如何在HTML5 canvas的drawImage()函数中应用抗锯齿？

ctx.imageSmoothingQuality = "low|medium|high"

var c = document.createElement('canvas'),
ocx = c.getContext('二d');
c.width = img.width * 0.5;
c.height = img.height * 0.5;

ocxx.drawImage(img, 0, 0, c.width, c.height);

ocx.drawImage(c, 0, 0, c.width * 0.5, cc.height * 0.5);