sphereFit.m

function [Center,Radius,ErrorDist] = sphereFit(X)
% this fits a sphere to a collection of data using a closed form for the
% solution (opposed to using an array the size of the data set). 
% Minimizes Sum((x-xc)^2+(y-yc)^2+(z-zc)^2-r^2)^2
% x,y,z are the data, xc,yc,zc are the sphere's center, and r is the radius

% Assumes that points are not in a singular configuration, real numbers, ...
% if you have coplanar data, use a circle fit with svd for determining the
% plane, recommended Circle Fit (Pratt method), by Nikolai Chernov
% http://www.mathworks.com/matlabcentral/fileexchange/22643

% Input:
% X: n x 3 matrix of cartesian data
% Outputs:
% Center: Center of sphere 
% Radius: Radius of sphere
% Author:
% Alan Jennings, University of Dayton

% Modified to add distance to sphere -> ErrorDist

A=[mean(X(:,1).*(X(:,1)-mean(X(:,1)))), ...
    2*mean(X(:,1).*(X(:,2)-mean(X(:,2)))), ...
    2*mean(X(:,1).*(X(:,3)-mean(X(:,3)))); ...
    0, ...
    mean(X(:,2).*(X(:,2)-mean(X(:,2)))), ...
    2*mean(X(:,2).*(X(:,3)-mean(X(:,3)))); ...
    0, ...
    0, ...
    mean(X(:,3).*(X(:,3)-mean(X(:,3))))];
A=A+A.';
B=[mean((X(:,1).^2+X(:,2).^2+X(:,3).^2).*(X(:,1)-mean(X(:,1))));...
    mean((X(:,1).^2+X(:,2).^2+X(:,3).^2).*(X(:,2)-mean(X(:,2))));...
    mean((X(:,1).^2+X(:,2).^2+X(:,3).^2).*(X(:,3)-mean(X(:,3))))];
Center=(A\B).';

Radius=sqrt(mean(sum([X(:,1)-Center(1),X(:,2)-Center(2),X(:,3)-Center(3)].^2,2)));
% (P-Centre)^2 - Radius^2
ErrorDist = sum([X(:,1)-Center(1),X(:,2)-Center(2),X(:,3)-Center(3)].^2,2) - Radius^2;