Gamma Adjustment in an HTML5 Canvas

Because you can't pull an image off of someone's computer and upload it to your server with a proxy.

Without cross domain limitations on canvas, you could do that.

I wrote a bit about those rules a while back:

The difference is in being explicit with permissions.

That said, JSONP is one of the ugliest (but useful!) hacks ever invented.

Your proxy wouldn't send the user's cookies with its request.