/
ndgriddata.py
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/
ndgriddata.py
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"""
Convenience interface to N-D interpolation
.. versionadded:: 0.9
"""
from __future__ import division, print_function, absolute_import
import numpy as np
from .interpnd import LinearNDInterpolator, NDInterpolatorBase, \
CloughTocher2DInterpolator, _ndim_coords_from_arrays
from scipy.spatial import cKDTree
__all__ = ['griddata', 'NearestNDInterpolator', 'LinearNDInterpolator',
'CloughTocher2DInterpolator']
#------------------------------------------------------------------------------
# Nearest-neighbour interpolation
#------------------------------------------------------------------------------
class NearestNDInterpolator(NDInterpolatorBase):
"""
NearestNDInterpolator(points, values)
Nearest-neighbour interpolation in N dimensions.
.. versionadded:: 0.9
Methods
-------
__call__
Parameters
----------
x : (Npoints, Ndims) ndarray of floats
Data point coordinates.
y : (Npoints,) ndarray of float or complex
Data values.
rescale : boolean, optional
Rescale points to unit cube before performing interpolation.
This is useful if some of the input dimensions have
incommensurable units and differ by many orders of magnitude.
.. versionadded:: 0.14.0
Notes
-----
Uses ``scipy.spatial.cKDTree``
"""
def __init__(self, x, y, rescale=False):
NDInterpolatorBase.__init__(self, x, y, rescale=rescale,
need_contiguous=False,
need_values=False)
self.tree = cKDTree(self.points)
self.values = y
def __call__(self, *args):
"""
Evaluate interpolator at given points.
Parameters
----------
xi : ndarray of float, shape (..., ndim)
Points where to interpolate data at.
"""
xi = _ndim_coords_from_arrays(args, ndim=self.points.shape[1])
xi = self._check_call_shape(xi)
xi = self._scale_x(xi)
dist, i = self.tree.query(xi)
return self.values[i]
#------------------------------------------------------------------------------
# Convenience interface function
#------------------------------------------------------------------------------
def griddata(points, values, xi, method='linear', fill_value=np.nan,
rescale=False):
"""
Interpolate unstructured D-dimensional data.
Parameters
----------
points : ndarray of floats, shape (n, D)
Data point coordinates. Can either be an array of
shape (n, D), or a tuple of `ndim` arrays.
values : ndarray of float or complex, shape (n,)
Data values.
xi : ndarray of float, shape (M, D)
Points at which to interpolate data.
method : {'linear', 'nearest', 'cubic'}, optional
Method of interpolation. One of
``nearest``
return the value at the data point closest to
the point of interpolation. See `NearestNDInterpolator` for
more details.
``linear``
tesselate the input point set to n-dimensional
simplices, and interpolate linearly on each simplex. See
`LinearNDInterpolator` for more details.
``cubic`` (1-D)
return the value determined from a cubic
spline.
``cubic`` (2-D)
return the value determined from a
piecewise cubic, continuously differentiable (C1), and
approximately curvature-minimizing polynomial surface. See
`CloughTocher2DInterpolator` for more details.
fill_value : float, optional
Value used to fill in for requested points outside of the
convex hull of the input points. If not provided, then the
default is ``nan``. This option has no effect for the
'nearest' method.
rescale : bool, optional
Rescale points to unit cube before performing interpolation.
This is useful if some of the input dimensions have
incommensurable units and differ by many orders of magnitude.
.. versionadded:: 0.14.0
Notes
-----
.. versionadded:: 0.9
Examples
--------
Suppose we want to interpolate the 2-D function
>>> def func(x, y):
... return x*(1-x)*np.cos(4*np.pi*x) * np.sin(4*np.pi*y**2)**2
on a grid in [0, 1]x[0, 1]
>>> grid_x, grid_y = np.mgrid[0:1:100j, 0:1:200j]
but we only know its values at 1000 data points:
>>> points = np.random.rand(1000, 2)
>>> values = func(points[:,0], points[:,1])
This can be done with `griddata` -- below we try out all of the
interpolation methods:
>>> from scipy.interpolate import griddata
>>> grid_z0 = griddata(points, values, (grid_x, grid_y), method='nearest')
>>> grid_z1 = griddata(points, values, (grid_x, grid_y), method='linear')
>>> grid_z2 = griddata(points, values, (grid_x, grid_y), method='cubic')
One can see that the exact result is reproduced by all of the
methods to some degree, but for this smooth function the piecewise
cubic interpolant gives the best results:
>>> import matplotlib.pyplot as plt
>>> plt.subplot(221)
>>> plt.imshow(func(grid_x, grid_y).T, extent=(0,1,0,1), origin='lower')
>>> plt.plot(points[:,0], points[:,1], 'k.', ms=1)
>>> plt.title('Original')
>>> plt.subplot(222)
>>> plt.imshow(grid_z0.T, extent=(0,1,0,1), origin='lower')
>>> plt.title('Nearest')
>>> plt.subplot(223)
>>> plt.imshow(grid_z1.T, extent=(0,1,0,1), origin='lower')
>>> plt.title('Linear')
>>> plt.subplot(224)
>>> plt.imshow(grid_z2.T, extent=(0,1,0,1), origin='lower')
>>> plt.title('Cubic')
>>> plt.gcf().set_size_inches(6, 6)
>>> plt.show()
"""
points = _ndim_coords_from_arrays(points)
if points.ndim < 2:
ndim = points.ndim
else:
ndim = points.shape[-1]
if ndim == 1 and method in ('nearest', 'linear', 'cubic'):
from .interpolate import interp1d
points = points.ravel()
if isinstance(xi, tuple):
if len(xi) != 1:
raise ValueError("invalid number of dimensions in xi")
xi, = xi
# Sort points/values together, necessary as input for interp1d
idx = np.argsort(points)
points = points[idx]
values = values[idx]
ip = interp1d(points, values, kind=method, axis=0, bounds_error=False,
fill_value=fill_value)
return ip(xi)
elif method == 'nearest':
ip = NearestNDInterpolator(points, values, rescale=rescale)
return ip(xi)
elif method == 'linear':
ip = LinearNDInterpolator(points, values, fill_value=fill_value,
rescale=rescale)
return ip(xi)
elif method == 'cubic' and ndim == 2:
ip = CloughTocher2DInterpolator(points, values, fill_value=fill_value,
rescale=rescale)
return ip(xi)
else:
raise ValueError("Unknown interpolation method %r for "
"%d dimensional data" % (method, ndim))