HOWTO: get rid of packed B-frames in AVI (MPEG-4 Visual) video with FFmpeg

You've probably seen some AVI (Audio Video Interleave) files with MPEG-4 video (more specifically, MPEG-4 Visual / ASP, usually encoded by DivX or Xvid codecs) inside; they are often used for DVD-rips. Such AVI video files often contain "packed B-frames", which is not an optimal frame storage way, but just a workaround for the VfW (Video for Windows) framework shortcomings (i. e., bugs).
To ensure you have a "bad-style" MPEG-4 AVI file, you can check it with a media analyzer utility like MediaInfo (in this case, it'll show you something like "Muxing mode : Packed bitstream").
Fortunately, that wrong thing is easy to "unpack" with an up-to date version of FFmpeg (v2.7+):

$ ffmpeg -i in_bad.avi -codec copy -bsf:v mpeg4_unpack_bframes out_good.avi

The conversion is lossless (neither video nor audio quality is harmed; the file should remain compatible with the hardware players... or even become more compatible), but often can even reduce the resulting AVI container size.

BTW, after "bitstream unpacking", the MPEG-4 video stream becomes standards-compliant, so you can put it directly into the popular MP4 container without transcoding (but it requires the audio stream to be MP4-compatible too, e. g. MP3 is OK);
e. g., here are 2.5 3 remuxing examples using the same FFmpeg:

$ ffmpeg -i in10_good.avi -codec copy out10_good.mp4
$ ffmpeg -i in20_bad.avi -codec copy -bsf:v mpeg4_unpack_bframes out20_better.mp4
$ ffmpeg -i in20_bad.avi -codec copy -bsf:v mpeg4_unpack_bframes -movflags faststart out25_best.mp4

You can download the source code and binary builds of the current FFmpeg (for MS Windows, Apple Mac OS X and Linux) freely from the official site.
Usually, you will find a compatible version in the repositories of your favorite Linux distribution;
however, in Debian, it's available only starting from the release 9 "Stretch" (and fortunately, it's currently in the backports for 8 "Jessie" too; and for use in Ubuntu, you need release 15.10 "Wily Werewolf", at least):

# sudo apt-get install ffmpeg

References:

1. FFmpeg
   1. FFmpeg Bitstream Filters Documentation
   2. Download FFmpeg
   3. FFmpeg Changelog
   4. #2913 (Bitstream filter to fix "invalid and inefficient vfw-avi packed B frames") – FFmpeg
2. MediaInfo
3. Debian Package Tracker - ffmpeg
4. ffmpeg package : Ubuntu
5. Wikipedia
   1. Ripping
   2. MPEG-4 Part 2
   3. DivX
   4. Xvid
   5. Audio Video Interleave
   6. Video for Windows
   7. Video compression picture types
   8. Inter frame

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Last updated: 2017-12-28