用canvas怎么压缩图片？

<input type="file" id="upload" /><p>压缩前：</p><img id="oldImg" src=""/><p>压缩后：</p><img id="newImg" src=""/>

var oldImg = document.getElementById("oldImg");var newImg = document.getElementById("newImg");//创建一个隐藏的canvasvar canvas = document.createElement("canvas");
canvas.id = "myCanvas";
canvas.style.display = "none";
document.body.appendChild(canvas);            
var cxt = canvas.getContext('2d');var upload = document.getElementById("upload");
upload.onchange = function(){//获取input file里面的图片路径，该路径为blob格式var url = getObjectURL(this.files[0]);
    oldImg.src = url;
    canvasImg.src = oldImg.src;
    canvasImg.onload = function(){//这是一个过渡的img,当这个img加载完成时绘制到canvas上面var m = oldImg.height/oldImg.width;
        canvas.width = 375;
        canvas.height = canvas.width*m;
        cxt.drawImage(canvasImg,0,0,canvas.width,canvas.height);//canvas绘制完成则转换为base64并传到新的图片上，再删除canvasvar Pic = document.getElementById("myCanvas").toDataURL("image/png");
        newImg.src = Pic;
        document.body.removeChild(canvas);}
}//建立一個可存取到該file的urlfunction getObjectURL(file) {var url = null;if(window.createObjectURL != undefined) { // basicurl = window.createObjectURL(file);
    } else if(window.URL != undefined) { // mozilla(firefox)url = window.URL.createObjectURL(file);
    } else if(window.webkitURL != undefined) { // webkit or chromeurl = window.webkitURL.createObjectURL(file);
    }return url;
}