[FFmpeg-trac] #2771(avformat:closed): -segment_time generates unexpected file lengths

[FFmpeg]

#2771: -segment_time generates unexpected file lengths
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             Reporter:  Acc72       |                    Owner:
                 Type:  defect      |                   Status:  closed
             Priority:  normal      |                Component:  avformat
              Version:  git-master  |               Resolution:  invalid
             Keywords:  segment     |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |
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Changes (by saste):

 * keywords:   => segment
 * resolution:   => invalid
 * status:  new => closed
 * component:  undetermined => avformat


Comment:

 Replying to [ticket:2771 Acc72]:
 > Hello,
 >
 > I'm attempting to utilize the segmenter on an A/V stream, from my USB
 webcam. With the -segment_time option, I've configured ffmpeg to output a
 circular series of four (4) mp4 files, each three (3) seconds in length.
 However, ffmpeg generates files which are sixteen (16) seconds in length.
 Occasionally, random files are generated at shorter lengths (2 sec, 3 sec,
 8 sec, etc...). However, most of them are generated sixteen (16) seconds
 in length.
 >
 > Results change, if I play with the value for -segment_time:
 > (-Segment_time < 16) all 4 files generated @ 16 seconds.
 > (-segment_time >= 16) 1st file generated @ 33 seconds and all subsequent
 files generated @ 16 seconds.
 >
 > Not sure why this is happening???

 This is expected, from the docs:

 Every segment starts with a keyframe of the selected reference stream,
 which is set through the reference_stream option.

 Note that if you want accurate splitting for a video file, you need to
 make the input key frames correspond to the exact splitting times expected
 by the segmenter, or the segment muxer will start the new segment with the
 key frame found next after the specified start time.

 ...

 You can use the ffmpeg -force_key_frames option to force key-frames, read
 the examples in the segment manual.

 [...]

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Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/2771#comment:4>
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