canvas转换图像格式及尺寸

///用于上传的图片 
<input type="file" name="imageFile" id="imageFile" />
//压缩按钮
     <input type="button"  onclick="change()" value="压缩图"/>

javaScripts处理:

window.change = function () {
              var oFile = document.getElementById('imageFile').files[0];    //读取文件
              var reader = new FileReader(),
                  img = new Image();
              ///加载地址
              reader.readAsDataURL(oFile);
              reader.onload = function () {        //文件加载完成
                  var url = reader.result;
                  img.src = url;
              };
              img.onload = function () {
                  init();
              }
              function init() {
                  var w = img.width, h = img.height;
                  var canvas = document.createElement("canvas");
                  var ctx = canvas.getContext('2d');
                  //压缩比例为3
                  canvas.width = w / 3;
                  canvas.height = h / 3;
                  //将图片转化为canvas;并显示出来
                  ctx.drawImage(img, 0, 0, w, h, 0, 0, canvas.width, canvas.height);

                  var quality = 0.3;        //图片的质量，这里设置的是0.3
                  var data = canvas.toDataURL("image/png"); //获取画布图片，并且要png格式
               //下载图片的两个方法
               //window.location.href = data;   
                  //saveFile(data, new Date().getTime() + ".png");
                  document.body.appendChild(canvas);
                  
              }

             //下载图片
              function saveFile(data, filename) {
                  var save_link = document.createElementNS('http://www.w3.org/1999/xhtml', 'a');
                  save_link.href = data;
                  save_link.download = filename;

                  var event = document.createEvent('MouseEvents');
                  event.initMouseEvent('click', true, false, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
                  save_link.dispatchEvent(event);
              };
          }