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https://issues.apache.org/jira/browse/MATH-985?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Gilles updated MATH-985:
------------------------
Attachment: math985.png
I've attached a plot of your example, comparing the old and new code.
There are still discrepancies with the original data, but this happens "only"
on the border of the interpolation range. I don't know whether this is expected
or not.
I'm going to replace the unit tests that now fail; that they succeeded with a
buggy code proves their uselessness :(. You are warmly welcome to suggest some
tests that properly demonstrate expectations from this algorithm.
> BicubicSpline interpolation returns unexpected values
> (BicubicSplineInterpolator/BicubicSplineInterpolationFunction)
> --------------------------------------------------------------------------------------------------------------------
>
> Key: MATH-985
> URL: https://issues.apache.org/jira/browse/MATH-985
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.1.1
> Environment: Windows 7 64 bit, 64 bit JVM
> Reporter: Johnathan Kool
> Attachments: math985.png
>
>
> I've been testing out the Tricubic spline functions, and have been getting
> some strange values. I dug down a bit, and it seems like they start at the
> Bicubic level. SplineInterpolator/PolynomialSplineFunction seem to be
> returning correct values. I set up a block of data that increases linearly,
> so you'd expect the interpolated values to follow the same trend, except the
> values tend to overshoot to half the distance between knot points, and then
> undershoot for the remaining half. Probably the easiest thing would be to
> show some tests. First - 1D which works fine:
> {code:title=SplineTest.java|borderStyle=solid}
> import org.apache.commons.math3.analysis.interpolation.SplineInterpolator;
> import org.apache.commons.math3.analysis.polynomials.PolynomialSplineFunction;
> public class SplineTest {
> double[] x = new double[]{.52,.54,.56,.58,.6};
> double[] y = new double[]{76,77,78,79,80};
>
> public static void main(String[] args){
> SplineTest st = new SplineTest();
> st.go();
> }
>
> public void go(){
> SplineInterpolator si = new SplineInterpolator();
> PolynomialSplineFunction sf = si.interpolate(x, y);
>
> System.out.println(sf.value(0.52));
> System.out.println(sf.value(0.5225));
> System.out.println(sf.value(0.525));
> System.out.println(sf.value(0.5275));
> System.out.println(sf.value(0.53));
> System.out.println(sf.value(0.5325));
> System.out.println(sf.value(0.535));
> System.out.println(sf.value(0.5375));
> System.out.println(sf.value(0.54));
> System.out.println(sf.value(0.5425));
> System.out.println(sf.value(0.545));
> System.out.println(sf.value(0.5475));
> System.out.println(sf.value(0.55));
> System.out.println(sf.value(0.5525));
> System.out.println(sf.value(0.555));
> System.out.println(sf.value(0.5575));
> System.out.println(sf.value(0.56));
> }
> }
> {code}
> and next, 2D which doesn't:
> {code:title=BicubicSplineTest.java|borderStyle=solid}
> import
> org.apache.commons.math3.analysis.interpolation.BicubicSplineInterpolatingFunction;
> import
> org.apache.commons.math3.analysis.interpolation.BicubicSplineInterpolator;
> public class BicubicSplineTest {
> double[] x = new double[]{0,1,2};
> double[] y = new double[]{.52,.54,.56,.58,.6};
> double[] v1 = new double[]{76,77,78,79,80};
> double[][] v2 = new double[][]{v1,v1,v1};
>
> public static void main(String[] args){
> BicubicSplineTest bt = new BicubicSplineTest();
> bt.go();
> }
>
> public void go(){
> BicubicSplineInterpolator bi = new BicubicSplineInterpolator();
> BicubicSplineInterpolatingFunction bf = bi.interpolate(x, y,
> v2);
>
> System.out.println(bf.value(1, 0.52));
> System.out.println(bf.value(1, 0.5225));
> System.out.println(bf.value(1, 0.525));
> System.out.println(bf.value(1, 0.5275));
> System.out.println(bf.value(1, 0.53));
> System.out.println(bf.value(1, 0.5325));
> System.out.println(bf.value(1, 0.535));
> System.out.println(bf.value(1, 0.5375));
> System.out.println(bf.value(1, 0.54));
> System.out.println(bf.value(1, 0.5425));
> System.out.println(bf.value(1, 0.545));
> System.out.println(bf.value(1, 0.5475));
> System.out.println(bf.value(1, 0.55));
> System.out.println(bf.value(1, 0.5525));
> System.out.println(bf.value(1, 0.555));
> System.out.println(bf.value(1, 0.5575));
> System.out.println(bf.value(1, 0.56));
> }
> }
> {code}
> The data points increase from 76 to 80 in a linear way. Incrementing by 1/8
> the distance to the next point, the 1D spline returns:
> 76.0
> 76.125
> 76.25
> 76.375
> 76.5
> 76.625
> 76.75
> 76.875
> 77.0
> 77.125
> 77.25
> 77.375
> 77.5
> 77.625
> 77.75
> 77.875
> 78.0
> The 2D spline returns:
> 76.0
> 80.14453124999996
> 80.84375000000003
> 79.24609375000007
> 76.50000000000003
> 73.75390625000007
> 72.15625000000001
> 72.85546874999996
> 76.99999999999997
> 81.14453124999993
> 81.84374999999997
> 80.24609375000006
> 77.50000000000003
> 74.75390625000009
> 73.15625000000004
> 73.85546874999997
> 78.0
> Even though it's still effectively a 1D problem. I'm not sure exactly what's
> causing it - maybe something when multiplying the coefficients, but thought I
> should flag it.
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