title: Drawing over green pixels only on a canvas image
body: <p>i'm capturing a video frame to a canvas using</p>
  <pre><code>var video = document.getElementById('myvideo');
  var canvas = document.getElementById('mycanvas');
  var outputcanvas = document.createElement(&quot;CANVAS&quot;);
  outputcanvas.width = 640;
  outputcanvas.height = 480;
  var ctx = outputcanvas.getContext(&quot;2d&quot;);
  ctx.drawImage(video,0,0,640,480);
  </code></pre>
  <p>I then have some black and white drawings on mycanvas that i want to draw over the video frame, however i want to avoid drawing over any blues.</p>
  <p>Does anyone know of a way i can manipulate the image on my canvas and place it over the video frame I have just added?</p>
  <p>I have alpha set, so only the drawings themselves are placed onto the outputcanvas using:</p>
  <pre><code>ctx.drawImage(canvas,0,0,640,480);
  </code></pre>
  <p>any ideas?</p>