android - C++，Android NDK：如何将我的原始音频数据正确保存到文件并再次加载

Question

我正在开发一个播放音频的 Android 应用程序。为了最大限度地减少延迟，我通过 JNI 使用 C++ 来使用 C++ 库双簧管播放应用程序。

目前，在播放之前，应用程序必须解码给定的文件（例如 mp3），然后播放解码后的原始音频流。如果文件较大，这会导致播放开始前的等待时间。所以我想事先进行解码，保存它，当请求播放时，只需播放保存文件中的解码数据。我几乎不知道如何在 C++ 中进行正确的文件 i/o，并且很难理解它。有可能我的问题可以通过正确的库来解决，我不确定。

所以目前我正在像这样保存我的文件：

bool Converter::doConversion(const std::string& fullPath, const std::string& name) {

    // here I'm setting up the extractor and necessary inputs. Omitted since not relevant

    // this is where the decoder is called to decode a file to raw audio
    constexpr int kMaxCompressionRatio{12};
    const long maximumDataSizeInBytes = kMaxCompressionRatio * (size) * sizeof(int16_t);
    auto decodedData = new uint8_t[maximumDataSizeInBytes];

    int64_t bytesDecoded = NDKExtractor::decode(*extractor, decodedData);
    auto numSamples = bytesDecoded / sizeof(int16_t);
    auto outputBuffer = std::make_unique<float[]>(numSamples);

    // This block is necessary to get the correct format for oboe.
    // The NDK decoder can only decode to int16, we need to convert to floats
    oboe::convertPcm16ToFloat(
            reinterpret_cast<int16_t *>(decodedData),
            outputBuffer.get(),
            bytesDecoded / sizeof(int16_t));

    // This is how I currently save my outputBuffer to a file. This produces a file on the disc.
    std::string outputSuffix = ".pcm";
    std::string outputName = std::string(mFolder) + name + outputSuffix;
    std::ofstream outfile(outputName.c_str(), std::ios::out | std::ios::binary);
    outfile.write(reinterpret_cast<const char *>(&outputBuffer), sizeof outputBuffer);

    return true;
}

所以我相信我拿了我的浮点数组，将它转换为一个字符数组并保存它。我不确定这是否正确，但这是我对它的最佳理解。反正后面有个文件。 编辑：正如我在分析我保存的文件时发现的那样，我只存储了 8 个字节。

现在我如何再次加载这个文件并恢复我的 outputBuffer 的内容？

目前我有这一点，这显然是不完整的：

StorageDataSource *StorageDataSource::openPCM(const char *fileName, AudioProperties targetProperties) {

    long bufferSize;
    char * buffer;

    std::ifstream stream(fileName, std::ios::in | std::ios::binary);

    stream.seekg (0, std::ios::beg);
    bufferSize = stream.tellg();
    buffer = new char [bufferSize];
    stream.read(buffer, bufferSize);
    stream.close();

如果这是正确的，我该怎么做才能将数据恢复为原始类型？如果我做错了，它如何以正确的方式工作？

标签： androidc++ioandroid-ndkoboe

Answer 1

感谢@Michael的评论，我想出了如何做到这一点。

这就是我现在保存数据的方式：

bool Converter::doConversion(const std::string& fullPath, const std::string& name) {

    // here I'm setting up the extractor and necessary inputs. Omitted since not relevant

    // this is where the decoder is called to decode a file to raw audio
    constexpr int kMaxCompressionRatio{12};
    const long maximumDataSizeInBytes = kMaxCompressionRatio * (size) * sizeof(int16_t);
    auto decodedData = new uint8_t[maximumDataSizeInBytes];

    int64_t bytesDecoded = NDKExtractor::decode(*extractor, decodedData);
    auto numSamples = bytesDecoded / sizeof(int16_t);
 
    // converting to float has moved to the reading function, so now i save decodedData directly.

    std::string outputSuffix = ".pcm";
    std::string outputName = std::string(mFolder) + name + outputSuffix;
    std::ofstream outfile(outputName.c_str(), std::ios::out | std::ios::binary);
    
    outfile.write((char*)decodedData, numSamples * sizeof (int16_t));
    return true;
}

这就是我再次读取存储文件的方式：

    long bufferSize;
    char * inputBuffer;

    std::ifstream stream;
    stream.open(fileName, std::ifstream::in | std::ifstream::binary);

    if (!stream.is_open()) {
        // handle error
    }

    stream.seekg (0, std::ios::end); // seek to the end
    bufferSize = stream.tellg(); // get size info, will be 0 without seeking to the end
    stream.seekg (0, std::ios::beg); // seek to beginning

    inputBuffer = new char [bufferSize];

    stream.read(inputBuffer, bufferSize); // the actual reading into the buffer. would be null without seeking back to the beginning
    stream.close();

    // done reading the file.
    
    auto numSamples = bufferSize / sizeof(int16_t); // calculate my number of samples, so the audio is correctly interpreted
    
    auto outputBuffer = std::make_unique<float[]>(numSamples);

    // the decoding bit now happens after the file is open. This avoids confusion
    // The NDK decoder can only decode to int16, we need to convert to floats
    oboe::convertPcm16ToFloat(
            reinterpret_cast<int16_t *>(inputBuffer),
            outputBuffer.get(),
            bufferSize / sizeof(int16_t));


    // here I continue working with my outputBuffer

我没有或没有得到的重要信息/理解 C++ 是

a）指针的大小与它指向的数据的大小不同，b）寻找流的工作原理。在我的缓冲区中找到任何数据之前，我需要把针放回起点。