demux: add option to create CC tracks eagerly

We don't hope to auto-detect them at load time, as that would be too
much of a pain - even FFmpeg requires fetching and parsing of video
packets, and exposes the information only via deprecated API.

But there still needs to be a way to select them by default. This is
also needed to get the first CC packet at all (without seeking back).

This commit also attempts to clean up locking a bit, which is a PITA,
but it's better be careful & clean.

1 files changed, 12 insertions, 0 deletions

diff --git a/DOCS/man/options.rst b/DOCS/man/options.rst
index 8408245044..a6c2136947 100644
--- a/DOCS/man/options.rst
+++ b/DOCS/man/options.rst
@@ -2134,6 +2134,18 @@ Subtitles
 
     Default: ``no``.
 
+``--sub-create-cc-track=<yes|no>``
+    For every video stream, create a closed captions track (default: no). The
+    only purpose is to make the track available for selection at the start of
+    playback, instead of creating it lazily. This applies only to
+    ``ATSC A53 Part 4 Closed Captions`` (displayed by mpv as subtitle tracks
+    using the codec ``eia_608``). The CC track is marked "default" and selected
+    according to the normal subtitle track selection rules. You can then use
+    ``--sid`` to explicitly select the correct track too.
+
+    If the video stream contains no closed captions, or if no video is being
+    decoded, the CC track will remain empty and will not show any text.
+
 Window
 ------