grayslice.m

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function bout=grayslice(I,z)
%GRAYSLICE Create indexed image from intensity image by thresholding.
%   X=GRAYSLICE(I,N) thresholds the intensity image I using
%   threshold values 1/n, 2/n, ..., (n-1)/n, returning an indexed
%   image in X.
%
%   X=GRAYSLICE(I,V), where V is a vector of values between 0 and
%   1, thresholds I using the values of V as thresholds,
%   returning an indexed image in X.
%
%   You can view the thresholded image using IMSHOW(X,MAP) with a
%   colormap of appropriate length.
%
%   Class Support
%   -------------
%   The input image I can be of class uint8 or double. Note that
%   the threshold values are always between 0 and 1, even if I is
%   of class uint8. In this case, each threshold value is
%   multiplied by 255 to determine the actual threshold to use.
%
%   The class of the output image X depends on the number of
%   threshold values, as specified by N or length(V). If the
%   number of threshold values is less than 256, then X is of
%   class uint8, and the values in X range from 0 to N or
%   length(V). If the number of threshold values is 256 or
%   greater, X is of class double, and the values in X range from
%   1 to N+1 or length(V)+1.
%
%   Example
%   -------
%   Use multilevel thresholding to enhance the intelligibility of
%   an astronomical image.
%
%       I = imread('ngc4024m.tif');
%       X = grayslice(I,16);
%       imshow(I), figure, imshow(X,hot(16))
%
%   See also GRAY2IND.

%   Clay M. Thompson 1-28-93
%   modified by Chris Griffin 7-8-96
%   Copyright 1993-1998 The MathWorks, Inc.  All Rights Reserved.
%   $Revision: 5.11 $  $Date: 1997/11/24 15:35:03 $


error(nargchk(1,2,nargin));

if nargin<2, z = 10; end

if ((prod(size(z)) == 1) & ((round(z)==z) | (z>1)))
   % arg2 is scalar: Integer number of equally spaced levels.
   n = z; 
   if isa(I,'uint8'), 
      z = 255*(0:(n-1))/n; 
   else % I is double
      z = (0:(n-1))/n;
   end
else
   % arg2 is vector containing threshold levels
   n = length(z)+1;
   if isa(I,'uint8')
      zmax = double(max(255,max(I(:))));
   else    % I was a double
      zmax = max(1,max(I(:)));
   end
   zmin = double(min(0,min(I(:)))); 
   % if z is uint8, promote it to double for the call to sort
   if isa(z, 'uint8'), z = double(z); end;
   z = [zmin,max(zmin,min(zmax,sort(z(:)')))]; % sort and threshold z
end

% For syntax: grayslice(I,n) , we don't need to promote I to double

% Get output matrix of appropriate size and type
if n<256
   b = repmat(uint8(0), size(I));  
else 
   b = zeros(size(I)); 
end

% Loop over all intervals, except the last
for i=1:length(z)-1,
   % j is the index value we will output, so it depend upon storage class
   if isa(b,'uint8'), 
      j = i-1; 
   else 
      j = i;  
   end
   d = find(I>=z(i) & I<z(i+1));
   if ~isempty(d), 
      b(d) = j; 
   end
end

% Take care of that last interval
d = find(I >= z(end));
if ~isempty(d)
   % j is the index value we will output, so it depend upon storage class
   if isa(b, 'uint8'), 
      j = length(z)-1; 
   else 
      j = length(z); 
   end
   b(d) = j; 
end

if nargout==0,
   imshow(b,jet(n))
   return
end
bout = b;