colorseg.m

function I = colorseg(varargin)
%COLORSEG Performs segmentation of a color image.
%   S = COLORSEG('EUCLIDEAN', F, T, M) performs segmentation of color
%   image F using a Euclidean measure of similarity. M is a 1-by-3
%   vector representing the average color used for segmentation (this
%   is the center of the sphere in Fig. 6.26 of DIPUM). T is the
%   threshold against which the distances are compared.
%
%   S = COLORSEG('MAHALANOBIS', F, T, M, C) performs segmentation of
%   color image F using the Mahalanobis distance as a measure of
%   similarity. C is the 3-by-3 covariance matrix of the sample color
%   vectors of the class of interest. See function covmatrix for the
%   computation of C and M. 
%
%   S is the segmented image (a binary matrix) in which 0s denote the
%   background. 

%   Copyright 2002-2004 R. C. Gonzalez, R. E. Woods, & S. L. Eddins
%   Digital Image Processing Using MATLAB, Prentice-Hall, 2004
%   $Revision: 1.5 $  $Date: 2003/11/21 14:28:34 $

% Preliminaries.
% Recall that varargin is a cell array.
f = varargin{2};
if (ndims(f) ~= 3) | (size(f, 3) ~= 3)
   error('Input image must be RGB.');
end
M = size(f, 1); N = size(f, 2);
% Convert f to vector format using function imstack2vectors.
[f, L] = imstack2vectors(f);
f = double(f);
% Initialize I as a column vector.  It will be reshaped later
% into an image.
I = zeros(M*N, 1); 
T = varargin{3};
m = varargin{4};
m = m(:)'; % Make sure that m is a row vector.

if length(varargin) == 4 
   method = 'euclidean';
elseif length(varargin) == 5 
   method = 'mahalanobis';
else 
   error('Wrong number of inputs.');
end

switch method
case 'euclidean'
   % Compute the Euclidean distance between all rows of X and m. See
   % Section 12.2 of DIPUM for an explanation of the following
   % expression. D(i) is the Euclidean distance between vector X(i,:)
   % and vector m. 
   p = length(f);
   D = sqrt(sum(abs(f - repmat(m, p, 1)).^2, 2));
case 'mahalanobis'
   C = varargin{5};
   D = mahalanobis(f, C, m);
otherwise 
   error('Unknown segmentation method.')
end

% D is a vector of size MN-by-1 containing the distance computations
% from all the color pixels to vector m. Find the distances <= T.
J = find(D <= T);

% Set the values of I(J) to 1.  These are the segmented
% color pixels.
I(J) = 1;

% Reshape I into an M-by-N image.
I = reshape(I, M, N);