% CANNY - Canny edge detection
%
% Function to perform Canny edge detection. Code uses modifications as
% suggested by Fleck (IEEE PAMI No. 3, Vol. 14. March 1992. pp 337-345)
%
% Usage: [gradient or] = canny(im, sigma)
%
% Arguments: im - image to be procesed
% sigma - standard deviation of Gaussian smoothing filter
% (typically 1)
%
% Returns: gradient - edge strength image (gradient amplitude)
% or - orientation image (in degrees 0-180, positive
% anti-clockwise)
%
% See also: NONMAXSUP, HYSTHRESH
% Copyright (c) 1999-2003 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.
% April 1999 Original version
% January 2003 Error in calculation of d2 corrected
function [gradient, or] = canny(im, sigma)
[rows, cols] = size(im);
im = double(im); % Ensure double
hsize = [6*sigma+1, 6*sigma+1]; % The filter size.
gaussian = fspecial('gaussian',hsize,sigma);
im = filter2(gaussian,im); % Smoothed image.
h = [ im(:,2:cols) zeros(rows,1) ] - [ zeros(rows,1) im(:,1:cols-1) ];
v = [ im(2:rows,:); zeros(1,cols) ] - [ zeros(1,cols); im(1:rows-1,:) ];
d1 = [ im(2:rows,2:cols) zeros(rows-1,1); zeros(1,cols) ] - ...
[ zeros(1,cols); zeros(rows-1,1) im(1:rows-1,1:cols-1) ];
d2 = [ zeros(1,cols); im(1:rows-1,2:cols) zeros(rows-1,1); ] - ...
[ zeros(rows-1,1) im(2:rows,1:cols-1); zeros(1,cols) ];
X = h + (d1 + d2)/2.0;
Y = v + (d1 - d2)/2.0;
gradient = sqrt(X.*X + Y.*Y); % Gradient amplitude.
or = atan2(-Y, X); % Angles -pi to + pi.
neg = or<0; % Map angles to 0-pi.
or = or.*~neg + (or+pi).*neg;
or = or*180/pi; % Convert to degrees.
