How do I generate a thumbnail client-side in a modern browser?

Question

I'm looking for an elegant way to generate a thumbnail for use with the FileAPI. Currently I get a DataURL representing an image. Problem is, if the image is very large, than moving it around and rerendering it becomes CPU intensive. I can see 2 options to get around this.

• Generate a thumbnail on the client
• Generate a thumbnail on the server, send the thumbnail back to the client (AJAX).

With HTML5 we have a canvas element? Does anyone know how to use it to generate thumbnails from pictures? They don't have to be perfect -- sampling quality is acceptable. Is there a jQuery plugin that will do this for me? Are there any other way to speed up the clientside use of large images?

I'm using HTML5, and Firefox 3.6+: there is no need to support anything other than Firefox 3.6+, please don't provide suggestions for IE 6.0

Answer 1

Here’s what you can do:

function getThumbnail(original, scale) {
  var canvas = document.createElement("canvas");

  canvas.width = original.width * scale;
  canvas.height = original.height * scale;

  canvas.getContext("2d").drawImage(original, 0, 0, canvas.width, canvas.height);

  return canvas
}

Now, to create thumbnails, you simply do the equivalent of this:

var image = document.getElementsByTagName("img")[0];
var thumbnail = getThubmnail(image, 1/5);

document.body.appendChild(thumbnail);

Note: Remember to make sure that the image is loaded (using onload) before trying to make a thumbnail of it.

Answer 2

Okay, the way I can see this working is drawing the image to the canvas at a smaller size, then exporting the canvas. Say you want a 64px thumbnail:

var thumbSize = 64;
var canvas = document.getElementById("canvas");
canvas.width = thumbSize;
canvas.height = thumbSize;
var c = canvas.getContext("2d");
var img = new Image();
img.onload = function(e) {
    c.drawImage(this, 0, 0, thumbSize, thumbSize);
    document.getElementById("thumb").src = canvas.toDataURL("image/png");
};
img.src = fileDataURL;

With this code, an image element with the id "thumb" is used as the thumbnail element. fileDataURL is the data URL that you got from the file API.

More information on drawing images to the canvas: http://diveintohtml5.info/canvas.html#images

And on exporting canvas data: http://msdn.microsoft.com/en-us/library/ie/ff975241(v=vs.85).aspx