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• Writing multithreaded video and audio packets with FFmpeg

  27 février 2017, par Robert Jones

  I couldn’t find any information on the way av_interleaved_write_frame deals with video and audio packets.

  I have multiple audio and video packets coming from 2 threads. Each thread calls a write_video_frame or write_audio_frame, locks a mutex, initialize an AVPacket and writes data to an .avi file.

  Initialization of AVCodecContext and AVFOrmatContext is ok.

  — Edit 1 —

  Audio and video are coming from an external source (microphone and camera) and are captured as raw data without any compression (even for video).
  I use h264 to encode video and no compression for Audio (PCM).

  Audio captured is : 16bits, 44100khz, stereo

  Video captured is 25FPS

  Question :

  1) Is it a problem if I write multiple video packets at once (let’s say 25 packets/sec) and just one audio packet/sec.

  Answer : Apparently not, the function av_interleaved_write_frame should be able to manage that kind of data as soon as pts and dts is well managed

  This means I call av_interleaved_write_frame 25 times for video writing and just 1 for audio writing per second. Could this be a problem ? If it is how can I deal with this scenario ?

  2) How can I manage pts and dts in this case ? It seems to be a problem in my application since I cannot correctly render the .avi file. Can I use real time stamps for both video and audio ?

  Answer : The best thing to do here is to use the timestamp given when capturing audio / video as pts and dts for this kind of application. So these are not exactly real time stamps (from wall clock) but media capture timestamps.

  Thank you for your precious advices.

• Is there a way to detect if a video is of poor quality with ffmpeg ? [on hold]

  29 avril 2015, par Daniela Carrasco

  I am working on a project where I am tracking the amount of time someone spends looking at a video.

  There are some videos in the mix that lag horribly and so it makes it seem as if the person is looking for longer than they actually are because the image is frozen with their eyes still looking at it, even though in real time, they aren’t looking.

  I thought that using frames per second would help me detect this but it seems to be inconsistent. I am using VCode and FFmpeg as well as FFprobe to try and figure this out.

• ffmpeg copy stream preserving FPS

  10 mars 2017, par James Taylor

  I have a stream that I know is outputting at a certain frame rate (30 FPS). I want to use ffmpeg to make a copy of this stream and save it to disk.

  I have the following command :

  ffmpeg -i http://input/ -c copy -map 0 \
  
      -f segment -strftime 1 -segment_time 900 \
  
      -segment_atclocktime 1 -segment_format mp4 %Y-%m-%d_%H-%M-%S.mp4

  But when I run the command, I see the following :

  frame=   32 fps=3.9 q=-1.0 Lsize=N/A time=00:00:01.27 bitrate=N/A

  Where it appears the FPS is hovers around 4.0 FPS and time moves slower than real time.

  I tried added -re (copy the rate of the input stream) and -r 30 (manually set the rate to 30 FPS) flag specified before the input file, but it didn’t seem to work.

  I also read a similar question here using -framerate 30, but that option doesn’t exist in the man pages and is an Invalid option.

  Any help would be greatly appreciated !

  ---

  So I let the modified command (removing the flags -c copy -map 0) run for exactly 5 minutes. Running ffprobe yields :

  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '2017-03-10_01-09-12.mp4':
  
    Metadata:
  
      major_brand     : isom
  
      minor_version   : 512
  
      compatible_brands: isomiso2avc1mp41
  
      encoder         : Lavf57.2.100
  
    Duration: 00:00:15.43, start: 0.066016, bitrate: 13416 kb/s
  
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc), 1024x768, 13414 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
  
      Metadata:
  
        handler_name    : VideoHandler

  Again, this only produces 15 seconds of video and I can’t seem to get a 1:1 relationship between the input stream of 30 FPS and an output stream also in 30 FPS in real time. Playing the video yields something that’s sped up.