fitline.m

MATLAB Functions for Multiple View Geometry

% FITLINE - Least squares fit of a line to a set of points
%
% Usage:   [C, dist] = fitline(XY)
%
% Where:   XY  - 2xNpts array of xy coordinates to fit line to data of
%                the form 
%                [x1 x2 x3 ... xN
%                 y1 y2 y3 ... yN]
%
% Returns: C    - 3x1 array of line coefficients in the form
%                 c(1)*X + c(2)*Y + c(3) = 0
%          dist - Array of distances from the fitted line to the supplied
%                 data points.  Note that dist is only calculated if the
%                 function is called with two output arguments.
%
% The magnitude of C is scaled so that line equation corresponds to
%   sin(theta)*X + (-cos(theta))*Y + rho = 0
% where theta is the angle between the line and the x axis and rho is the
% perpendicular distance from the origin to the line.  Rescaling the
% coefficients in this manner allows the perpendicular distance from any
% point (x,y) to the line to be simply calculated as
%   r = abs(c(1)*X + c(2)*Y + c(3))
%
%
% If you want to convert this line representation to the classical form 
%     Y = a*X + b
% use
%  a = -c(1)/c(2)
%  b = -c(3)/c(2)
%
% Note, however, that this assumes c(2) is not zero

% Copyright (c) 2003-2005 Peter Kovesi
% School of Computer Science & Software Engineering
% The University of Western Australia
% http://www.csse.uwa.edu.au/
% 
% Permission is hereby granted, free of charge, to any person obtaining a copy
% of this software and associated documentation files (the "Software"), to deal
% in the Software without restriction, subject to the following conditions:
% 
% The above copyright notice and this permission notice shall be included in 
% all copies or substantial portions of the Software.
%
% The Software is provided "as is", without warranty of any kind.

% June      2003 - Original version
% September 2004 - Rescaling to allow simple distance calculation.

function [C, dist] = fitline(XY)
  
  [rows,npts] = size(XY);    
  
  if rows ~=2
    error('Data is not 2D');
  end
  
  if npts < 2
    error('Too few points to fit line');
  end
  
  % Set up constraint equations of the form  AC = 0,
  % where C is a column vector of the line coefficients
  % in the form   c(1)*X + c(2)*Y + c(3) = 0.
  
  A = [XY' ones(npts,1)];  % Build constraint matrix
  
  if npts == 2             % Pad A with zeros
    A = [A; zeros(1,3)]; 
  end

  [u d v] = svd(A);        % Singular value decomposition.
  C = v(:,3);              % Solution is last column of v.

  % Rescale coefficients so that line equation corresponds to
  %   sin(theta)*X + (-cos(theta))*Y + rho = 0
  % so that the perpendicular distance from any point (x,y) to the line
  % to be simply calculated as 
  %   r = abs(c(1)*X + c(2)*Y + c(3))

  theta = atan2(C(1), -C(2));

  % Find the scaling (but avoid dividing by zero)
  if abs(sin(theta)) > abs(cos(theta))
      k = C(1)/sin(theta);
  else
      k = -C(2)/cos(theta);
  end
  
  C = C/k;
  
  % If requested, calculate the distances from the fitted line to
  % the supplied data points 
  if nargout==2   
      dist = abs(C(1)*XY(1,:) + C(2)*XY(2,:) + C(3));
  end