[FFmpeg-user] How to reassemble raw packet data back into a new MP4 file?

[Ferdi Scholten]

On 21-09-2024 04:39, Alan Chen via ffmpeg-user wrote:
> I am using ffmpeg.
>
> I use the following command to get all packet info:
>
> ffprobe -print_format json -show_packets -show_streams -show_entries
> packet=pts,duration,size,codec_type,pos test.mp4 > test.mp4.info
>
> Then based on each packet info, such as the one below:
>
>          {
>              "codec_type": "video",
>              "pts": 0,
>              "duration": 512,
>              "size": "6419",
>              "pos": "175"
>          }
>
> I extract the raw data of the packet from the test.mp4 file, take the above
> packet as a example, I extract the data at offset = 175 and size = 6419,
> then save the packet into a new file, video_sample0.dat.
>
> I write an app to write all the packets into different audio and video
> files, i.e., audio_sample0.dat, audio_sample1.dat, ..., video_sample0.dat,
> video_sample1.dat...
>
> Then I use ffprobe with -show_data option to show the raw data of each
> packet, then compare the output with my packet files and confirm they are
> identical.
>
> After that, I remove some useless packets. Then try to recreate a new MP4
> file with the remaining packets.
>
> I take one packet file as a test:
>
> ffmpeg -f m4v -i video_sample0.dat -c copy output.mp4
>
> But get the following error:
>
> ffmpeg version 7.0.2 Copyright (c) 2000-2024 the FFmpeg developers
>    built with gcc 14.1.0 (Rev3, Built by MSYS2 project)
>    configuration: --enable-shared
>    libavutil      59.  8.100 / 59.  8.100
>    libavcodec     61.  3.100 / 61.  3.100
>    libavformat    61.  1.100 / 61.  1.100
>    libavdevice    61.  1.100 / 61.  1.100
>    libavfilter    10.  1.100 / 10.  1.100
>    libswscale      8.  1.100 /  8.  1.100
>    libswresample   5.  1.100 /  5.  1.100
> [mpeg4 @ 00628400] time_increment_bits 0 is invalid in relation to the
> current bitstream, this is likely caused by a missing VOL header
> [mpeg4 @ 00628400] time_increment_bits set to 5 bits, based on
> bitstream analysis
> [mpeg4 @ 00628400] time_increment_bits 4 is invalid in relation to the
> current bitstream, this is likely caused by a missing VOL header
> [mpeg4 @ 00628400] time_increment_bits set to 5 bits, based on
> bitstream analysis
> [mpeg4 @ 00628400] looks like this file was encoded with
> (divx4/(old)xvid/opendivx) -> forcing low_delay flag
> [m4v @ 003e8d00] Could not find codec parameters for stream 0 (Video:
> mpeg4, yuv420p): unspecified size
> Consider increasing the value for the 'analyzeduration' (0) and
> 'probesize' (5000000) options
> Input #0, m4v, from 'video_sample0.dat':
>    Duration: N/A, bitrate: N/A
>    Stream #0:0: Video: mpeg4, yuv420p, 25 fps, 1200k tbr, 1200k tbn
> File 'output.mp4' already exists. Overwrite? [y/N] y
> Stream mapping:
>    Stream #0:0 -> #0:0 (copy)
> [mp4 @ 00628400] dimensions not set
> [out#0/mp4 @ 0062b380] Could not write header (incorrect codec
> parameters ?): Invalid argument
> Conversion failed!
>
> So, how to reassemble the raw packet data back to a new MP4 file?
> _______________________________________________
> Start reading and understanding this:https://www.iso.org/standard/55688.html
It is the first of 32 parts to read in order to understand how MPEG-4 
works and how to (re)create it. Unfortunately, this information is not 
free (patented) so you'll have to pay to read it.