Convert rgb image to an indexed image without decreasing the colors in Matlab
<p>I am doing steganography in indexed image. One of my problems is to convert the rgb image to an indexed image without decreasing the colors. I found out rgb2ind function in matlab converts rgb to indexed using minimum variance quantization and dithering. What I want is to make this indexed image colors exactly like rgb colors. How can i do that?</p>

<p>For the example, in this code i have this matrix M and convert it with rgb2ind function. I'm expecting the result variable Mrgb is equal with the matrix M.</p>

<pre><code>% Matrix M 
M(:,:,1) =  [225  230  250  243   20    3  244  247  255  255
242  252  239    0  239  224   27  252  255  255
224  239   28  243  236  231  240   11  255  255
243  252   15  224   15   12  251    0  255  255
224   11  252  227    0   11  232  251    0  255
243    4  255  228   19    4  243  224    0  255
 0  255  236  251  228  251  224  243  255    0
 19  236  255  224   27   12  251  236  255    0
  0  255  255  255  255  255  255  255  255    0
255    0    0    0    0    0    0    0    0  255];

M(:,:,2) =[255  255  255  255    0    0  255  255  255  255
255  255  255    0  255  255    0  255  255  255
255  255    0  255  255  255  255    0  255  255
255  255    0  255    0    0  255    0  255  255
255    0  255  255    0    0  255  255    0  255
255    0  255  255    0    0  255  255    0  255
0  255  255  255  255  255  255  255  255    0
0  255  255  255    0    0  255  255  255    0
0  255  255  255  255  255  255  255  255    0
255    0    0    0    0    0    0    0    0  255];

M(:,:,3) = [255  255  255  255    0    0  255  255  255  255
255  255  255    0    0    0    0  255  255  255
255  255    0    0    0    0    0    0  255  255
255  255    0    0    0    0    0    0  255  255
255    0    0    0    0    0    0    0    0  255
255    0    0    0    0    0    0    0    0  255
0    0    0    0    0    0    0    0    0    0
0    0    0    0    0    0    0    0    0    0
0    0    0    0    0    0    0    0    0    0
255    0    0    0    0    0    0    0    0  255];

[Mind map] = rgb2ind(M,255);
Mrgb = ind2rgb(Mind, map);
Mrgb=im2uint8(Mrgb);
isequal(M, Mrgb)
</code></pre>

<p><strong>UPDATE</strong></p>

<p>I update the matrix M in 4-D. I tried code from Dan's answer and show the image with <code>imshow</code>, but the image become purple when it supposed to be yellow. How can I fix it? </p>

<pre><code>M(:,:,1,1) = [ 225  230  250  243   20    3  244  247  255  255
  242  252  239    0  239  224   27  252  255  255
  224  239   28  243  236  231  240   11  255  255
  243  252   15  224   15   12  251    0  255  255
  224   11  252  227    0   11  232  251    0  255
  243    4  255  228   19    4  243  224    0  255
    0  255  236  251  228  251  224  243  255    0
   19  236  255  224   27   12  251  236  255    0
    0  255  255  255  255  255  255  255  255    0
  255    0    0    0    0    0    0    0    0  255];


M(:,:,2,1) = [  255  255  255  255    0    0  255  255  255  255
  255  255  255    0  255  255    0  255  255  255
  255  255    0  255  255  255  255    0  255  255
  255  255    0  255    0    0  255    0  255  255
  255    0  255  255    0    0  255  255    0  255
  255    0  255  255    0    0  255  255    0  255
    0  255  255  255  255  255  255  255  255    0
    0  255  255  255    0    0  255  255  255    0
    0  255  255  255  255  255  255  255  255    0
  255    0    0    0    0    0    0    0    0  255];


M(:,:,3,1) = [ 255  255  255  255    0    0  255  255  255  255
  255  255  255    0    0    0    0  255  255  255
  255  255    0    0    0    0    0    0  255  255
  255  255    0    0    0    0    0    0  255  255
  255    0    0    0    0    0    0    0    0  255
  255    0    0    0    0    0    0    0    0  255
    0    0    0    0    0    0    0    0    0    0
    0    0    0    0    0    0    0    0    0    0
    0    0    0    0    0    0    0    0    0    0
  255    0    0    0    0    0    0    0    0  255];


M(:,:,1,2) = [  255  255  255  255    0    0  255  255  255  255
  255  255  255    0  255  255    0  255  255  255
  255  255    0  255  255  255  255    0  255  255
  255  255    0  255  255  255  255    0  255  255
  255    0  255  255  255  255  255  255    0  255
  255    0  255  255  255  255  255  255    0  255
    0  255  255  255  255  255  255  255  255    0
    0  255  255  255  255  255  255  255  255    0
    0  255  255  255  255  255  255  255  255    0
  255    0    0    0    0    0    0    0    0  255];


M(:,:,2,2) = [  255  255  255  255    0    0  255  255  255  255
  255  255  255    0  255  255    0  255  255  255
  255  255    0  255  255  255  255    0  255  255
  255  255    0  255  255  255  255    0  255  255
  255    0  255  255  255  255  255  255    0  255
  255    0  255  255  255  255  255  255    0  255
    0  255  255  255  255  255  255  255  255    0
    0  255  255  255  255  255  255  255  255    0
    0  255  255  255  255  255  255  255  255    0
  255    0    0    0    0    0    0    0    0  255];


M(:,:,3,2) = [  255  255  255  255    0    0  255  255  255  255
  255  255  255    0    0    0    0  255  255  255
  255  255    0    0    0    0    0    0  255  255
  255  255    0    0    0    0    0    0  255  255
  255    0    0    0    0    0    0    0    0  255
  255    0    0    0    0    0    0    0    0  255
    0    0    0    0    0    0    0    0    0    0
    0    0    0    0    0    0    0    0    0    0
    0    0    0    0    0    0    0    0    0    0
  255    0    0    0    0    0    0    0    0  255];



[map, ~, Mind] = unique(reshape(M,[],3), 'rows');
[row, col, ~, frame]=size(M);
Mind = reshape(Mind, [row, col, frame]);

%%convert to rgb 
Mrgb=zeros(row, col, 3, frame);
z = size(map,1);
Mrgb = cat(3, map(Mind), map(Mind + z), map(Mind + 2*z));
Mrgb = reshape(Mrgb, [row, col,3, frame]);

map=uint8(map);
map=im2double(map);

imshow(Mind(:,:,1), map)
</code></pre>