title: HTML Canvas Tracing
body: <p>I'm trying to build something in HTML5/Canvas to allow tracing over an image and alert if deviating from a predefined path.</p>
  
  <p>I've figured out how to load an external image into the canvas, and allow mousedown/mousemovement events over it to draw over the image, but what I'm having trouble getting my head around is comparing the two.</p>
  
  <p>Images are all simple black on white outlines, so from what I can tell a getPixel style event can tell if there is black underneath where has been drawn upon or underneath where the mouse is on.</p>
  
  <p>I could do it with just the mouse position, but that would require defining the paths of every image outline (and there are a fair number, hence ideally wanting to do it by analyzing the underlying image)..</p>
  
  <p>I've been told that its possible with Flash, but would like to avoid that if possible so that compatability with non-flash platforms (namely the ipad) can be maintained as they are the primary target for the page to run.</p>
  
  <p>Any insight or assistance would be appreciated!</p>