文件上传之base64编码转换

我主要是参考这个帖子：
http://ask.dcloud.net.cn/question/2089

下面的path是“file:///mnt/sdcard/DCIM/Camera/20150208_154024.jpg”

    console.log("path=" + path);  
    var img = new Image();  
    img.src = path; // 传过来的图片路径在这里用。  
    img.onload = function() {  
        var that = this;  
        console.log("wh = " + that.width + "," + that.height);  
        //生成比例   
        var w = that.width,  
            h = that.height,  
            scale = w / h;  
        w = 480 || w; //480  你想压缩到多大，改这里  
        h = w / scale;  
        console.log("wh = " + w + "," + h);  
        //生成canvas  
        var canvas = document.createElement('canvas');  

        var ctx = canvas.getContext('2d');  

        canvas.width = w;  
        canvas.height = h;  

        ctx.drawImage(that, 0, 0, w, h);  

        var base64 = canvas.toDataURL('image/jpeg', 1 || 0.8); //1最清晰，越低越模糊。  
        alert(base64);

代码和参考的帖子是一样的。但是 base64的内容却只是 "data:,“

搜索了一下，说 Android 2.3 以下的 canvas 不支持 toDataURL，请问是这个原因吗？
还是其他哪个地方代码有问题？