osc: make transparency of background-box configurable

see boxalpha option

DOCS/man/en/osc.rst

@@ -6,7 +6,7 @@ offer basic mouse-controllability. It is intended to make interaction easier
 for new users and to enable precise and direct seeking.
 
 The OSC is enabled by default if mpv was compiled with lua support. It can be
-disabled using ``--osc=no``.
+disabled entirely using the ``--osc=no`` option.
 
 Using the OSC
 -------------
@@ -176,6 +176,10 @@ Configurable parameters
     | Default: 0
     | Horizontal alignment, -1 (left) to 1 (right)
 
+``boxalpha``
+    | Default: 80
+    | Alpha of the background box, 0 (opaque) to 255 (fully transparent)
+
 ``hidetimeout``
     | Default: 500
     | Duration in ms until the OSC hides if no mouse movement, negative value

mpvcore/lua/osc.lua

@@ -18,6 +18,7 @@ local user_opts = {
     vidscale = true,                        -- scale the controller with the video?
     valign = 0.8,                           -- vertical alignment, -1 (top) to 1 (bottom)
     halign = 0,                             -- horizontal alignment, -1 (left) to 1 (right)
+    boxalpha = 80,                          -- alpha of the background box, 0 (opaque) to 255 (fully transparent)
     hidetimeout = 500,                      -- duration in ms until the OSC hides if no mouse movement, negative value disables autohide
     fadeduration = 200,                     -- duration of fade out in ms, 0 = no fade
     deadzonesize = 0,                       -- size of deadzone
@@ -675,8 +676,8 @@ function osc_init()
     --
 
     local metainfo = {}
-    metainfo.alpha1 = 80
-    metainfo.alpha3 = 80
+    metainfo.alpha1 = user_opts.boxalpha
+    metainfo.alpha3 = user_opts.boxalpha
     register_box(posX, posY, 5, osc_w, osc_h, osc_r, osc_styles.box, metainfo)
 
     --