Automatically Crop HTML5 canvas to contents

Question

Let's say this is my canvas, with an evil-looking face drawn on it. I want to use toDataURL() to export my evil face as a PNG; however, the whole canvas is rasterised, including the 'whitespace' between the evil face and canvas edges.

+---------------+
|               |
|               |
|     (.Y. )    |
|      /_       |
|     \____/    |
|               |
|               |
+---------------+

What is the best way to crop/trim/shrinkwrap my canvas to its contents, so my PNG is no larger than the face's 'bounding-box', like below? The best way seems to be scaling the canvas, but supposing the contents are dynamic...? I'm sure there should be a simple solution to this, but it's escaping me, with much Googling.

+------+
|(.Y. )|
| /_   |
|\____/|
+------+

Thanks!

Answer 1

Edited (see comments)

function cropImageFromCanvas(ctx) {
  var canvas = ctx.canvas, 
    w = canvas.width, h = canvas.height,
    pix = {x:[], y:[]},
    imageData = ctx.getImageData(0,0,canvas.width,canvas.height),
    x, y, index;

  for (y = 0; y < h; y++) {
    for (x = 0; x < w; x++) {
      index = (y * w + x) * 4;
      if (imageData.data[index+3] > 0) {
        pix.x.push(x);
        pix.y.push(y);
      } 
    }
  }
  pix.x.sort(function(a,b){return a-b});
  pix.y.sort(function(a,b){return a-b});
  var n = pix.x.length-1;

  w = 1 + pix.x[n] - pix.x[0];
  h = 1 + pix.y[n] - pix.y[0];
  var cut = ctx.getImageData(pix.x[0], pix.y[0], w, h);

  canvas.width = w;
  canvas.height = h;
  ctx.putImageData(cut, 0, 0);

  var image = canvas.toDataURL();  //open cropped image in a new window
  var win=window.open(image, '_blank');
  win.focus();
}