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

[FFmpeg]

#4048: segment: Handle WebVTT as per HLS spec
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             Reporter:  11rcombs  |                     Type:  enhancement
               Status:  new       |                 Priority:  normal
            Component:  avformat  |                  Version:  git-master
             Keywords:            |               Blocked By:
             Blocking:            |  Reproduced by developer:  0
Analyzed by developer:  0         |
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 As per the [http://tools.ietf.org/html/draft-pantos-http-live-
 streaming-13#section-4 HLS spec]:
 {{{
    Subtitle segments MUST be formatted as WebVTT [WebVTT] files.  Each
    subtitle segment MUST contain all subtitle cues that are intended to
    be displayed during the period indicated by the segment EXTINF
    duration.  The start time offset and end time offset of each cue MUST
    indicate the total display time for that cue, even if that time range
    extends beyond the EXTINF duration.  A WebVTT segment MAY contain no
    cues; this indicates that no subtitles are to be displayed during
    that period.
 }}}
 This means that the segment (or HLS) muxer needs to handle two particular
 cases, when muxing WebVTT for HLS:
 1. No frames exist during a segment's duration, so a blank segment should
 be written.
 2. A frame spans two or more different segments, so it should be written
 in all of them.
 This behavior could be triggered by an option, or automatically when the
 underlying format is WebVTT.

 Relatedly, the WebVTT muxer should provide an option to write the X
 -TIMESTAMP-MAP header for HLS synchronization.

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Ticket URL: <https://trac.ffmpeg.org/ticket/4048>
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