node.js - NodeJS base64 图像编码/解码不太工作

Question

我一直在尝试处理将发布到 nodeJS(和 express 框架)的图像保存到数据库，并且遇到了一些麻烦。忽略所有的 web 处理，我认为我已经将问题缩小到 base64 编码在 node.js 中发生的方式。我相信下面这个过于简单的例子应该可以工作，但输出图像总是被破坏。

示例 (1) 加载图像 (2) 保存 if (image_orig) 的副本，以确认 Node 可以正确读取文件。这总是有效的。 (3) 我获取图像并对其内容进行 base64 编码，(4) 然后对其进行解码。最终输出图像 (image_decoded) 总是损坏的。

帮助! (OSX Lion 上的 node.js 0.6.0)

console.log("starting");
process.chdir(__dirname);

var fs = require("fs");

var image_origial = "image.jpg";
fs.readFile(image_origial, function(err, original_data){
    fs.writeFile('image_orig.jpg', original_data, function(err) {});
    var base64Image = new Buffer(original_data, 'binary').toString('base64');
    var decodedImage = new Buffer(base64Image, 'base64').toString('binary');
    fs.writeFile('image_decoded.jpg', decodedImage, function(err) {});
});

Answer 1

我认为您对编码参数的用法有点误解。如果您要指定编码“二进制”，那么您需要始终如一地执行此操作。但实际上你根本不需要它。您似乎混淆了 Buffer 与二进制字符串的用法。

// This tells node to load the file into a Buffer 'original_data' because you
// have not specified an encoding for the returned values. If you provided an
// encoding, then original_data would be a string with that encoding.
fs.readFile(image_origial, function(err, original_data){

    // This tells node to take that buffer, and write it to the new filename.
    // Again no encoding is provided, so it will assume a Buffer or utf8 string.
    fs.writeFile('image_orig.jpg', original_data, function(err) {});

    // This tells node to create a new buffer from the old buffer, which means
    // it will iterate over original_data copying the bytes one at a time. But
    // they will be identical buffers. It will ignore the 'binary' argument
    // since the object you are passing isn't a string.
    // Then it encodes the content of that Buffer to base64, which is fine.
    var base64Image = new Buffer(original_data, 'binary').toString('base64');

    // Here you decode the base64 to a buffer, which is fine, but then you
    // convert the buffer into a string with encoding 'binary'. This means that
    // it is a string object whose code points are bytes of the buffer.
    var decodedImage = new Buffer(base64Image, 'base64').toString('binary');

    // Here you try to write that String object to a file. Since the argument you
    // have given is a string and you have not given an encoding argument for the
    // write command, then it will assume that 'utf8' is the encoding. It will try to
    // decode your binary string into a utf8 encoded buffer, and write that buffer.
    // This will cause it to fail because that encoding conversion is wrong.
    // Really through, 'binary' is just wrong to use. Buffers are already binary.
    fs.writeFile('image_decoded.jpg', decodedImage, function(err) {});
});

下一个示例将起作用，但效率非常低，因为不需要一直更改编码，但我只是想清楚地表明它。如果你真的想要有一个特定的编码，你需要确保你是一致的。这些函数中的每一个都有一个编码参数。

fs.readFile(image_origial, 'binary', function(err, original_data){
    fs.writeFile('image_orig.jpg', original_data, 'binary', function(err) {});
    var base64Image = new Buffer(original_data, 'binary').toString('base64');
    var decodedImage = new Buffer(base64Image, 'base64').toString('binary');
    fs.writeFile('image_decoded.jpg', decodedImage, 'binary', function(err) {});
});

这是正确的做法。将所有内容都保存为 Buffer，除非您将其设为 base64。

fs.readFile(image_origial, function(err, original_data){
    fs.writeFile('image_orig.jpg', original_data, function(err) {});
    var base64Image = original_data.toString('base64');
    var decodedImage = new Buffer(base64Image, 'base64');
    fs.writeFile('image_decoded.jpg', decodedImage, function(err) {});
});