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

[dg1727 at hushmail.com]

Hello, 

I am trying to concatenate (on Ubuntu Linux), without transcoding, 
some FLV files which have H.264 video and AAC audio.  They were 
originally split from a longer video, so the frame rates must have 
been the same originally.  

First I changed the container from FLV to MP4 [1] as follows:  

for f in *.flv; do \
ffmpeg -i "$f" -vcodec copy -acodec copy "${f%.flv}.mp4"; \
done

Next, I tried to concatenate the MP4 files using MP4Box.  The 
resulting file had the audio out of sync with the video.  Then I 
found, using "MP4Box -info", that the audio duration of each file 
was different from the video duration, so [2] MP4Box didn't 
concatenate the videos in the way I expected.  

I browsed the discussion in [3], which told me that sometimes the 
video-frames-per-second field of a video file is in error so that 
software computes the wrong video duration from this field.  
Indeed, calculating from the audio duration of one of my files 
produced a video FPS of about 24.9787 instead of the 25.0000 
indicated by the reported video duration.  It seems like 24.9787 is 
the correct value.  I then thought of the following proposed 
feature:  

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.  

Also, I found a couple of software tools that gave me warnings 
like, "This file uses H.264.  If there are B-frames, something may 
not work right."  So I would also like the following feature:  

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.  

Do proposed features A and B already exist?  If not, should I make 
tickets in Trac to request that these features be added to FFmpeg?  

If these features exist in software other than FFmpeg, I would be 
interested to know that, but I would still like these features to 
be in FFmpeg to make it more of a "one-stop shop."  I hope these 
features would not be hard to add to FFmpeg anyway.  

I appreciate any comments.  

[1] http://ubuntuforums.org/showthread.php?t=1687798&page=2
[2] comment 5 on 
http://sourceforge.net/projects/gpac/forums/forum/287547/topic/48156
01
[3] http://forum.doom9.org/archive/index.php/t-157642.html