[FFmpeg-trac] #3107(undetermined:new): Problem using muxing example

FFmpeg trac at avcodec.org
Fri Nov 15 20:47:07 CET 2013

#3107: Problem using muxing example
             Reporter:  ahsan        |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
              Version:  unspecified  |  undetermined
             Keywords:               |               Resolution:
             Blocking:               |               Blocked By:
Analyzed by developer:  0            |  Reproduced by developer:  0

Comment (by easonic):

 Replying to [comment:10 ahsan]:
 > I merged, demux and mux example codes so that i can take in any
 .mp4/.avi/mpg files and then re-encode them in the same format
 > I am having following issues
 > 1. MUX example code uses avformat_alloc_output_context2() to create the
 output context for example for .mpg it will choose MPEG1 encoder, the .mpg
 file i used is encoded using MPEG2 encoder.
 > I was wondering how can i select different encoders, is there any API
 call which i can use to re-set the encoder name in the context encoder
 > 2. I uses the fmt_ctx from decoder to set the encoder parameters, i try
 to memcpy the context (decoder to encoder for the codec) but it fails by
 setting the different parameters from fmt_ctx to the c
 > c->width = fmt_ctx->streams[video_stream_idx]->codec->width;
 > c->height = fmt_ctx->streams[video_stream_idx]->codec->height;
 > c->coded_width = fmt_ctx->streams[video_stream_idx]->codec>coded_width;
 >  ........................
 > c->gop_size = fmt_ctx->streams[video_stream_idx]->codec->gop_size;
 >               /* emit one intra frame every twelve frames at most */
 > c->bit_rate = (fmt_ctx->streams[video_stream_idx]->codec->bit_rate == 0)
 ? 400000 :
 > fmt_ctx->streams[video_stream_idx]->codec->bit_rate;
 >  /* i am seting it to 400kbps in case it is set to 0 as i am seeing it
 for some stream */
 > ........................
 > I am seeing different issue
 > 1. GOP Size fmt_ctx most of the time has gop_size = 12 which does not
 seems like the case for the original/input stream
 > 2. bit-rate i am getting in the output .mp4 is way higher then the one i
 see from the fmt_ctx, it seems like encoded bit-stream is in 50Mbits/sec
 where as input .mp4 was 300kbps or 400kbps bit-rate
 > I am wondering what is the best way to set the encoder context using the
 input bitstream
 > [.mp4, .avi, .mpg etc] files
 > Please let me know if you are able to figure it out

 Hi, ahsan

 Sorry for the delay, really busy for the project. Um, do you resolve that
 issue? I met maybe the same problem here, what I did was just:
 but found that the '''output avformatcontext''' complains about the
 Thus, I traced the problem, found that after
 '''avformat_write_header(outFmtCtx, NULL);''', some parameter in the
 '''output avformatcontext->streams[video]->codec''' will change; thus I
 set the parameter to zero before '''avformat_write_header(outFmtCtx,
 NULL);''' and that works.
 For my experiment, I found that to get the correct duration and time_base
 (you can treat it as after parsing the codeccontext parameter), I need to
 set videostream's codec to:
   * IMPORTANT!!! Set the stream->codec information:
   * Copy all the CodecFormat info except the time_base and codec_tag
   * video_stream->codec->time_base: set to inputFmtCtx->vStream->time_base
   *                                 video only!
   * video_stream->codec->codec_tag: calculate by av_write_header
   * video_stream->codec->flags    : set global if the output format need
   *                                 global
 Um, and for audio, just skip the time_base part, and it works for me, too.

 If you need my code (what I was doing was to remux files without
 reencoding.), I will email to you later~

Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/3107#comment:11>
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