Fallback fonts with atlas clearing. (#1790)

* