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• How do I add a delay to a live stream sourced from webcam (v4l2) with FFMPEG ?

  20 novembre 2018, par David

  How can I use FFMPEG to add a delay to a stream being sent from a (v4l2) webcam to a media server ?

  The use case here is something like a security camera where I want to be able to stream video to a server when something is detected in the video. The easiest way to ensure the event of interest is captured on the video is to use FFMPEG to stream from the camera to a virtual loopback device with an added delay. That loopback device can then be used to initiate live streaming when an even of interest occurs.

  In GStreamer, I would accomplish a delay of this sort with the queue element’s min-threshold-time parameter. For example the following (much-simplified) example pipeline adds a 2 second delay to the output coming from a v4l2 webcam before displaying it :

  gst-launch-1.0 v4l2src device=/dev/video1 ! queue max-size-buffers=0 max-size-time=0 max-size-bytes=0 min-threshold-time=2000000000 ! xvimagesink

  How do I accomplish the same thing with FFMPEG ? There are some technical challenges that prevent us from using GStreamer for this.

  I have investigated the itsoffset option for this, but as far as I can tell it is only usable for already-recorded files, and it is not clear what a good alternative would be.

• How to convert Live stream to HLS multi-bitrate ? [on hold]

  22 avril 2015, par Griffin

  I am currently working on Live HLS Streaming. I am trying to convert streams from USB tuner to HLS with adaptive bitrate. My activity is to segment a live stream from a TV tuner card into chunks of different bit-rates.

  Is there any open source tools available in Linux which does the same ? Help appreciated.

• Why android video player that based on ffmpeg can't sync video' time and audio's time ?

  6 juillet 2021, par ZSpirytus

  Background

  


  Recently, I use ffmpeg to write my first Android video player. But video channel's time is faster than audio channel's time about 2 3 times.

  


  Question

  


  Why android video player audio and video is out of sync ? Video is about faster than audio 2 3 times. Thanks for your reading and answers.

  


  Code

  


  In short, I use PacketDispatcher to read AVPacket from http hlv source :

  


  void PacketDispatcher::RealDispatch() {
    while (GetStatus() != DISPATCHER_STOP) {
        ...

        AVPacket *av_packet = av_packet_alloc();
        int ret = av_read_frame(av_format_context, av_packet);

        // PacketDispatcher is who read the AVPacket from http hlv source 
        // and dispatch to decoder by its stream index.
        decoder_map[av_packet->stream_index]->Push(av_packet);
    }
}


  


  And then, Decoder written by Producer-Consumer Pattern, Decoder maintain a queue that store all the AVPacket received from PacketDispatcher. The code like this :

  


  // write to the queue
void BaseDecoder::Push(AVPacket *av_packet) {
    ...
    av_packet_queue.push(av_packet);
    ...
}

// real decode logic
void BaseDecoder::RealDecode() {
    ...
    while (true) {
        // read frame from codec
        AVFrame *av_frame = av_frame_alloc();
        ret = avcodec_receive_frame(av_codec_ctx, av_frame);

        void *decode_result = DecodeFrame(av_frame);
        // send to render
        m_render->Render(decode_result);
    }
}


  


  And then, I do rendering logic in Render. Render also written by Producer-Consumer Pattern, it maintain a queue that store AVFrame received from Decoder, the code like this :

  


  // write AVFrame
void BaseRender::Render(void *frame_data) {
    ...
    frame_queue.push(frame_data);
    ...
}

// render to surface or Open SL
void BaseRender::RealRender() {
    if (m_render_synchronizer && m_render_synchronizer->Sync(frame_data)) {
        continue;
    }
}


  


  Finally, the synchronizer will decide to sleep time or drop video frame according to the frame pts, frame pts is :

  


  frame_data->pts = av_frame->best_effort_timestamp * av_q2d(GetTimeBase());


  


  Also, video extra delay is :

  


  frame_data->video_extra_delay = av_frame->repeat_pict * 1.0 / fps * 2.0;


  


  RenderSynchronizer code like this :

  


  bool RenderSynchronizer::Sync(void *frame_data) {&#xA;    auto base_frame_data = static_cast<baseframedata>(frame_data);&#xA;    if (base_frame_data->media_type == AVMEDIA_TYPE_AUDIO) {&#xA;        audio_pts = pcm_data->pts;&#xA;        return false;&#xA;    } else if (base_frame_data->media_type == AVMEDIA_TYPE_VIDEO) {&#xA;        video_pts = rgba_data->pts;&#xA;        return ReceiveVideoFrame(static_cast<rgbadata>(frame_data));&#xA;    }&#xA;    return false;&#xA;}&#xA;&#xA;bool RenderSynchronizer::ReceiveVideoFrame(RGBAData *rgba_data) {&#xA;    if (audio_pts &lt;= 0 || video_pts &lt;= 0) {&#xA;        return false;&#xA;    }&#xA;&#xA;    double diff = video_pts - audio_pts;&#xA;    if (diff > 0) {&#xA;        if (diff > 1) {&#xA;            av_usleep((unsigned int) (rgba_data->extra_delay * 1000000.0));&#xA;        } else {&#xA;            av_usleep((unsigned int) ((diff &#x2B; rgba_data->extra_delay) * 1000000.0));&#xA;        }&#xA;        return false;&#xA;    } else if (diff &lt; 0) {&#xA;        LOGD(TAG, "drop video frame");&#xA;        return true;&#xA;    } else {&#xA;        return false;&#xA;    }&#xA;}&#xA;</rgbadata></baseframedata>