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• Speed up ffmpeg transcoding of fullHD input to hls

  18 juin 2020, par DotNetter

  I try to convert full hd mp4 (bitrate of 12Mb) file to hls live stream.
Chose to use default 2sec chunks.
I encountered with a issue that make online live transcoding and streaming impossible : ffmpeg require more than 2 second to create HLS chunk. So from time to time my stream gets interrupted for a seconds or smth to wait ffpeg produce next chunk.

  



  Adopt following command (fragment of bash script) :

  



  args=(
    -i hd.mp4 
    -vsync 1
    -tune zerolatency 
    -preset ultrafast
    -c:v libx264 
    -force_key_frames "expr:gte(t,n_forced*2)"
    -crf 23 
    -b:v:0 768k 
    -vf "scale=720:trunc(ow/a/2)*2" 
    -b:a:0 96k
    -b:v:1 1024k 
    -vf "scale=960:trunc(ow/a/2)*2" 
    -b:a:1 128k
    -b:v:2 2500k
    -maxrate 2800k 
    -bufsize 2000k 
    -vf "scale=1280:trunc(ow/a/2)*2"
    -b:a:2 128k
    -b:v:3 4000k
    -maxrate 4500k 
    -bufsize 3800k
    -map 0:a
    -map 0:v 
    -map 0:a 
    -map 0:v 
    -map 0:a 
    -map 0:v 
    -map 0:a 
    -map 0:v
    -f hls 
    -hls_list_size 4
    -var_stream_map "a:0,v:0 a:1,v:1 a:2,v:2 a:3,v:3"
    -master_pl_name live.m3u8 
    live_%v.m3u8
)

ffmpeg "${args[@]}"


  



  What's interesting, when I removed filtering for fullHD ,other words removed this filter

  



  -b:v:3 4000k
    -maxrate 4500k 
    -bufsize 3800k 


  



  and corresponding stream mapping, and limited the highest output up to only HD everything got stabilized.

  


• avformat/dashdec : drop arbitrary DASH manifest size limit

  3 septembre 2020, par Jan Ekström

  avformat/dashdec : drop arbitrary DASH manifest size limit
  Currently the utilized AVBPrint API is internally limited to unsigned
  
  integers, so if we limit the file size as well as the amount to read
  
  to UINT_MAX - 1, we do not require additional limiting to be performed
  
  on the values.
  This change is based on the fact that initially the 8*1024 value added
  
  in 96d70694aea64616c68db8be306c159c73fb3980 was only for the case where
  
  the file size was not known. It was not a maximum file size limit.
  In 29121188983932f79aef8501652630d322a9974c this was reworked to be
  
  a maximum manifest file size limit, while its commit message appears
  
  to only note that it added support for larger manifest file sizes.
  This should enable various unfortunately large MPEG-DASH manifests,
  
  such as Youtube's multi-megabyte live stream archives to load up
  
  as well as bring back the original intent of the logic.
  

  • [DH] libavformat/dashdec.c

• How do I half the frame-rate of a DNxHR mxf file with FFMpeg losslessly ?

  19 mai 2020, par in03

  I have limited experience with FFMpeg, so there's a chance I'm missing something simple here. Imagine the following scenario :

  



  I have a DNxHR HQ UHD 50P .mxf file, 10 seconds/500 frames long. I want to remove every second frame from this file with FFMpeg so that I end up with a DNxHR HQ UHD 25P .mxf file, 10 seconds/250 frames long.

  



  I do not want the file to be re-encoded in any way and any audio streams should remain untouched. I imagine this is possible since DNxHR is an intra-frame codec. If this isn't possible with FFMpeg, I'm happy to hear alternative approaches with similar tools.

  



  I last tried the following command ffmpeg -i TEST.mxf -r 25 -c:v copy TEST_25P.mxf. Which retains the original duration of the file, halves the frame-rate, bitrate and frame-count as expected, but does so by slowing the clip down to half-speed and keeping the file at the same duration, effectively deleting the 2nd half of the file...

  



  Any suggestions area appreciated.

  



  EDIT :

  



  I am exporting files from DaVinci Resolve which can only export files at the project's locked framerate. We edit in 50P because at some point we will upgrade our platform to serve 50P content, but for the time being we can only serve 25P. Converting our 50P deliverables to 25P is much easier than converting 25P projects to 50P projects.