[FFmpeg-trac] #4048(avformat:new): segment: Handle WebVTT as per HLS spec

[FFmpeg]

#4048: segment: Handle WebVTT as per HLS spec
-------------------------------------+------------------------------------
             Reporter:  11rcombs     |                    Owner:
                 Type:  enhancement  |                   Status:  new
             Priority:  normal       |                Component:  avformat
              Version:  git-master   |               Resolution:
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+------------------------------------
Changes (by 11rcombs):

 * keywords:  hls webvtt =>


Comment:

 It's not so much a bug as the lack of support for a newly-specified
 behavior. But let's use this {{{test.vtt}}} file as an example:
 {{{
 00:00.000 --> 00:07.000
 Test line 1, spans segments 0 and 1

 00:07.000 --> 00:08.000
 Test line 2, contained in segment 1

 00:16.000 --> 00:18.000
 Test line 3, should be in segment 3 after an empty segment 2

 00:18.000 --> 00:40.000
 Test line 4, should span several segments
 }}}

 Let's run {{{ffmpeg -i in.vtt -f segment -segment_time 5 -segment_format
 webvtt -scodec copy out-%05d.vtt}}}.

 We get:
 out-00000.vtt
 {{{
 WEBVTT

 00:00.000 --> 00:07.000
 Test line 1, spans segments 0 and 1
 }}}
 out-00001.vtt
 {{{
 WEBVTT

 00:07.000 --> 00:08.000
 Test line 2, contained in segment 1
 }}}
 out-00002.vtt
 {{{
 WEBVTT

 00:16.000 --> 00:18.000
 Test line 3, should be in segment 3 after an empty segment 2
 }}}
 out-00003.vtt
 {{{
 WEBVTT

 00:18.000 --> 00:40.000
 Test line 4, should span several segments
 }}}

 The correct output for HLS would be:
 out-00000.vtt
 {{{
 WEBVTT

 00:00.000 --> 00:07.000
 Test line 1, spans segments 0 and 1
 }}}
 out-00001.vtt
 {{{
 WEBVTT

 00:00.000 --> 00:07.000
 Test line 1, spans segments 0 and 1

 00:07.000 --> 00:08.000
 Test line 2, contained in segment 1
 }}}
 out-00002.vtt
 {{{
 WEBVTT

 }}}
 out-00003.vtt
 {{{
 WEBVTT

 00:18.000 --> 00:40.000
 Test line 4, should span several segments
 }}}
 out-00004.vtt
 {{{
 WEBVTT

 00:18.000 --> 00:40.000
 Test line 4, should span several segments
 }}}
 <etc…, with more duplicates up to out-00007.vtt>

 The two behaviors that need adding (writing empty segments, and
 duplicating packets spanning multiple segments) seem like they could
 reasonably be separate options. The writing-empty-segments behavior seems
 sensible for generic subtitle segmenting (in order to have predictable
 segment durations), whereas duplicating packets is pretty WebVTT-in-HLS-
 specific.

--
Ticket URL: <https://trac.ffmpeg.org/ticket/4048#comment:3>
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