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• How can I make ffmpeg output proper SDP data for a data stream

  8 juillet 2021, par Shalom Crown

  I have a program based on the FFMPEG libraries, to add KLV data to a video stream. When I try to sent the output to an RTSP server (rtsp-simple-server), I get a 400 response.

  


  The apparent reason is that the SDP data for the KLV stream is missing the rtpmap.

  


  I would like to add the missing data either by setting the proper parameters in the contexts, or by specifying the data explicitly.

  


  This is the ANNOUNCE captured with Wireshark

  


  ANNOUNCE rtsp://0.0.0.0:8554/test RTSP/1.0
Content-Type: application/sdp
CSeq: 2
User-Agent: Lavf58.29.100
Content-Length: 270

v=0
o=- 0 0 IN IP4 127.0.0.1
s=No Name
c=IN IP4 127.0.0.1
t=0 0
a=tool:libavformat 58.29.100
m=video 0 RTP/AVP 96
b=AS:10000
a=rtpmap:96 H264/90000
a=fmtp:96 packetization-mode=1
a=control:streamid=0
m=application 0 RTP/AVP 97
b=AS:90
a=control:streamid=1


  


  Code fragment for initializing the data stream :

  


  AVStream* data_track = avformat_new_stream(muxer, nullptr);

if (data_track == nullptr) {
    LOG_ERROR << "failed to open data output stream";
    return nullptr;
}

muxer->oformat->video_codec = AV_CODEC_ID_H264;

AVCodec *klvEncoder = avcodec_find_encoder(AV_CODEC_ID_SMPTE_KLV);

if (klvEncoder == nullptr) {
    klvEncoder = avcodec_find_encoder(AV_CODEC_ID_BIN_DATA);
}

if (klvEncoder == nullptr) {
    klvEncoder = avcodec_find_encoder(AV_CODEC_ID_TEXT);
}

if (klvEncoder != nullptr) {
    klvEncoderContext = avcodec_alloc_context3(encoder);

    avcodec_parameters_from_context(data_track->codecpar, encoderContext);
} else {
    avcodec_parameters_copy(data_track->codecpar, pVideoStream->codecpar);
}

data_track->codecpar->codec_tag =  KLV_ID_TAG;
data_track->codecpar->codec_type = AVMEDIA_TYPE_DATA;
data_track->codecpar->codec_id = AV_CODEC_ID_SMPTE_KLV;
data_track->codecpar->bit_rate = 90000;
data_track->codecpar->format =  AV_SAMPLE_FMT_U8;
data_track->stream_identifier = KLV_ID_TAG;
data_track->id = 0x101;
data_track->time_base = video_track->time_base;
data_track->avg_frame_rate = video_track->time_base;


  


• C++/CLI FFmpeg av_read_frame returns a size but no data ?

  26 novembre 2013, par tommed

  I have written some C code to access ffmpeg and wrapped it in a C++/CLI (.NET managed) class. The program fetches a live video stream and extracts frames and converts them to PNG files.

  Unfortunately the images that are saved to disk are always black (opening them in Notepad++ shows that they are full of nulls).

  I am using the assemblies aformat/codec-55.dll and the development headers and libs for compilation from ffmpeg-20131120-git-e502783-win64-dev. The whole project is compiled using Managed C++ (Cpp/cli) .NET 4.0 for 64-bit.

  After some investigation the problem appears to be that av_read_frame fills the AVPacket->size value correctly, but the AVPAcket->data is always null. When the frame is finished (got==1) then the data for the AVFrame is just a matrix of nulls. :(

  Here is the code :
  Example code (sorry, but it didnt paste well into SO)

  I think the problem is at line 34 when the packet is returned like so :
  

  Please, how can I get this to work ? What have I done wrong ?

• FFmpeg av_read_frame returns a size but no data ?

  11 décembre 2013, par tommed

  I have written some C code to access ffmpeg and wrapped it in a C++/CLI (.NET managed) class. The program fetches a live video stream and extracts frames and converts them to PNG files.

  Unfortunately the images that are saved to disk are always black (opening them in Notepad++ shows that they are full of nulls).

  I am using the assemblies aformat/codec-55.dll and the development headers and libs for compilation from ffmpeg-20131120-git-e502783-win64-dev. The whole project is compiled using Managed C++ (Cpp/cli) .NET 4.0 for 64-bit.

  After some investigation the problem appears to be that av_read_frame fills the AVPacket->size value correctly, but the AVPAcket->data is always null. When the frame is finished (got==1) then the data for the AVFrame is just a matrix of nulls. :(

  Here is the code :
  Example code (sorry, but it didnt paste well into SO)

  I think the problem is at line 34 when the packet is returned like so :
  

  Please, how can I get this to work ? What have I done wrong ?