ffmpeg mp4解码管道输出的问题

测试代码：

HANDLE hReadPipe, hWritePipe;
SECURITY_ATTRIBUTES sa;
int testFfmpegPipe()        // 测试ffmpeg管道io
{
    char cmdString[200]{ "ffmpeg -i D:\vc\images\small.mp4 -f image2pipe -pix_fmt bgr24 -vcodec rawvideo -" };

    // 创建管道
    sa.nLength = sizeof(SECURITY_ATTRIBUTES);
    sa.lpSecurityDescriptor = NULL;             //使用系统默认的安全描述符
    sa.bInheritHandle = TRUE;                   //一定要为TRUE，不然句柄不能被继承。
    CreatePipe(&hReadPipe, &hWritePipe, &sa, 0); //创建pipe内核对象,设置好hReadPipe,hWritePipe.

    // 创建dos子进程
    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    si.cb = sizeof(STARTUPINFO);
    GetStartupInfo(&si);
    si.hStdError = hWritePipe; //设定其标准错误输出为hWritePipe
    si.hStdOutput = hWritePipe; //设定其标准输出为hWritePipe
    si.wShowWindow = SW_HIDE;
    si.dwFlags = STARTF_USESHOWWINDOW | STARTF_USESTDHANDLES;
    if (!CreateProcess(NULL, cmdString
        , NULL, NULL, TRUE, NULL, NULL, NULL, &si, &pi)) {
        //MessageBox("Error on CreateProcess()");
        return 1;
    }
    CloseHandle(hWritePipe);

    Mat rgba = Mat::zeros(cv::Size(560,320), CV_8UC3);
    const uint32_t framebytes = 560 * 320 * 3 * 1.1;
    char buffer[framebytes] = { 0 };
    DWORD bytesRead;
    uint32_t a = 0;
    while (true) {
        if (ReadFile(hReadPipe, buffer, framebytes, &bytesRead, NULL) == NULL)//从hReadPipe中读出数据.
            break;
        if (bytesRead<5000) {
            cout << buffer << endl;
        }
        cout << bytesRead << endl;
    }

    return 0;
}

视频是 560*320，166帧

上面主进程读取的字节数如下：共，2834行，与理想的166帧166行相差甚远，每帧560*320*3=537600也和下面的不一样。

开头和结果这些小字节的都是解码的什么鬼哦

打印出来才发现，是ffmpeg的各种提示信息。。。。

ffmpeg version 4.3.1-full_build-www.gyan.dev
 Copyright (c) 2000-2020 the FFmpeg developers

  built with gcc 10.2.0 (Rev1, Built by MSYS2 project)

  configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-libxml2 --enable-gmp --enable-lzma --enable-libsnappy --enable-zlib --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-sdl2 --enable-libdav1d --enable-libzvbi --enable-librav1e --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-libaom --enable-libopenjpeg --enable-libvpx --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-ffnvcodec --enable-nvdec --enable-nvenc --enable-d3d11va --enable-dxva2 --enable-libmfx --enable-libcdio --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libwavpack --enable-libilbc --enable-libgsm --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint

  libavutil      56. 51.100 / 56. 51.100

  libavcodec     58. 91.100 / 58. 91.100

  libavformat    58. 45.100 / 58. 45.100

  libavdevice    58. 10.100 / 58. 10.100

  libavfilter     7. 85.100 /  7. 85.100

  libswscale      5.  7.100 /  5.  7.100

  libswresample   3.  7.100 /  3.  7.100

  libpostproc    55.  7.100 / 55.  7.100

把头尾的这些小字节去掉，总数刚好对的上，看来这个buffer是固定了32768=2^15不够一帧，要自己循环处理。

上面16个32768 + 13312 刚好等于一帧大小。

好了，需要ffmpeg屏蔽中间提示信息，加上这个就好了：

 -loglevel quiet

加上帧大小，循环读出，满足一帧容量后输出，最终代码如下：

HANDLE hReadPipe, hWritePipe;
SECURITY_ATTRIBUTES sa;
int testFfmpegPipe()        // 测试ffmpeg管道io
{
    char cmdString[200]{ "ffmpeg -loglevel quiet -i D:\vc\images\small.mp4 -f image2pipe -pix_fmt bgr24 -vcodec rawvideo -" };

    // 创建管道
    sa.nLength = sizeof(SECURITY_ATTRIBUTES);
    sa.lpSecurityDescriptor = NULL;             //使用系统默认的安全描述符
    sa.bInheritHandle = TRUE;                   //一定要为TRUE，不然句柄不能被继承。
    CreatePipe(&hReadPipe, &hWritePipe, &sa, 0); //创建pipe内核对象,设置好hReadPipe,hWritePipe.

    // 创建dos子进程
    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    si.cb = sizeof(STARTUPINFO);
    GetStartupInfo(&si);
    si.hStdError = hWritePipe; //设定其标准错误输出为hWritePipe
    si.hStdOutput = hWritePipe; //设定其标准输出为hWritePipe
    si.wShowWindow = SW_HIDE;
    si.dwFlags = STARTF_USESHOWWINDOW | STARTF_USESTDHANDLES;
    if (!CreateProcess(NULL, cmdString
        , NULL, NULL, TRUE, NULL, NULL, NULL, &si, &pi)) {
        //MessageBox("Error on CreateProcess()");
        return 1;
    }
    CloseHandle(hWritePipe);

    Mat rgba = Mat::zeros(cv::Size(560,320), CV_8UC3);
    const uint32_t framebytes = 560 * 320 * 3;
    char buffer[framebytes] = { 0 };
    uint32_t readCount = 0;
    DWORD bytesRead;
    
    while (true) {
        if (ReadFile(hReadPipe, buffer, framebytes, &bytesRead, NULL) == NULL) { //从hReadPipe中读出数据.
            break;
        }
        memcpy(rgba.data + readCount, buffer, bytesRead);
        readCount += bytesRead;
        if (readCount == framebytes) {  // 拿到完整一帧了
            cout << "frame finished." << endl;
            readCount = 0;
            waitKey(100);
            imshow("frame", rgba);
            //imwrite("D:\vc\images\xx.bmp", rgba);
        }
    }

    return 0;
}