Recherche avancée

Sur d’autres sites (12515)

• ffmpeg probesize and analyzeduration flags

  7 mai 2014, par Xerphiel

  Could someone explain to me in simple terms what these flags/options do ?

  Running "ffmpeg -h full | grep probesize/analyzeduration" yields the following documentation :

  -probesize         <int>        .D.... set probing size (from 32 to INT_MAX) (default 5e+06)
  
  -analyzeduration   <int>        .D.... specify how many microseconds are analyzed to probe the input (from 0 to INT_MAX) (default 5e+06)
  
  </int></int>

  This is beyond my understanding, so am hoping for a simpler explanation.

  All the discussions on this subject I’ve found via googling do not describe the basic function of the options.

  Any help is appreciated.

  Thank you.

• Wrong video duration when trying to make screen capture with libav\ffmpeg

  16 juin 2023, par Ларионов Фёдор

  I am performing screen capture using libav/c++ with the gdbigrab InputFormat and saving the transcoded video with h.264 to an .mp4 container. Despite doing all the necessary conversions from pts/dts frames and packets, the final video has an issue with its duration. When playing the video, it stops a few seconds before the end and rewinds back to the end.

  


  To diagnose the problem, I used FFprobe to examine the recorded video stream and found the following issues :

  


  ffprobe my_video.mp4 -show_streams

  


  Provides the following information :

  


  time_base=1/12800
start_pts=28096
start_time=2.195000
duration_ts=38945
duration=3.042578


  


  Although I tracked the frame and burst timestamps while recording, starting at zero and ending at the expected pts for the number of frames recorded, I tried to set the start and end time of the stream manually. I observed this problem with different time_base and different video durations, do some minor and major fixes that does not affected this issue. Interestingly, the start time always has the same offset, and the expected/specified video duration in the code is close to the sum of the duration of the video stream and its start time. What am I missing ?

  


  


  UPDATE :

  


  


  There is the source of problem :

  


  [mov,mp4,m4a,3gp,3g2,mj2 @ 000002a69f15b380] Processing st: 0, edit list 0 - media time: -1, duration: 27059
[mov,mp4,m4a,3gp,3g2,mj2 @ 000002a69f15b380] Processing st: 0, edit list 1 - media time: 0, duration: 38912 


  


  which i get by that command ffmpeg -hide_banner -y -v 9 -loglevel 99 -i sc_test.mp4

  


  The question is where this edits happend.

  


• Change keyframe interval losslessly to produce mpeg-dash content

  25 mars 2023, par Eeel

  I have a case where i have a mkv file encoded to a 2.5 GB file with x264/CRF settings, in this file group of pictures are not of equal duration (i.e 2,4 or 6 seconds ...).

  


  Now i want to create a mpeg-dash version of the file by extracting the video track to mp4 and set the keyframe interval to 2sec losslessly preserving size (minus other audio and subtitle tracks)

  


  What i already have that work :

  


  Fix keyframe to 2s with x264 :

  


  -codec:v libx264 -force_key_frames 'expr:gte(t,n_forced*2)' -movflags faststart


  


  But it require also encoding settings that change the quality :

  


  -b:v 3500k -maxrate 6000k -bufsize 3500k


  


  In this case i don't need to change the quality from CRF encoding, just change keyframe interval and set faststart flag.

  


  note :
I have detailed bitrate informations about the file obtained by parsing output of ffprobe : average, min and max bitrate in kb/s.
Please avoid solution using -crf 0, mpeg-dash files are later crypted using MP4Box that crash with CRF encoded files.

  


  How can i do this using ffmpeg ?

  


  Edit :

  


  Sorry my question is not clear, i'm fine with re-encode, i want to preserve quality as much as possible.

  


  Using a little more than source mean bitrate for the -b:v parameter the resulting file size is almost same as source, metric test results are not as good i expect :

  


  PSNR 41.03 (expected 44+)
VMAF 76.32 (expected 92+)


  


  Is my bitrate approch good ? Is there anything i can try to improve metric results ?