混合时 C++ ffmpeg 库帧率不正确

Question

我正在尝试创建一个函数，它将合并一个音频文件和一个视频文件并将它们输出到 mp4。除了输出的帧率不正确外，我已经成功地做到了这一点。与原版有非常细微的差别。 30.13 而它应该正好是 30。当我将这些文件与 ffmpeg 程序组合时，结果正好是 30。

我相信它与接收乱序数据时的 dts/pts 更正有关，但 ffmpeg 程序也以类似的方式执行此操作。所以我不确定从这里去哪里。我查看了 ffmpeg 源代码并复制了他们的一些 dts 更正，但仍然没有成功。我在这里做错了什么？

bool mux_audio_video(const char* audio_filename, const char* video_filename, const char* output_filename){
    av_register_all();

    AVOutputFormat* out_format = NULL;
    AVFormatContext* audio_context = NULL, *video_context = NULL, *output_context = NULL;
    int video_index_in = -1, audio_index_in = -1;
    int video_index_out = -1, audio_index_out = -1;


    if(avformat_open_input(&audio_context, audio_filename, 0, 0) < 0)
            return false;
    if(avformat_find_stream_info(audio_context, 0) < 0){
            avformat_close_input(&audio_context);
            return false;
    }
    if(avformat_open_input(&video_context, video_filename, 0, 0) < 0){
            avformat_close_input(&audio_context);
            return false;
    }
    if(avformat_find_stream_info(video_context, 0) < 0){
            avformat_close_input(&audio_context);
            avformat_close_input(&video_context);
            return false;
    }

    if(avformat_alloc_output_context2(&output_context, av_guess_format("mp4", NULL, NULL), NULL, output_filename) < 0){
            avformat_close_input(&audio_context);
            avformat_close_input(&video_context);
            return false;
    }
    out_format = output_context->oformat;

    //find first audio stream in the audio file input
    for(size_t i = 0;i < audio_context->nb_streams;++i){
            if(audio_context->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_AUDIO){
                    audio_index_in = i;

                    AVStream* in_stream = audio_context->streams[i];
                    AVCodec* codec = avcodec_find_encoder(in_stream->codecpar->codec_id);
                    AVCodecContext* tmp = avcodec_alloc_context3(codec);
                    avcodec_parameters_to_context(tmp, in_stream->codecpar);
                    AVStream* out_stream = avformat_new_stream(output_context, codec);
                    audio_index_out = out_stream->index;
                    if(output_context->oformat->flags & AVFMT_GLOBALHEADER){
                            tmp->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
                    }
                    tmp->codec_tag = 0;
                    avcodec_parameters_from_context(out_stream->codecpar, tmp);
                    avcodec_free_context(&tmp);

                    break;
            }
    }
    //find first video stream in the video file input
    for(size_t i = 0;i < video_context->nb_streams;++i){
            if(video_context->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO){
                    video_index_in = i;

                    AVStream* in_stream = video_context->streams[i];
                    AVCodec* codec = avcodec_find_encoder(in_stream->codecpar->codec_id);
                    AVCodecContext* tmp = avcodec_alloc_context3(codec);
                    avcodec_parameters_to_context(tmp, in_stream->codecpar);
                    AVStream* out_stream = avformat_new_stream(output_context, codec);
                    video_index_out = out_stream->index;
                    if(output_context->oformat->flags & AVFMT_GLOBALHEADER){
                            tmp->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
                    }
                    tmp->codec_tag = 0;
                    avcodec_parameters_from_context(out_stream->codecpar, tmp);
                    avcodec_free_context(&tmp);

                    break;
            }
    }

    //setup output
    if(!(out_format->flags & AVFMT_NOFILE)){
            if(avio_open(&output_context->pb, output_filename, AVIO_FLAG_WRITE) < 0){
                    avformat_free_context(output_context);
                    avformat_close_input(&audio_context);
                    avformat_close_input(&video_context);
                    return false;
            }
    }
    if(avformat_write_header(output_context, NULL) < 0){
            if(!(out_format->flags & AVFMT_NOFILE)){
                    avio_close(output_context->pb);
            }
            avformat_free_context(output_context);
            avformat_close_input(&audio_context);
            avformat_close_input(&video_context);
            return false;
    }

