I have corrupt mpeg4 files which require frame reshuffling (sample at https://drive.google.com/file/d/1GD7YiVS2z8h0r6UdsPPIXFgnh9RukoeR/view?usp=drive_link )
I had some success using ffmpeg’s shuffleframes filter but it requires video reencoding, and I would like to avoid that.

I analyzed those mpeg4 files and observed they have no CTTS box, so I tried adding one with the required offsets, to no avail: VLC 3.0.20 still plays the frames in the wrong order :-(.

To confirm whether CTTS worked as I thought, I crafted the following test video with 100 numbered frames @2fps using ffmpeg: https://drive.google.com/file/d/1vp3uKZYgoOFqlMk7GLKOC-_vxzJCMwGQ/view?usp=sharing
I analyzed it and can confirm it includes a CTTS box which defines frame offsets(*), and VLC plays it fine. Good.

Now the weird thing is: if I edit that test MPEG-4 and just remove the CTTS box (resulting in the following file: https://drive.google.com/file/d/1JMEcyBoz5r0uVDt08ZOumDVngT1PYWm8/view?usp=sharing ), VLC still plays the video in the right order !

So my question is: how does VLC proceed ? I guess it uses some other (redundant) information ?
And, when both that information and CTTS are present and disagree, which ones takes precedence ?

Or, in short, how can I fix frame order in a consistent way ?

Thanks,

Vincent

(*) start of CTTS decoding from test file:

Entry{count=1, offset=16384},   2f => frame encoded at pos.  0 => presented at pos.  0+2=2
Entry{count=1, offset=32768},   4f => frame encoded at pos.  1 => presented at pos.  1+4=5
Entry{count=2, offset=8192},    1f => frame encoded at pos.  2 => presented at pos.  2+1=3
                                1f => frame encoded at pos.  3 => presented at pos.  3+1=4
Entry{count=1, offset=40960},   5f => frame encoded at pos.  4 => presented at pos.  4+5=9
Entry{count=1, offset=16384},   2f => frame encoded at pos.  5 => presented at pos.  5+2=7
Entry{count=1, offset=0},       0f => frame encoded at pos.  6 => presented at pos.  6+0=6
Entry{count=1, offset=8192},    1f => frame encoded at pos.  7 => presented at pos.  7+1=8
Entry{count=1, offset=32768},   4f => frame encoded at pos.  8 => presented at pos.  8+4=12
Entry{count=2, offset=8192},    1f => frame encoded at pos.  9 => presented at pos.  9+1=10
                                1f => frame encoded at pos. 10 => presented at pos. 10+1=11
Entry{count=1, offset=32768},   4f => frame encoded at pos. 11 => presented at pos. 11+4=15
Entry{count=2, offset=8192},    1f => frame encoded at pos. 12 => presented at pos. 12+1=13
                                1f => frame encoded at pos. 13 => presented at pos. 13+1=14
Entry{count=1, offset=32768},   4f => frame encoded at pos. 14 => presented at pos. 14+4=18
Entry{count=2, offset=8192},    1f => frame encoded at pos. 15 => presented at pos. 15+1=16
                                1f => frame encoded at pos. 16 => presented at pos. 16+1=17
Entry{count=1, offset=24576},   3f => frame encoded at pos. 17 => presented at pos. 17+3=20
Entry{count=1, offset=8192},    1f => frame encoded at pos. 18 => presented at pos. 18+1=19