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• FFMPEG on Heroku worker dynos takes too long - runs into timeout error

  26 janvier 2021, par John

  I am encoding a large video file that on my 5-year-old laptop runs at 7X and finishes in about 5 minutes. But with 2X worker dynos on heroku it cant even finish before the 4 hour background job time limit. I didn't have any issues until 2 days ago and now its causing lots of probelms. If I run heroku run rails console and run the job manually, I see it start at 7X but then quickly go down to 2X and then 0.5X speed. Anyone know what is causing this issue ?

  


  I'm encoding a VP8 webm file to MP4. If anyone knows a faster way to do this that might also fix this issue that would be great too !

  


  ffmpeg -y -i #{temp_webm_file.path} -movflags faststart -preset ultrafast #{temp_mp4_file.path}

  


• How to map frame extracted with ffmpeg and subtitle of a video ? (frame accuracy problem)

  14 novembre 2019, par Abitbol

  would like to generate text files for frames extracted with ffmpeg, containing subtitle of the frame if any, on a video for which I have burn the subtitles using ffmpeg also.

  I use a python script with pysrt to open the subrip file and generate the text files.
  What I am doing is that each frames is named with the frame number by ffmpeg, then and since they are extracted at a constant rate, I can easily retrieve the time position of the frame using the formula t1 = fnum/fps, where fnum is the number of the frame retrieved with the filename, and fps is the frequency passed to ffmpeg for the frame extraction.

  Even though I am using the same subtitle file to retrieve the text positions in the timeline, that the one that has been used in the video, I still get accuracy errors. Most I have some text files missing or some that shouldn’t be present.

  Because time is not really continuous when talking about frames, I have tried recalibrating t using the fps of the video wih the hardcoded subtitles, let’s call that fps vfps for video fps (I have ensured that the video fps is the same before and after subtitle burning). I get the formula : t2 = int(t1*vfps)/vfps.
  It still is not 100% accurate.

  For example, my video is at 30fps (vfps=30) and I extracted frames at 4fps (fps=4).
  The extracted frame 166 (fnum=166) shows no subtitle. In the subrip file, the previous subtitle ends at t_prev=41.330 and the next subtitle begins at t_next=41.400, which means that t_sub should satisfy : t_prev < t_sub and t_sub < t_next, but I can’t make this happen.

  Formulas I have tried :

  t1 = fnum/fps  # 41.5 > t_next
  
  t2 = int(fnum*vfps/fps)/vfps  # 41.5 > t_next
  
  # is it because of a indexing problem? No:
  
  t3 = (fnum-1)/fps  # 41.25 < t_prev
  
  t4 = int((fnum-1)*vfps/fps)/vfps  # 41.23333333 < t_prev
  
  t5 = int(fnum*vfps/fps - 1)/vfps  # 41.466666 > t_next
  
  t6 = int((fnum-1)*vfps/fps + 1)/vfps  # 41.26666 < t_prev

  Command used :

  # burning subtitles
  
  # (previously)
  
  # ffmpeg -r 25 -i nosub.mp4 -vf subtitles=sub.srt withsub.mp4
  
  # now:
  
  ffmpeg -i nosub.mp4 -vf subtitles=sub.srt withsub.mp4
  
  # frames extraction
  
  ffmpeg -i withsub.mp4 -vf fps=4 extracted/%05.bmp -hide_banner

  Why does this happen and how can I solve this ?

  One thing I have noticed is that if I extract frames of the original video and the subtitle ones, do a difference of the frames, the result is not only the subtitles, there are variations in the background (that shouldn’t happen). If I do the same experience using the same video two times, the difference is null, which means that the frame extraction is consistant.

  Code for the difference :

  ffmpeg -i withsub.mp4 -vf fps=4 extracted/%05.bmp -hide_banner
  
  ffmpeg -i no_sub.mp4 -vf fps=4 extracted_no_sub/%05.bmp -hide_banner
  
  for img in no_sub/*.bmp; do
  
      convert extracted/${img##*/} $img -compose minus -composite diff/${img##*/}
  
  done

  Thanks.

• avcodec/nvdec : Use RefStruct-pool API for decoder pool

  6 août 2022, par Andreas Rheinhardt

  avcodec/nvdec : Use RefStruct-pool API for decoder pool
  It involves less allocations, in particular no allocations
  
  after the entry has been created. Therefore creating a new
  
  reference from an existing one can't fail and therefore
  
  need not be checked. It also avoids indirections and casts.
  Also note that nvdec_decoder_frame_init() (the callback
  
  to initialize new entries from the pool) does not use
  
  atomics to read and replace the number of entries
  
  currently used by the pool. This relies on nvdec (like
  
  most other hwaccels) not being run in a truely frame-threaded
  
  way.
  Tested-by : Timo Rothenpieler <timo@rothenpieler.org>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/nvdec.c
  • [DH] libavcodec/nvdec.h