Makes HTML5 canvas elements support @2x displays by doubling their size, then scaling them down to 50%. Requires jQuery.

retinacanvas.js

// Function
function retinaCanvas($canvas, context) {
	$canvas.each(function() {
		$(this).prop({
			width: $(this).width() * 2,
			height: $(this).height() * 2
		});
		context.scale(2,2);
	});
}

// Usage
var $canvas = $('canvas'); // Define canvas element
var context = $canvas[0].getContext('2d'); // Define context
retinaCanvas($canvas, context); // Make @2x compliant