[FFmpeg-devel] FIX: HLS Discontinuity / Non-Monotonous DTS

[Bodecs Bela]

2018.03.16. 19:52 keltezéssel, wm4 írta:
> On Fri, 16 Mar 2018 09:58:17 -0400
> Joe Koberg <joe at koberg.us> wrote:
>
>> Hello,
>>
>> This patch adds code to track and correct timestamp discontinuities, fixing
>> "non-monotonous dts" errors and timing issues with HLS playlists.
>>
>> For some time, FFmpeg has not properly handled discontinuous timestamps in
>> the MPEG-TS stream from Apple HLS playlists. Symptoms include inability to
>> remux to containers like MP4 or stream to RTMP endpoints; choppy, broken,
>> or sped-up playback; and voluminous "Non-monotonous DTS", "went backwards",
>> and "out of order" messages logged.  This is particularly troublesome in
>> the context of HLS, as the format allows insertion of independently-sourced
>> fragments for ad-insertion or to concatenate ongoing video to a stream.
>>
>> Over the years, many users have encountered and reported this issue in
>> different ways.
>>
>>    * https://trac.ffmpeg.org/ticket/5419 "HLS EXT-X-DISCONTINUITY tag is not
>> supported"
>>    * https://trac.ffmpeg.org/ticket/6613
>>    * https://trac.ffmpeg.org/ticket/6810
>>    * https://trac.ffmpeg.org/ticket/6709
>>    * https://trac.ffmpeg.org/ticket/6365
>>    * https://trac.ffmpeg.org/ticket/5236
>>    * https://github.com/Bilibili/ijkplayer/issues/2565
>>    * https://stackoverflow.com/questions/49289394/downloading-ts-stream-with-
>> ext-x-discontinuity-sequence-ffmpeg
>>    * etc...
>>
>>
>> It appears that the -dts_delta_threshold checking code is quite similar in
>> intent, and adjacent to this code, but the HLS demuxer lacks the
>> AVFMT_TS_DISCONT flag, effectively disabling that option.  Even when
>> AVFMT_TS_DISCONT is set on the input format (e.g. MPEG-TS),
>> "-dts_delta_threshold 0" produces out-of-sync audio in our tests.  That
>> code has been left as-is for now.  The flag probably should be set on the
>> HLS format, though.
>>
>> In this fix, any backwards DTS jump is considered an error, as is any
>> forward jump of DTS or PTS by more than `dts_monotonicity_threshold`
>> microseconds (really, AV_TIME_BASE units, defaulting to 1000000 = 1
>> second). The delta from the predicted value is updated and applied to all
>> subsequent packets, with the assumption that video was stitched together
>> from continuous runs.
>>
>> With this patch in place and the -force_dts_monotonicity flag set, we can
>> successfuly transcode arbitrary HLS playlists to MP4, FLV, or other
>> timestamped containers when we could not before.  The playback presentation
>> of the result is smooth, properly timed, and in-sync in Quicktime Player,
>> VLC, and ffplay with correctly reported and the file seekable.
>>
>> Interaction with flags such as -copyts, -copytb, and others has not been
>> fully analyzed.
>>
>> We have prepared a test HLS playlist that reveals the issue.
>>
>> How to reproduce:
>>
>>    ffmpeg -i https://s3.amazonaws.com/playon-test-videos/discont_
>> test_new/discont_test.m3u8 -c copy -y output.mp4
>>
>> With patch:
>>
>>    ffmpeg -force_dts_monotonicity -i https://s3.amazonaws.com/
>> playon-test-videos/discont_test_new/discont_test.m3u8 -c copy -y output.mp4
>>
>>
>> Please let me know if I've missed anything!
> So I haven't looked too closely at this. It's possible that ffmpeg.c
> should handle discontinuities better, or has a regression with handling
> it. But besides that, shouldn't the HLS demuxer just be changed to
> parse the discontinuity markers, and add them as packet flags? Then
> these "clever" heuristics (which are just really hard to maintain)
> wouldn't be needed, and also applications using libavformat as library
> would benefit from it.
>
> Your patch may or may not have a benefit on top of it (for normal .ts
> files etc.), and the HLS demuxer may or may not have additional issues,
> but I think that's orthogonal.
>
> There isn't such a discontinuity packet flag - that would have to be
> added.
maybe this  discontinuity packet flag could be used in concat demuxer also?
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