如何在HTML5 canvas drawImage()中应用抗锯齿？
对于抗锯齿，您需要设置重采样质量。
ctx.imageSmoothingQuality = "low|medium|high"
使用屏幕外的画布将图像缩小一半-
var c = document.createElement('canvas'),
ocx = c.getContext('2d');
c.width = img.width * 0.5;
c.height = img.height * 0.5;
ocxx.drawImage(img, 0, 0, c.width, c.height);//将图像再次缩小到一半并重复
ocx.drawImage(c, 0, 0, c.width * 0.5, cc.height * 0.5);