From bug-octave-request at bevo dot che dot wisc dot edu Tue Feb 17 03:16:45 2004
Subject: imaging bugs
From: Stefan van der Walt <stefan at sun dot ac dot za>
To: bug-octave at bevo dot che dot wisc dot edu
Date: Tue, 17 Feb 2004 11:15:15 +0200

'imread' returns incorrect map:
-------------------------------

octave:32> [I, map] = imread('../images/lenna256.jpg');
octave:33> min(min(map))
ans = -0.0078125

Map should be in [0, 1]


'histeq' not correct:
---------------------

octave:68> a = histeq(I/256, 256);
octave:69> max(max(a))
ans = 61

Also, the resulting image is incorrect.  I have written comments for
the case when running this function on a gray scale lena256:

function J = histeq (I, n)
   [r,c] = size (I);
   [X,map] = gray2ind(I); % map is 64 scales of gray, thus X maximum 64
   [nn,xx] = imhist(I);   % intensity image: 256 bins [0, 255]
   Icdf = ceil (n * cumsum (1/prod(size(I)) * nn));
   J = reshape(Icdf(X),r,c);  % !! Incorrect -- Icdf is 1x256,
                              % !! while X is maximum 64
   plot(Icdf,'b;;');
endfunction


Maybe this would provide a working solution?

   J = reshape(Icdf(X/length(map)*length(xx)),r,c);



-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------