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• How to output fragmented mp4 with ffmpeg ?

  12 octobre 2016, par S B

  ffmpeg -i infile.avi out.mp4 outputs non-fragmented MP4.

  How do I obtain fragmented mp4 ?

  Update
  A fragmented mp4 file is internally divided into several back-to-back chunks or MPEG-4 movie fragments. Each chunk has its own moof atom - so there are several moof atoms interleaved in the file instead of a single moov at the end as in the case of an unfragmented mp4. This makes it easier to stream over slow networks where buffering is involved

  There are several tools like mp4box that convert a normal mp4 to a fragmented one. Unfortunately we cannot use something like this

  ffmpeg <options to="to" output="output" mp4="mp4"> | mp4box
  
  </options>

  since ffmpeg does not produce seekable output while producing mp4 containers.

• image -> video -> image not lossless using avconv

  15 décembre 2015, par nrob

  I’m trying to test the amount of information lost with some different video codecs. I’ve got a python script which uses PyPNG to write a series of 8 bit RGB images. I then encode it using avconv, for instance

  avconv -r 1 -i ../frames/data%03d.png -c:v ffv1 -qscale:v 0 -r 1 

  outffv1.avi

  I then decode this back into pngs like so

  avconv -r 1 -i outffv1.avi -r 1 ./outffv1/frame%03d.png

  But when I compare the images before and after the video compression, they are different (mean absolute error of 15%). The thing that is confusing me is that this is true (give or take) independent of the codec.

  For instance, I get similar answers for libtheora for a range of qscale values.

  The png encoding i.e. write to png, and immediately load back in without and video compression step, is lossless.

• ffmpeg reverts audio volume even after removing the audio track

  7 avril 2023, par DniweTamp

  I tried making a simple batch file that separates and normalizes audio track, and merges it with the reencoded video track afterwards, but whenever I try to add the edited audio track back the volume just goes back to the original level.

  


  It happens specifically when I try to add the audio back to video, the volume level stays normalized if I reencode said audio to .aac or .ogg files, but when I try to add it to the video, even using -c:a copy, the volume just reverts to the original level

  


  Below is an image that shows the volume levels of audio tracks

  


  I detach audio, normalize it (result is top track), add it back to the video, and it becomes what's on the bottom (which is also the same volume as the original video's audio)


  


  Here's the contents of my batch file, expected result was a reencoded video, with normalized audio

  


  I tried removing audio track with both -an and -map 0 -map -0:a, same result

  


  I can confirm that the normalize program does its job correctly, by checking volume levels in audacity

  


  ffmpeg -i %1 -y temp.wav
normalize.exe temp.wav
ffmpeg -i %1 -c:v libx264 -map 0 -map -0:a -profile:v main -pix_fmt yuv420p -movflags +faststart -preset medium -crf 22 -map_metadata -1 -y video.temp.mp4
ffmpeg -i %1 -i temp.wav -c:v copy -y video.h264.mp4
del temp.wav
del video.temp.mp4


  


  Edit : and here's the video file I tried this on.