# coding=utf-8
""" Module for fitting a sphere to a list of 3D points """
#scipy has a nice least squares optimisor
from scipy.optimize import leastsq
import numpy
[docs]def fit_sphere_least_squares(x_values, y_values, z_values, initial_parameters):
"""
Uses scipy's least squares optimisor to fit a sphere to a set
of 3D Points
:return: x: an array containing the four fitted parameters
:return: ier: int An integer flag. If it is equal to 1, 2, 3 or 4, the
solution was found.
:param: (x,y,z) three arrays of equal length containing the x, y, and z
coordinates.
:param: an array containing four initial values (centre, and radius)
"""
return leastsq(_calculate_residual_sphere, initial_parameters,
args=(x_values, y_values, z_values))
def _calculate_residual_sphere(parameters, x_values, y_values , z_values):
"""
Calculates the residual error for an x,y,z coordinates, fitted
to a sphere with centre and radius defined by the parameters tuple
:return: The residual error
:param: A tuple of the parameters to be optimised, should contain
[x_centre, y_centre, z_centre, radius]
:param: arrays containing the x,y, and z coordinates.
"""
#extract the parameters
x_centre, y_centre, z_centre, radius = parameters
#use numpy's sqrt function here, which works by element on arrays
distance_from_centre = numpy.sqrt((x_values - x_centre)**2 +
(y_values - y_centre)**2 +
(z_values - z_centre)**2)
return distance_from_centre - radius