Simple mpv script to fix subtitle delay (ie. signs starting too soon, too late)

fix_subdelay.lua

--[[
	Simple script to fix sub-delay on offset subs
	
	First, framestep to where the desired subtitle (sign) SHOULD start, press ALT+Z
	Next, framestep to where the desired subtitle (sign) DOES start, press ALT+Z
	Done.

	You may press ALT+Z twice in succession to zero out sub-delay.
]]--

local start_time = nil

function on_script_action()
  local playback_time = mp.get_property_native("playback-time")

  if start_time == playback_time then
  	mp.osd_message("Identical frames, clearing sub-delay")
  	mp.set_property_native("sub-delay", 0)
  	start_time = nil
  
  elseif start_time ~= nil then
  	-- Take current sub-delay into account
  	local sub_delay = mp.get_property_native("sub-delay")
  	local delta = start_time - playback_time + sub_delay
  	mp.set_property_native("sub-delay", delta)
  	mp.osd_message("Set sub-delay to " .. tostring(delta), 2)
  	start_time = nil

  else
  	start_time = playback_time
  	mp.osd_message("Picked expected frame, now pick the actual frame", 2)
  end
end

mp.add_key_binding("alt+z", "fix_subdelay_action", on_script_action)