    int64_t video_pts = 0, audio_pts = 0;
    int64_t last_video_dts = 0, last_audio_dts = 0;

    while(true){
            AVPacket packet;
            av_init_packet(&packet);
            packet.data = NULL;
            packet.size = 0;
            int64_t* last_dts;
            AVFormatContext* in_context;
            int stream_index = 0;
            AVStream* in_stream, *out_stream;

            //Read in a frame from the next stream
            if(av_compare_ts(video_pts, video_context->streams[video_index_in]->time_base,
                             audio_pts, audio_context->streams[audio_index_in]->time_base) <= 0)
            {
                    //video
                    last_dts = &last_video_dts;
                    in_context = video_context;
                    stream_index = video_index_out;

                    if(av_read_frame(in_context, &packet) >= 0){
                            do{
                                    if(packet.stream_index == video_index_in){
                                            video_pts = packet.pts;
                                            break;
                                    }
                                    av_packet_unref(&packet);
                            }while(av_read_frame(in_context, &packet) >= 0);
                    }else{
                            break;
                    }
            }else{
                    //audio
                    last_dts = &last_audio_dts;
                    in_context = audio_context;
                    stream_index = audio_index_out;

                    if(av_read_frame(in_context, &packet) >= 0){
                            do{
                                    if(packet.stream_index == audio_index_in){
                                            audio_pts = packet.pts;
                                            break;
                                    }
                                    av_packet_unref(&packet);
                            }while(av_read_frame(in_context, &packet) >= 0);
                    }else{
                            break;
                    }
            }
            in_stream = in_context->streams[packet.stream_index];
            out_stream = output_context->streams[stream_index];

            av_packet_rescale_ts(&packet, in_stream->time_base, out_stream->time_base);

            //if dts is out of order, ffmpeg throws an error. So manually fix. Similar to what ffmpeg does in ffmpeg.c
            if(packet.dts < (*last_dts + !(output_context->oformat->flags & AVFMT_TS_NONSTRICT)) && packet.dts != AV_NOPTS_VALUE && (*last_dts) != AV_NOPTS_VALUE){
                    int64_t next_dts = (*last_dts)+1;
                    if(packet.pts >= packet.dts && packet.pts != AV_NOPTS_VALUE){
                            packet.pts = FFMAX(packet.pts, next_dts);
                    }
                    if(packet.pts == AV_NOPTS_VALUE){
                            packet.pts = next_dts;
                    }
                    packet.dts = next_dts;
            }
            (*last_dts) = packet.dts;

            packet.pos = -1;
            packet.stream_index = stream_index;

            //output packet
            if(av_interleaved_write_frame(output_context, &packet) < 0){
                    break;
            }
            av_packet_unref(&packet);

    }

    av_write_trailer(output_context);

    //cleanup
    if(!(out_format->flags & AVFMT_NOFILE)){
            avio_close(output_context->pb);
    }
    avformat_free_context(output_context);
    avformat_close_input(&audio_context);
    avformat_close_input(&video_context);
    return true;
}

Answer 1

我发现了问题。我只需要将 last_video_dts 和 last_audio_dts 初始化为 int64_t 的最小值而不是 0.

int64_t last_video_dts, last_audio_dts;
last_video_dts = last_audio_dts = std::numeric_limits<int64_t>::lowest();

现在的输出与ffmpeg程序的输出基本一致

编辑：

正如kamilz所说，使用AV_NOPTS_VALUE.

更好更便携

int64_t last_video_dts, last_audio_dts;
last_video_dts = last_audio_dts = AV_NOPTS_VALUE;

混合时 C++ ffmpeg 库帧率不正确

C++ ffmpeg library framerate incorrect when muxing

c++

ffmpeg

libav