On Fri, 13 Jun 2025, softworkz . wrote:
[...]
(Please note, that the default is 0, which means that nothing is
dropped and there's no change in behavior when it's 0).
Probably it's best to look at an example. Let's say we have:
- a 300s video
- that we want to stream via HLS
- Segment-Duration: 3s - makes 100 segments
- Now we want to create the segments on-demand only,
so we deliver a synthetic playlist with 100 3s segments,
even though we don't have any segment yet
- Once specific segments are needed, we create them on-the-fly
That's a situation that the commit message is about:
Existing segments 0-30 and 70-99 => we already have them on disk
31-69 need to be created
This option allows to stop precisely after 69.
Otherwise, it would start overwriting segment 70 before stopping
via 'q' or break signal.
So, in order to generate segments 31-69, you will set
segment_start_number to 31 and the segment_limit to 38.
This causes the muxer to write and complete segment 69
in the exact same way like when it would be creating segment
70, but without starting to write segment 70 - which would
destroy
the existing segment 70 (which is good already).
Buy you have to seek in the input to achieve that, don't you? And
you can
just as easily specify the input duration to not overwrite segment
70...
Here we're getting at the core of the problem: When specifying -to or
a duration, you cannot achieve the exact same "cut" like the segment
muxer does it. It usually cuts by video key frames. And when it does
the cut it also cuts all other streams at that moment. Timestamp
offsets between video and audio are common, often the audio is
somewhat ahead of the video (specifically in case of TV broadcast
streams), but it varies. Also, when transcoding before segmenting,
it's not easy to predict the exact video key frame times.
For each segment n to segment n+1 the cut must be done in a way
that for each stream, there is neither a duplicate nor a missing
frame. That is what makes it non-trivial.
In the end, it always comes down to the same conclusion: the only
way to get the exact same cut point through all streams like the
segment muxer does it - is to let the segment muxer do it.
At least that was my conclusion from intensive work on this, some
years ago.
But this is also a problem for the first segment. You can set
segment_start_number to 31, but you still have to know where to seek
exactly, because setting segment_start_number alone won't make the segment
muxer "ignore" data for the first 30 segments...
So what does not make sense to me is that you want to rely on segment
muxer behaviour for ending the segments, but not for starting them. The
way I see it you either know exactly where the segments supposed to start
and end, in which case you don't really need this option, because you
can set -ss and -to as you need. Or you don't, in which case this option
alone won't help you.
If you want to implement the segment limit, you have to make sure
the
ffmpeg encoding process stops after the last segment. One idea is
to
return an error if the segment limit is reached.
Maybe an EOF/EOS error?
I am not sure if it is legal for a muxer to return AVERROR_EOF... Maybe
I'd simply return AVERROR_INVALIDDATA.
Another possibility is that if you limit the number of segments, then you
make all remaining data go to the last segment. I don't know if that
helps or hurts any actual use case :)
Regards,
Marton
Thanks,
sw
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
To unsubscribe, visit link above, or email
ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
To unsubscribe, visit link above, or email
ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
