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• How to get FFmpeg to consistently apply speed effect to first few frames ?

  16 janvier 2021, par melody_florum

  I am using FFmpeg to apply speed effects to short gifs and videos and I noticed strange stuttering on looping gifs when sped up. I manually tested and found what was going on. FFmpeg failed to apply the speed filter to the first few frames of the video.

  


  I started with this file, which loops between the numbers "1 2 3 4" every frame.

  


  123410fps.mp4

  


  I then ran the command ffmpeg -i 123410fps.mp4 -vf setpts=0.5*PTS 123410fpsout.mp4

  


  That left me with this file : 123410fpsout.mp4

  


  On close analysis, I discovered that the output file didn't show a repeating 1 3 1 3 or 2 4 2 4 pattern, but shows 1 2 3 4 1 3 1 3 1 3... the speed effect didn't apply for the first few frames

  


  I apologize if it sounds like i'm being super nitpicky, but my usage for this is on very short gifs and videos where this effect is very apparent. How can I prevent this effect from happening and have a 100% consistent speed effect applied to the video ? I don't mind using other non-ffmpeg software but a CLI solution that works easily for most videos is ideal.

  


  In case anyone is wondering, I don't use ffmpeg to encode gifs, i use gifski from ffmpeg exported image sequences. The problem shows up even when no gifs are used, so it's not a gifski problem.

  


• FFmpeg - How do I set audio language and subtitles when converting videos ?

  7 mai 2022, par mereMolf

  I have a handful of mkv video files that happen to have English and Japanese audio tracks along with english subtitles. I want to convert them all to webm files and keep only the Japanese audio with English subtitles.

  


  How do I do that ?

  


  Specifically, I use QWinFF, a GUI frontend to FFmpeg.
Under the Advanced options, I use the following parameters

  


  -f webm -vcodec libvpx -g 120 -level 216 -profile:v 0 -qmax 25 -qmin 10 -rc_buf_aggressivity 0.95 -vb 480k -acodec libvorbis -aq 90


  


  Any help would be appreciated.
Thanks.

  


• Can I run a program from Command Prompt on only a numbered subset of files ?

  28 août 2020, par Andrew Shibata

  I have an experiment with 2 groups : English speakers and Mandarin-English bilinguals. I have a batch file that works for converting audio files (using ffmpeg) for the English monolingual group. The Mandarin speaker group also has a few additional trials where they record some more sentences in Mandarin and I'd like to either run ffmpeg separately on just those trials or move that subset to a different folder after the fact.

  


  The file formats are [subject ID]-[trial #]-vowels_record.ogg. The English trials are trial #s 6-35 and the Mandarin trials are trial #s 36-50.

  


  The following code WORKS for my English only dataset :

  


  for %%a in ("*vowels_record.ogg") do ffmpeg -i "%%a" "vowel_wavs\%%~na.wav

  


  This is my attempt at figuring out how to get just the relevant files into relevant folders :

  


  for %%a in ("*{6..35}-vowels_record.ogg") do ffmpeg -i "%%a" "vowel_wavs\%%~na.wav"
for %%a in ("*{36..50}-vowels_record.ogg") do ffmpeg -i "%%a" "mandarin_wavs\%%~na.wav"


  


  I have some regex experience, but not as much with making .bat files and I'm unsure what the right way is to target the range of files I'm looking for.