TOOLS: improve autocrop.lua

It now inserts no filters and does nothing until the hot-key is pressed.
This makes it more suitable to be put in ~/.mpv/lua.

When the hot-key is pressed, it now inserts the cropdetect filter and
waits 1 second (or a --lua-opts specified duration) before gathering
the cropdetect metadata and inserting the appropriate crop filter. A
second press of the hotkey removes the crop.

TOOLS/lua/autocrop.lua

@@ -1,11 +1,77 @@
-mp.command('vf add @autocrop.cropdetect:lavfi=graph="cropdetect=limit=24:round=2:reset=0"')
+script_name=string.gsub(mp.get_script_name(),"lua/","")
+cropdetect_label=string.format("%s-cropdetect",script_name)
+crop_label=string.format("%s-crop",script_name)
 
-function update_crop_handler()
-    cropdetect_metadata=mp.get_property_native("vf-metadata/autocrop.cropdetect")
-    mp.command(string.format('vf add @autocrop.crop:crop=%s:%s:%s:%s',
+-- number of seconds to gather cropdetect data
+detect_seconds = tonumber(mp.get_opt(string.format("%s.detect_seconds", script_name)))
+if not detect_seconds then
+    detect_seconds = 1
+end
+
+function del_filter_if_present(label)
+    -- necessary because mp.command('vf del @label:filter') raises an
+    -- error if the filter doesn't exist
+    local vfs = mp.get_property_native('vf')
+    for i,vf in pairs(vfs) do
+	if vf['label'] == label then
+	    table.remove(vfs, i)
+	    mp.set_property_native('vf', vfs)
+	    return true
+	end
+    end
+    return false
+end
+
+function autocrop_start()
+    -- if there's a crop filter, just remove it and exit
+    if del_filter_if_present(crop_label) then
+        return
+    end
+    -- insert the cropdetect filter
+    ret=mp.command(
+	string.format(
+	    'vf add @%s:lavfi=graph="cropdetect=limit=24:round=2:reset=0"',
+	    cropdetect_label
+	)
+    )
+    -- wait to gather data
+    mp.add_timeout(detect_seconds, do_crop)
+end
+
+function do_crop()
+    require 'mp.msg'
+    -- get the metadata
+    local cropdetect_metadata = mp.get_property_native(
+	string.format('vf-metadata/%s', cropdetect_label)
+    )
+    -- use it to crop if its valid
+    if cropdetect_metadata then
+	if cropdetect_metadata['lavfi.cropdetect.w']
+	    and cropdetect_metadata['lavfi.cropdetect.h']
+	    and cropdetect_metadata['lavfi.cropdetect.x']
+	    and cropdetect_metadata['lavfi.cropdetect.y']
+	then
+            mp.command(string.format('vf add @%s:crop=%s:%s:%s:%s',
+                                     crop_label,
                                      cropdetect_metadata['lavfi.cropdetect.w'],
                                      cropdetect_metadata['lavfi.cropdetect.h'],
                                      cropdetect_metadata['lavfi.cropdetect.x'],
                                      cropdetect_metadata['lavfi.cropdetect.y']))
+        else
+            mp.msg.error(
+		"Got empty crop data. You might need to increase detect_seconds."
+	    )
+	end
+    else
+        mp.msg.error(
+            "No crop data. Was the cropdetect filter successfully inserted?"
+        )
+        mp.msg.error(
+            "Does your version of ffmpeg/libav support AVFrame metadata?"
+        )
+    end
+    -- remove the cropdetect filter
+    del_filter_if_present(cropdetect_label)
 end
-mp.add_key_binding("C","update_crop",update_crop_handler)
+
+mp.add_key_binding("C","auto_crop",autocrop_start)