title: How to draw existing <img> to canvas
body: <p>I know I can do this using javascript:</p>
  
  <pre><code>var ctx = getCanvas('testCanvas').getContext('2d'); // get Canvas context
  var img = new Image();
  img.src = 'test.png';
  img.onload = function(){
      ctx.drawImage(img, 200, 200); // x, y, width, height
  }
  </code></pre>
  
  <p>But how to draw existing  tag to canvas:</p>
  
  <p>In html:</p>
  
  <pre><code>&lt;img src='test.png'&gt;
  </code></pre>
  
  <p>Why I want to do this? I want to optimize image loading using pagespeed</p>