FFmpeg5入门教程10.21：音视频解混合（demuxer）为MP3和H264

本文介绍将视频压缩数据(h264)和音频数据(mp3)从视频文件(mp4)中解出来。

在10.04：解码视频流过程中介绍了解码视频流的基本流程，但是只解码视频流。而在FFmpeg5入门教程10.14：音频解码为pcm中介绍了解码音频流的基本流程，并且是只解码音频流。而一个视频文件中包含音视频至少两条流，我们在解码的时候故意过滤了我们不需要的流。

随便找一个视频，查看视频信息：

$ ffprobe Sample.mkv
...
[flv @ 0x55894ec85440] Missing AMF_END_OF_OBJECT in AMF_DATA_TYPE_MIXEDARRAY, found 0
Input #0, flv, from 'Sample.mkv':
  Metadata:
    creator         : www.qiyi.com
    metadatacreator : Yet Another Metadata Injector for FLV - Version 1.2
    hasKeyframes    : true
    hasVideo        : true
    hasAudio        : true
    hasMetadata     : true
    canSeekToEnd    : false
    datasize        : 40600924
    videosize       : 36538352
    audiosize       : 3942540
    lasttimestamp   : 645
    lastkeyframetimestamp: 637
    lastkeyframelocation: 40109260
  Duration: 00:10:44.87, start: 0.000000, bitrate: 503 kb/s
  Stream #0:0: Video: h264 (High), yuv420p(tv, smpte170m/smpte170m/bt709, progressive), 624x480, 451 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc
  Stream #0:1: Audio: aac (HE-AAC), 44100 Hz, stereo, fltp, 47 kb/s

可以看到有两条流，分别是Stream #0:0 为h264即视频流，Stream #0:1为aac即音频流。接下来我们将其解出来并分别保存为音视频文件。

流程为：

实际的操作为读取两条流，然后创建两条流，分别转换时间戳保存数据。

首先是打开输入

//打开输入流
ret = avformat_open_input(&ifmtCtx, inFilename, 0, 0);
if (ret < 0)
{
  printf("can't open input file\n");
  break;
}

//获取流信息
ret = avformat_find_stream_info(ifmtCtx, 0);
if (ret < 0)
{
  printf("can't retrieve input stream information\n");
  break;
}

创建输出上下文

avformat_alloc_output_context2(&ofmtCtxVideo, NULL, NULL, outFilenameVideo);
if (!ofmtCtxVideo)
{
  printf("can't create video output context");
  break;
}

打开输出上下文

int open_codec_context(int *streamIndex, AVFormatContext *&ofmtCtx, AVFormatContext *ifmtCtx, AVMediaType type)
{
    AVStream *outStream = NULL, *inStream = NULL;
    int ret = -1, index = -1;

    index = av_find_best_stream(ifmtCtx, type, -1, -1, NULL, 0);
    if (index < 0)
    {
        printf("can't find %s stream in input file\n", av_get_media_type_string(type));
        return ret;
    }

    inStream = ifmtCtx->streams[index];

    outStream = avformat_new_stream(ofmtCtx, NULL);
    if (!outStream)
    {
        printf("failed to allocate output stream\n");
        return ret;
    }

    ret = avcodec_parameters_copy(outStream->codecpar, inStream->codecpar);
    if (ret < 0)
    {
        printf("failed to copy codec parametes\n");
        return ret;
    }

    outStream->codecpar->codec_tag = 0;

    //    if (ofmtCtx->oformat->flags & AVFMT_GLOBALHEADER)
    //    {
    //        outStream->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
    //    }

    *streamIndex = index;

    return 0;
}

打开输出文件

//打开输出文件:视频
if (!(ofmtCtxVideo->oformat->flags & AVFMT_NOFILE))
{
  if (avio_open(&ofmtCtxVideo->pb, outFilenameVideo, AVIO_FLAG_WRITE) < 0)
  {
    printf("can't open output file: %s\n", outFilenameVideo);
    break;
  }
}

写文件头

//写文件头
if (avformat_write_header(ofmtCtxVideo, NULL) < 0)
{
  printf("Error occurred when opening video output file\n");
  break;
}

然后就是最关键的时间戳转换

while (1)
{
  AVFormatContext *ofmtCtx;
  AVStream *inStream, *outStream;

  if (av_read_frame(ifmtCtx, &packet) < 0)
  {
    break;
  }

  inStream = ifmtCtx->streams[packet.stream_index];

  if (packet.stream_index == videoIndex)
  {
    outStream = ofmtCtxVideo->streams[0];
    ofmtCtx = ofmtCtxVideo;
  }
  else if (packet.stream_index == audioIndex)
  {
    outStream = ofmtCtxAudio->streams[0];
    ofmtCtx = ofmtCtxAudio;
  }
  else
  {
    continue;
  }

  //convert PTS/DTS
  packet.pts = av_rescale_q_rnd(packet.pts, inStream->time_base, outStream->time_base,(AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));
  packet.dts = av_rescale_q_rnd(packet.dts, inStream->time_base, outStream->time_base,(AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX));
  packet.duration = av_rescale_q(packet.duration, inStream->time_base, outStream->time_base);
  packet.pos = -1;
  packet.stream_index = 0;

  //write
  if (av_interleaved_write_frame(ofmtCtx, &packet) < 0)
  {
    printf("Error muxing packet\n");
    break;
  }

  av_packet_unref(&packet);
}