that the coefficients being used are the
correct. It's just not working though.
I looked into how Octave performs the same function. They do not do it this
way. They do it by building up from a series of linear cubic splines. I have
a mechanism for implementing it in that way and could take a crack at it if you
would like. I know I just joined the Apache project here so let me know what
the proper path forward would be to get this assigned to me and contribute it
back.
If I do get it, I intend to reform the unit tests so that they have more
appropriate tolerances for the functions listed. Even the ones that are
passing are doing so because their tolerances are jacked up way too high, which
isn't really testing the function numerical accuracy at all.
> 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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