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• Low-latency RTSP stream using OpenCV and ffmpeg seems to ignore flags

  15 juillet 2024, par Dominic

  I'm writing a Qt6 application in C++ which should display and optionally store mutltiple RTSP streams. Currently, I'm using OpenCV to capture the stream :

  


  m_capture = new cv::VideoCapture("rtsp://user:password@192.168.1.108:504/stream", cv::CAP_FFMPEG);
if (!m_capture->isOpened()) {
    //Error
}

cv::Mat frame;
m_abort = false;
while(!m_abort) 
{
    if (!m_capture->read(frame)) 
    {
        //Error
        return;
    }

    doStuff(frame);
}



  


  This works, but has a delay of about 3 seconds for one of the streams.

  


  Using this ffplay command in the terminal works without noticable delay, though :

  


  ffplay -fflags nobuffer -flags low_delay -rtsp_transport tcp "rtsp://user:password@192.168.1.108:504/stream"


  


  I've tried to pass forward these flags without luck :

  


  setenv("OPENCV_FFMPEG_CAPTURE_OPTIONS","rtsp_transport;tcp|fflags;nobuffer|flags;low_delay",1);


  


  Other than that, I have tried (as suggetested e.g. here)

  


  

  • using QMediaPlayer

  


  • setting the cv::VideoCapture cv::CAP_PROP_BUFFERSIZE to 3

  


  • using grab() and retrieve() instead of read()

  


  


  TL ;DR : How can I disable buffering in the OpenCV FFMPEG backend for RTSP streams to reduce latency in my Qt application ?

  


  I'm also open to entirely different approaches (e.g. using QMultiMedia), but haven't found a way to reduce latency with those either.

  


• avformat/iamf_parse : ignore Audio Elements with an unsupported type

  14 août 2024, par James Almer

  avformat/iamf_parse : ignore Audio Elements with an unsupported type
  Better fix for the NULL pointer dereference from d7f83fc2f423.
  Signed-off-by : James Almer <jamrial@gmail.com>
  

  • [DH] libavformat/iamf_parse.c
  • [DH] libavformat/iamfdec.c

• FFmpeg : When I ignore audio 1 and transcode audio 2 to aac I get vorbis. Is this a bug in FFmpeg ? [closed]

  26 septembre 2024, par CA Opa

  I have videos with 2 audio tracks. I don't want to copy or transcode audio track 1. I only want to copy video, transcode audio track 2 and copy subtitles using FFmpeg. When I -map track 2 (-map 0:a:1) and transcode to aac the result I get is not aac but vorbis. What is wrong with my program code ?

  


  Here is my code :

  


  "ffmpeg.exe" -i "U:\infile.mkv" -map 0:v:0 -c:v:0 copy -map 0:a:1 -c:a:1 aac -ac:a:1 2 -b:a:1 192000 -map 0:s:0 -c:s copy "U:\Outfile.mkv"


  


  This is what happens :

  


    Stream #0:0(eng): Video: h264 (High), yuv420p(tv, bt709/unknown/unknown, progressive), 1278x720 [SAR 1:1 DAR 71:40], 23.98 fps, 23.98 tbr, 1k tbn (default)
  Stream #0:1(rus): Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s (default)
      Metadata:
        title           : Многоголосый закадровый, студия "Селена Интернешнл"
  Stream #0:2(eng): Audio: flac, 48000 Hz, stereo, s16
  Stream #0:3(eng): Subtitle: subrip (srt)
[out#0/matroska @ 000001b979895440] Codec AVOption b:a:1 (set bitrate (in bits/s)) has not been used for any stream. The most likely reason is either wrong type (e.g. a video option with no video streams) or that it is a private option of some decoder which was not actually used for any stream.
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:2 -> #0:1 (flac (native) -> vorbis (libvorbis))
  Stream #0:3 -> #0:2 (copy)
Press [q] to stop, [?] for help
Output #0, matroska, to 'U:\Outfile.mkv':
  Metadata:
    encoder         : Lavf61.5.101
  Chapters:
    Chapter #0:0: start 0.000000, end 180.000000
      Metadata:
        title           : On a Cold Night...
  Stream #0:0(eng): Video: h264 (High) (H264 / 0x34363248), yuv420p(tv, bt709/unknown/unknown, progressive), 1278x720 [SAR 1:1 DAR 71:40], q=2-31, 23.98 fps, 23.98 tbr, 1k tbn (default)
  Stream #0:1(eng): Audio: vorbis (oV[0][0] / 0x566F), 48000 Hz, stereo, fltp
      Metadata:
        encoder         : Lavc61.11.100 libvorbis
  Stream #0:2(eng): Subtitle: subrip
[out#0/matroska @ 000001b979895440] video:174005KiB audio:1635KiB subtitle:1KiB other streams:0KiB global headers:4KiB muxing overhead: 0.059971%


  


  If I transcode both audio tracks it succeeds.

  


  "ffmpeg.exe" -i "infile.mkv" -map 0:v:0 -c:v:0 copy -map 0:a:0 -c:a:0 aac -ac:a:0 2 -b:a:0 192000 -map 0:a:1 -c:a:1 aac -ac:a:1 2 -b:a:1 192000 -map 0:s:0 -c:s copy "Outfile.mkv"


  


  If I just transcoding track 1 it works. Why does it fail when I only map audio track 2 ?