[FFmpeg-user] Possible feature additions for concatenating video files

[Geek.Song]

Forget FPS, MP4 supports variable FPS, you just need recalculate
timestamp(PTS/DTS) carefully.

On Fri, Jan 6, 2012 at 5:41 AM,  <dg1727 at hushmail.com> wrote:
> On Wed, 04 Jan 2012 18:45:59 -0500 Carl Eugen Hoyos
> <cehoyos at ag.or.at> wrote:
>><dg1727 <at> hushmail.com> writes:
>>
>>> A.  An option in FFmpeg to assume that the input file is
>>> correct (audio and video have the same duration) except that
>>> only the video-FPS field is wrong.  This option would change
>>> only the video-FPS field to make the computed video duration
>>> match the audio duration.
>>> Preferably, this option could be given to the "ffmpeg" command
>>> in the FLV-to-MP4 step listed above.
>>
>>If I understand that correctly, FFmpeg miscalculates (or
>>misdetects) the video (or audio) duration in one of your
>>samples, and the result of your initial
>>ffmpeg -i input -acodec copy -vcodec copy call results in an out-
>>of-sync file. If that is correct, please provide a sample.
>
> I'm not sure that FFmpeg is misdetecting anything.  I assume that
> the input *.mp4 file has a mistaken video-FPS number stored in it.
> I assume that "MP4Box -info" is _correctly_ multiplying the number
> of video frames by this mistaken video-FPS number and thus arriving
> at an _incorrect_ video duration; and that the same FPS is used in
> the concatenation, causing the video duration to be the same wrong
> value calculated by "MP4Box -info".
>
> Any sync problem doesn't appear until I try to concatenate the
> files.
>
> I have thought some more about my attempted concatenation.  Let me
> repeat the calculations I am doing:
>
>  (Number of video frames) / (video duration) = FPS that seems to
> be stored in the *.mp4
>
>  (Number of video frames) / (audio duration) = what seems to be
> the correct FPS
>
> I ran "MP4Box -info" on all 10 of my files and found that the
> "correct" FPS vary a little bit.  For the first 9 files, if the
> "correct" FPS is truncated to 3 decimal places, it is 24.978 in all
> cases.  Truncated to 5 decimal places, it ranges from 24.97812 to
> 24.97889.  The "correct" FPS for the 10th file is 24.9479.  (It
> looks like the last file has a few extra audio samples, and the
> correct FPS is probably 24.978.  If this file were forced to 24.978
> FPS, a fraction of a second of audio would extend past the video;
> this would be fine.)  If the frame rates were adjusted
> automatically according to my suggestion A above, then they would
> come out different (24.97812, 24.97889, 24.9479, etc.)  Since the
> resulting video files would have different frame rates, existing
> tools might not be able to concatenate them [4][5].
>
> [4]
> http://sourceforge.net/projects/gpac/forums/forum/287547/topic/48337
> 22
> [5]
> http://sourceforge.net/projects/gpac/forums/forum/287547/topic/48775
> 38
>
> For now, I am fine with determining the frame rate by hand instead
> of having FFmpeg calculate it as in my suggestion A (so far, I
> don't often need to do this video splicing).  It doesn't matter if
> the same frame rate as in the first 9 files is put into the last
> file; a discrepancy in the last file won't put anything out of
> sync.
>
> So I would like to force all 10 files to 24.978 FPS.
>
> Now I am having trouble finding the right command line to do this.
>
> My ffmpeg startup banner is:
>
> ffmpeg version 0.9.0.git, Copyright (c) 2000-2012 the FFmpeg
> developers
>  built on Jan  1 2012 14:20:29 with gcc 4.5.2
>  configuration:
>  libavutil      51. 33.100 / 51. 33.100
>  libavcodec     53. 49.101 / 53. 49.101
>  libavformat    53. 29.100 / 53. 29.100
>  libavdevice    53.  4.100 / 53.  4.100
>  libavfilter     2. 57.101 /  2. 57.101
>  libswscale      2.  1.100 /  2.  1.100
>  libswresample   0.  5.100 /  0.  5.100
>
> (
> I have a suggestion C.  Is it possible to have the version listed
> as something like 0.9.0.gitmaster20120101_140000.001 to be more
> precise?
> )
>
> Taking bits and pieces out of the ffmpeg manpage, I made the
> following 2 attempts:
>
> ffmpeg -i 1.flv -vcodec copy -acodec copy -r 24.978 1.mp4
>
> Result:  The output file had the same statistics as without the -r
> option.
>
> ffmpeg -i 1.flv -c:v libx264 -sameq -acodec copy -r 24.978 1a.flv
>
> Result:  Unknown encoder 'libx264'
>
> Then I ran
> ffmpeg -codecs | grep 264
> which showed:
>  D V D  h264            H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10
>
> Since the capabilities flags at the beginning (D V D) don't contain
> an E, the "h264" codec in my FFmpeg install is not capable of
> encoding.
>
> Do I need to download or compile my FFmpeg in a different way to
> gain access to H.264 encoding?  My package manager shows that I
> have 3 versions of libx264 installed.
>
> I prefer a way to modify the FPS during a stream-copy, to eliminate
> the chance of quality loss.
>
> What command line should I use to alter only the FPS field of the
> input FLV file, hopefully in the same command as changing the
> container from FLV to MP4?
>
> I also look forward to comment on my suggestion B:
>
> B.  A feature (maybe in ffprobe?) which reports the total number of
> I-frames, P-frames, and B-frames in a given video stream (any
> format, not just H.264).  This way, if the file has no B-frames, I
> can happily use tools on it that warn me that B-frames may cause
> problems.

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