client API, lua: add new API for setting OSD overlays

Lua scripting has an undocumented mp.set_osd_ass() function, which is
used by osc.lua and console.lua. Apparently, 3rd party scripts also use
this. It's probably time to make this a public API.

The Lua implementation just bypassed the libmpv API. To make it usable
by any type of client, turn it into a command, "osd-overlay".

There's already a "overlay-add". Ignore it (although the manpage admits
guiltiness). I don't really want to deal with that old command. Its main
problem is that it uses global IDs, while I'd like to avoid that scripts
mess with each others overlays (whether that is accidentally or
intentionally). Maybe "overlay-add" can eventually be merged into
"osd-overlay", but I'm too lazy to do that now.

Scripting now uses the commands. There is a helper to manage OSD
overlays. The helper is very "thin"; I only want to force script authors
to use the ID allocation, which may help with putting multiple scripts
into a single .lua file without causing conflicts (basically, avoiding
singletons within a script's environment). The old set_osd_ass() is
emulated with the new API.

The JS scripting wrapper also provides a set_osd_ass() function, which
calls internal mpv API. Comment that part (to keep it compiling), but
I'm leaving it to @avih to finish the change.

1 files changed, 35 insertions, 0 deletions

diff --git a/DOCS/man/lua.rst b/DOCS/man/lua.rst
index d98edf38a9..0829e8be4b 100644
--- a/DOCS/man/lua.rst
+++ b/DOCS/man/lua.rst
@@ -536,6 +536,41 @@ are useful only in special situations.
     Undo a previous registration with ``mp.register_script_message``. Does
     nothing if the ``name`` wasn't registered.
 
+``mp.create_osd_overlay(format)``
+    Create an OSD overlay. This is a very thin wrapper around the ``osd-overlay``
+    command. The function returns a table, which mostly contains fields that
+    will be passed to ``osd-overlay``. The ``format`` parameter is used to
+    initialize the ``format`` field. The ``data`` field contains the text to
+    be used as overlay. For details, see the ``osd-overlay`` command.
+
+    In addition, it provides the following methods:
+
+    ``update()``
+        Commit the OSD overlay to the screen, or in other words, run the
+        ``osd-overlay`` command with the current fields of the overlay table.
+
+    ``remove()``
+        Remove the overlay from the screen. A ``update()`` call will add it
+        again.
+
+    Example:
+
+    ::
+
+        ov = mp.create_osd_overlay("ass-events")
+        ov.data = "{\\an5}{\\b1}hello world!"
+        ov:update()
+
+    The advantage of using this wrapper (as opposed to running ``osd-overlay``
+    directly) is that the ``id`` field is allocated automatically.
+
+``mp.get_osd_size()``
+    Returns a tuple of ``osd_width, osd_height, osd_par``. The first two give
+    the size of the OSD in pixels (for video ouputs like ``--vo=xv``, this may
+    be "scaled" pixels). The third is the display pixel aspect ratio.
+
+    May return invalid/nonsense values if OSD is not initialized yet.
+
 mp.msg functions
 ----------------