Html5 canvas drawImage: how to apply antialiasing

Please have a look at the following example:

http://jsfiddle.net/MLGr4/47/

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

img = new Image();
img.onload = function(){
    canvas.width = 400;
    canvas.height = 150;
    ctx.drawImage(img, 0, 0, img.width, img.height, 0, 0, 400, 150);
}
img.src = "http://openwalls.com/image/1734/colored_lines_on_blue_background_1920x1200.jpg";

As you see, the image is not anti-aliased although it is said that drawImage applies anti aliasing automatically. I tried many different ways but it doesn't seem to work. Could you please tell me how I can get anti-aliased image? Thanks.