/* 
 * 图片压缩
 * img    原始图片
 * width   压缩后的宽度
 * height  压缩后的高度
 * ratio   压缩比率 
 */
 function compress(img, width, height, ratio) {        
   var canvas, ctx, img64;
        
   canvas = document.createElement('canvas');        
   canvas.width = width;
   canvas.height = height;
        
   ctx = canvas.getContext("2d");        
   ctx.drawImage(img, 0, 0, width, height);
        
   img64 = canvas.toDataURL("image/jpeg", ratio);
        
   return img64;
 }

var image = new Image();
image.src = "/img/test.jpg";
      
image.onload = function(){
  var img64 = compress(image, 500, 400, 0.7);
  document.getElementById("test").src = img64;
}