[MATH-1138] BicubicSplineInterpolator is returning incorrect interpolated values

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.3
• Fix Version/s: 3.4
• Assignee: Unassigned
• Reporter: Adam Bedrossian
• Created: 11/Jul/14 18:51
• Updated: 26/Dec/14 19:50
• Resolved: 17/Oct/14 08:54

Description

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.

Issue Links

• is related to: MATH-985 BicubicSpline interpolation returns unexpected values (BicubicSplineInterpolator/BicubicSplineInterpolationFunction)
• relates to: MATH-1166 BicubicInterpolation may have a bug