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manpage: document that backward playback from the end does not work

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Not specifying a --start or using --start=100% with
--play-direction=backward usually does not work. The demuxer gets no
packets and immediately enters EOF state, which then hangs because
backward playback mode neither considers this mode, nor propagates the
EOF.

As far as demuxer implementations are concerned, this behavior is OK and
even wanted. Seeking near the end with SEEK_FORWARD set is allowed not
to return any packets (so a normal relative forward seek as done by the
user would end playback). Seeking exactly to the end or past it without
SEEK_FORWARD set is probably also sane.

Another vaguely related issue is that a backward seek during playback
start does not "establish" the demux position correctly: if stream A
hits the next keyframe and seeks back, while stream B has not had a
chance to read a packet yet, then stream B will never try to read from
the old position. The effect is that stream B (and thus playback) will
effectively miss the seek target. This is "random" because it depends on
the order and number of packet read calls made by the decoders.

Fixing this is probably hard, and requires extending the already complex
state machine with more states, so turn the manpage into a TODO list for
now.
This commit is contained in:
wm4 2019-05-20 02:54:06 +02:00
parent 5ebbde7327
commit 6f7260d29c
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DOCS/man/options.rst
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@ -489,6 +489,14 @@ Playback Control
A workaround is to remux to a format like mkv, which enforces packet
boundaries. Making mpv cache the entire file in memory also works.
- Starting from the very end of the file often does not work. This includes
starting playback of a file while backward playback mode is enabled, and
no initial seek is set with ``--start``. You need to use something like
``--start=99%`` to try to get the end.
- Starting backward playback at a specific time position without pre-caching
the entire file usually misses the seek target due to a race condition.
Tuning:
- Remove all ``--vf``/``--af`` filters you have set. Disable deinterlacing.