title: Get pixel value on Gdk::Pixbuf, Set pixel value with Gdk::Cairo
body: <p>I'm using a <code>Gdk::Pixbuf</code> to display an image with <code>Gdk::Cairo</code> in C++ :</p>
  
  <pre><code>virtual bool on_draw(const Cairo::RefPtr&lt;Cairo::Context&gt;&amp; cr)
  {
      Glib::RefPtr&lt;Gdk::Pixbuf&gt; image = Gdk::Pixbuf::create_from_file(filename);
      Gdk::Cairo::set_source_pixbuf(cr, image, (width - image-&gt;get_width())/2, (height - image-&gt;get_height())/2);
      cr-&gt;paint();
      /* other displaying stuffs */
  }
  </code></pre>
  
  <p>This image is in B&amp;W and I need to bring out some pixels whose luminance is above a certain threshold. For that, I would like to color those pixels.</p>
  
  <p>First, I don't know (and I cannot find on the web) how to get the luminance of a certain pixel of my Pixbuf image.</p>
  
  <p>Second, I don't find another way to draw the pixel than drawing a line of length 1 (which is kind of ugly solution).</p>
  
  <p>Could you help me on this? If possible, I would like to avoid changing library...</p>
  
  <p>Thank you</p>