player: add display sync mode

If this mode is enabled, the player tries to strictly synchronize video
to display refresh. It will adjust playback speed to match the display,
so if you play 23.976 fps video on a 24 Hz screen, playback speed is
increased by approximately 1/1000. Audio wll be resampled to keep up
with playback.

This is different from the default sync mode, which will sync video to
audio, with the consequence that video might skip or repeat a frame once
in a while to make video keep up with audio.

This is still unpolished. There are some major problems as well; in
particular, mkv VFR files won't work well. The reason is that Matroska
is terrible and rounds timestamps to milliseconds. This makes it rather
hard to guess the framerate of a section of video that is playing. We
could probably fix this by just accepting jittery timestamps (instead
of explicitly disabling the sync code in this case), but I'm not ready
to accept such a solution yet.

Another issue is that we are extremely reliant on OS video and audio
APIs working in an expected manner, which of course is not too often
the case. Consequently, the new sync mode is a bit fragile.

1 files changed, 6 insertions, 0 deletions

diff --git a/DOCS/man/input.rst b/DOCS/man/input.rst
index 10be785ec5..9910d6b9c0 100644
--- a/DOCS/man/input.rst
+++ b/DOCS/man/input.rst
@@ -830,6 +830,12 @@ Property list
     Factor multiplied with ``speed`` at which the player attempts to play the
     file. Usually it's exactly 1. (Display sync mode will make this useful.)
 
+    OSD formatting will display it in the form of ``+1.23456%``, with the number
+    being ``(raw - 1) * 100`` for the given raw property value.
+
+``display-sync-active``
+    Return whether ``--video-sync=display`` is actually active.
+
 ``filename``
     Currently played file, with path stripped. If this is an URL, try to undo
     percent encoding as well. (The result is not necessarily correct, but