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• Use ffmpeg as external tool to stream 2 or more different sources via pipeline

  25 février 2021, par StackDOX

  I have an application running on an embedded system. This application has 2 video sources (and, theorically, 1 audio source). Concentrating to the video sources, I have 2 subprocess that computes different frames sets (unrelated each others). I want to send these frames to 2 differents streams.

  


  I would to avoid to write a lot of ffmpeg/libav code. I have ffmpeg compiled for the embeeded system and I can use it as tool. For example, I can write to the stdout the first frames set and pass it to the ffmpeg like this :

  


  ./my_app|ffmpeg -an -i - -vcodec copy -f rtp rtp://"remote_ip

  


  This basically works. But, now i would to send the other one frame set. How to do that ? Theorically I need anoter ffmpeg instance that read from another source that can't be the stdout of "my_app", because is already busy.

  


  ---

  


  I'm thinking to use 2 video files as support. I can record the 2 frames sets into 2 video files and then run 2 ffmpeg instances from these sources. In this case I think I need a way to limit the video files dimensions (like a circular buffer), because 2 streams can become really huge in time. It is a possibility ?
This can sound "weird" to me : I need to record a video source in realtime and stream it via ffmpeg (always in realtime). I don't know if it is a good idea, there are realtime problems for sure :

  


  loop:
my_app --write_into--> video_stream1.mp4 
ffmpeg <--read_from-- video_stream1.mp4

my_app --write_into--> video_stream2.mp4 
ffmpeg <--read_from-- video_stream2.mp4


  


  Have you some suggestion to address this kind of situation ?

  


  many thanks, bye.

  


• ffmpeg extract hdmv pgs subtitles from mkv to srt [closed]

  31 mars 2023, par breezybrea

  I've been trying to figure out how to extract hdmv pgs subtitles from an mkv file for a few days now. I must be doing something wrong. I'm a noob at this. Can someone please help ? I think i need to set an encoder or set a codec parameter to fix the issue.

  


  this is the subtitle I'm trying to rip

  


  Stream #0:4(eng): Subtitle: hdmv_pgs_subtitle (default)
    Metadata:
      title           : Signs / Songs
      BPS-eng         : 7215
      DURATION-eng    : 00:22:43.946000000
      NUMBER_OF_FRAMES-eng: 96
      NUMBER_OF_BYTES-eng: 1230263
      _STATISTICS_WRITING_APP-eng: mkvmerge v28.0.0 ('Voice In My Head') 64-bit
      _STATISTICS_WRITING_DATE_UTC-eng: 2018-10-22 23:45:00
      _STATISTICS_TAGS-eng: BPS DURATION NUMBER_OF_FRAMES NUMBER_OF_BYTES


  


  and the ffmpeg command i'm using is

  


  ffmpeg -i "FILE PATH".mvk -map 0:4 "FILE PATH".srt


  


  I've also tried

  


  ffmpeg -i "FILE PATH".mkv -map 0:4 pgssub "FILE PATH".srt

ffmpeg -i "FILE PATH".mkv -map 0:4 hdmv_pgs_Subtitles "FILE PATH".srt


  


  along with a few other variations and always get an error.

  


  

  • Subtitle encoding currently only possible from text to text or bitmap to bitmap

  


  • Unable to find a suitable output format for 'hdmv_pgs_subtitle'

  


  • Unsupported subtitles codec : dvd_subtitle

  


  • Could not find codec parameters for stream 4 (Subtitle : hdmv_pgs_subtitle (pgssub)) : unspecified size Consider increasing the value for the 'analyzeduration' and 'probesize' options

  


  • Could not write header for output file #0 (incorrect codec parameters ?) : Invalid argument

  


  • or the srt file it creates is 0kb.

  


  


  Can someone please tell me what i'm doing wrong and show me the correct code to use.
I'm on a mac and i've also tried using MKVToolNix and get a weird binary file that doesn't work cause it's probably in the wrong format.

  


• FFMPEG X264 reduce bitrate for unchanging input

  8 juin 2021, par GroovyDotCom

  ffmpeg -f lavfi -i smptebars=duration=1000:size=640x360:rate=30 -preset ultrafast -vcodec libx264 -tune zerolatency -b 900k -g 1000 -f mpegts video.ts


  


  As can be seen when running this command, the encode size continues to grow rapidly even though :

  


  

  1. The input source never changes. It is exactly the same image again and again.

  


  2. The encoder has been told to insert a keyframe only once every 1000 frames

  


  


  Is there some configuration option I could add to change this behaviour ? I can understand it would need to send an empty P frame but right now it seems to be sending much much more ?

  


  UPDATE
After Tim Roberts comment, I realized we can get a clearer picture of the file growth if I change the command to 1 FPS and do the encode in real-time.

  


  ffmpeg -f lavfi -re -i smptebars=duration=1000:size=640x360:rate=1 -preset ultrafast -vcodec libx264 -tune zerolatency -b 900k -g 1000 out.ts

  


  Now I see that it is going up almost exactly 1KB per frame. That seems like a lot for a P-frame that does nothing.

  


  So, just for kicks, I got rid of the TS muxing.

  


  ffmpeg -f lavfi -re -i smptebars=duration=1000:size=640x360:rate=1 -preset ultrafast -vcodec libx264 -tune zerolatency -b 900k -g 1000 out.h264

  


  Now the file grows by 1k every 17 frames - a 17X improvement !

  


  This seems to point to ffmpeg doing something weird in the TS muxer that inflates an 80 byte P-frame into 1K.