Hello all,
I’ve implemented a solution for a problem then creating WebVTT segments with
the segment muxer, but before I submit a patch I wanted to check back for
opinions about the approach.
THE REQUIREMENT
I need to do HLS segmentation for video/audio while additionally creating one
or more subtitle playlists and segments. Very simplified it’s like this:
ffmpeg -i INPUT
-map 0:0 -map 0:2 -c:v:0 copy -c:a:0 copy -f segment -segment_format mpegts
-segment_time 6 -y "segment%d.ts"
-map 0:3 -c:0 webvtt -f segment -segment_format webvtt -segment_time 6
-write_empty_segments 1 -y "sub_segment3%d.vtt"
THE PROBLEM
The mpegts segments are being created in a constant and regular way.
But not necessearily in the case of subtitles: When hitting a time range
without any subtitle event, the segmenter stops generating subtitle segments,
even when write_empty_segments is configured.
It’s not that write_empty_segments would not be working, though: As soon as a
new subtitle event reaches the muxer, it catches up by creating the missed
empty segments all at once. This might be OK for precreating content for
HLS/VOD scenarios but it’s a behavioral bug when you rely on having accurate
results while the process is running.
THE CAUSE
That’s rather trivial: When there are no subtitle events, there are no packets
arriving at the segment muxer (for vtt) and there’s probably nothiing that the
segment muxer could do about it.
A REASONABLE APPROACH?
After some rather ugly ideas I found that the best way to bring some more
action into the vtt segmenter might be to map the video stream in addition to
the subtitle stream – not with the purpose of outputting the video – just to
ensure regular activity in the segment muxer.
Also the segment muxer has implementation for having a ‚reference_stream‘ and
the video stream is supposed to be this anyway.
Remains one problem: The WebVTT muxer (libavformat/webvttenc.c) errors when
there’s more than a single stream. ("Exactly one WebVTT stream is needed.)
And that’s the change that I’ve done: Removed the single-stream limit from the
webvtt muxer and simply discard packets from other streams instead of failing.
What do you think about this?
Or does anybody have a better idea?
Thanks,
softworkz
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