通过canvas获取图片的像素数据，可以用浏览器做一个markman功能的插件

gistfile1.js

var canvas = document.createElement('canvas');
var ctx = canvas.getContext('2d');
var img = new Image();
img.src="";
ctx.drawImage(img, 0 ,0);
var imageData = ctx.getImageData(0,0,canvas.width,canvas.height);