title: Read only one RGB color in Octave
body: <p>I have to read a few hundred RGB images using the PNG format. I only need one of the colors ( either Red, Green or Blue ), and right now I'm doing something like this:</p>
  
  <pre><code>A = imread(file);
  A = A(:, :, 1);
  </code></pre>
  
  <p>I was wondering if it was possible to only read the values for one color, to make the reading faster. I need this operation to be as fast as possible.</p>