Retina canvas

var canvas = document.getElementById("myCanvas");
var context = canvas.getContext("2d");
// if backingStorePixelRatio is less than devicePixelRatio, everything needs to be scaled up
var ratio = (context.webkitBackingStorePixelRatio < window.devicePixelRatio) ? (window.devicePixelRatio || 1) : 1;
if (ratio > 1) {
    canvas.width *= ratio;
    canvas.height *= ratio;
    context.scale(ratio, ratio);
}