[FFmpeg-user] How to re encode broken mp4s

[Gabriele Greco]

I've some hundreds of broken h264 mp4s that have been encoded with nvenc
ffmpeg encoder with conflicting options (-vprofile baseline -bf3), these
files pretends to be baseline but they really have some B frames inside,
but those B frames are marked by the container as P, so that VLC and ffmpeg
fails to decode them and keep the previous I frame.

The files are reported by ffprobe as: I - P - P - P - P - P - P - I - P - P
- P - P - P - P - I ... but they are really I - B - B - B - B - B - B - I -
B - B - B - B - B - B - I ...

The curious thing is that:
- VLC, ffmpeg, ffplay and the iOS player cannot playback them smoothly (but
in a jerky 3/4 fps)
- quicktime, windows media player and the android video player can playback
them perfectly

So I'd like to try to fix them or re encode them if a fix is not possible.

Doing a simple step of:

ffmpeg -i source.mp4 -acodec copy -vcodec copy dest.mp4

... but it doesnt work neither:

ffmpeg -i source.mp4 -vcodec libx264 -acodec copy dest.mp

... and examining the output of:

ffmpeg -i source.mp4 -vframes 100 %04d.png

I saw that the problem is that ffmpeg decode all the frames that are not I
as the previous I frame, and this is probably correct because ffmpeg trusts
the container, while quicktime and co probabily go inside the h.264 data
structure...

Here is a sample (4MB):
https://www.dropbox.com/s/cic295zli9b3z33/sample.mp4?dl=0

There is a way to force ffmpeg to decode the h264 in an alterative way so
that the frames are interpreted with their correct type?

-- 
Bye,
 Gabry