[jira] [Commented] (MATH-985) BicubicSpline interpolation returns unexpected values (BicubicSplineInterpolator/BicubicSplineInterpolationFunction)

["Thomas Neidhart (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MATH-985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13879299#comment-13879299 ] 

Thomas Neidhart commented on MATH-985:
--------------------------------------

Can this issue be closed?

> 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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