scipy.interpolate.PPoly.from_spline#
- classmethod PPoly.from_spline(tck, extrapolate=None)[source]#
Construct a piecewise polynomial from a spline
- Parameters:
- tck
A spline, as returned by
splrep
or a BSpline object.- extrapolatebool or ‘periodic’, optional
If bool, determines whether to extrapolate to out-of-bounds points based on first and last intervals, or to return NaNs. If ‘periodic’, periodic extrapolation is used. Default is True.
Examples
Construct an interpolating spline and convert it to a
PPoly
instance>>> import numpy as np >>> from scipy.interpolate import splrep, PPoly >>> x = np.linspace(0, 1, 11) >>> y = np.sin(2*np.pi*x) >>> tck = splrep(x, y, s=0) >>> p = PPoly.from_spline(tck) >>> isinstance(p, PPoly) True
Note that this function only supports 1D splines out of the box.
If the
tck
object represents a parametric spline (e.g. constructed bysplprep
or aBSpline
withc.ndim > 1
), you will need to loop over the dimensions manually.>>> from scipy.interpolate import splprep, splev >>> t = np.linspace(0, 1, 11) >>> x = np.sin(2*np.pi*t) >>> y = np.cos(2*np.pi*t) >>> (t, c, k), u = splprep([x, y], s=0)
Note that
c
is a list of two arrays of length 11.>>> unew = np.arange(0, 1.01, 0.01) >>> out = splev(unew, (t, c, k))
To convert this spline to the power basis, we convert each component of the list of b-spline coefficients,
c
, into the corresponding cubic polynomial.>>> polys = [PPoly.from_spline((t, cj, k)) for cj in c] >>> polys[0].c.shape (4, 14)
Note that the coefficients of the polynomials polys are in the power basis and their dimensions reflect just that: here 4 is the order (degree+1), and 14 is the number of intervals—which is nothing but the length of the knot array of the original tck minus one.
Optionally, we can stack the components into a single
PPoly
along the third dimension:>>> cc = np.dstack([p.c for p in polys]) # has shape = (4, 14, 2) >>> poly = PPoly(cc, polys[0].x) >>> np.allclose(poly(unew).T, # note the transpose to match `splev` ... out, atol=1e-15) True