function convertBase64UrlToBlob(urlData){        var bytes=window.atob(urlData.split(',')[1]);        //去掉url的头，并转换为byte        //处理异常,将ascii码小于0的转换为大于0    var ab = new ArrayBuffer(bytes.length);    var ia = new Uint8Array(ab);    for (var i = 0; i < bytes.length; i++) {        ia[i] = bytes.charCodeAt(i);    }    return new Blob( [ab] , {type : 'image/png'});}