canvas裁剪之后的base64转换为上传文件blob对象

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'});
}