I've got a 3D dataset that I want to interpolate AND extrapolate linearly. The interpolation can be done easily with scipy.interpolate.LinearNDInterpolator
. The module can only fill in a constant/nan for values outside the parameter range, but I don't see why it would not offer an option to turn on extrapolation.
Looking at the code, I see that the module is written in cython. With no experience in cython, it is hard to play around with the code to implement extrapolation. I can write it in pure python code, but maybe someone else here has a better idea? My particular case involves a constant xy-grid, but the z-values keep changing a lot (-100,000) and therefore the interpolation must be fast as the interpolation will be runned for each time the z-values change.
To give a basic example, as requested, lets say that I have a grid like
xyPairs = [[-1.0, 0.0], [-1.0, 4.0],
[-0.5, 0.0], [-0.5, 4.0],
[-0.3, 0.0], [-0.3, 4.0],
[+0.0, 0.0], [+0.0, 4.0],
[+0.2, 0.0], [+0.2, 4.0]]
and lets say I want to calculate values at x = -1.5, -0.8, +0.5
and y = -0.2, +0.2, +0.5
. Currently, I am performing 1d interpolation/extrapolation along the x-axis for each y-value and then along the y-axis for each x-value. The extrapolation is done by the second function in ryggyr's answer
.
I propose a method, the code is awful but I hope it will help you. The idea is, if you know by advance the bounds in which you will have to extrapolate, you can add extra columns/rows at the edge of your arrays with linearly extrapolated values, and then interpolate on the new array. Here is an example with some data that will be extrapolated until x=+-50 and y=+-40:
Then you can interpolate on (xlarge,ylarge,zlarge). Since all operations are numpy slices operations, I hope it will be fast enough for you. When z data are updated, copy them in
zlarge[1:-1,1:-1]
and re-execute the 4 last lines.