Bilinear Interpolation on large arrays in Java

I am using Java's InterpolationBilinear class to help me resample an array. My current (and relatively small) test case is transforming a 10x10 array into a 20x20 array. My issue is that the interpolate(double[][], float x, float y) method in this class is only resampling the upper-left corner of the 2D array I send it (0x0, 0x1, 1x0, 1x1).

Currently, it looks like I'll have to write some co开发者_运维问答de to send the interpolate method a bunch of 2x2 arrays instead of the whole array. There just seems to be a better way, any tips?

I am not interested in using 3rd party libraries, only standard Java and code I can write myself.

Thanks!

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That's what the InterpolationBilinear class is supposed to do. See the documentation. The only thing it does is implement bilinear interpolation between four points arranged in a rectangle. The formula is explained here, and you can also implement it yourself pretty easily.

The question is: what are float x and float y in your example? For InterpolationBilinear.interpolate they're supposed to represent the fractional position between the four corners of the sample rectangle.

Or do you want float x and float y to represent element numbers in your array? e.g. (4.3,7.1) would mean interpolating between elements (4,7), (4,8), (5,7), and (5,8)? Then yes, you just have to plug those four values into interpolate (or your own implementation of that simple formula) along with the fractional positions 0.3 in x and 0.1 in y.

EDIT You have now clarified that x and y should represent the fractional position in the array, let's call it raw[][], and assume it's 10 x 10 i.e. indices go from 0 to 9 in both dimensions.

To interpolate at (x,y) you just need to find what square (x,y) falls into, and interpolate between the four corners. In the x direction, the indices will be the integers immediately above and below x*9, i.e. Math.floor(x*9) and Math.floor(x*9) + 1 -- same idea in y but with y*9. Now you have your four corners. Plug them into the formula. The fractional position will be something like xfrac = x*9 - Math.floor(x*9) and yfrac = y*9 - Math.floor(y*9). Plug those in the formula as well.

This has to be repeated for each point in your target array. Note that the x in the paragraph above will be equal to i/19 and the y equal to j/19 where i and j are the indices of your target array.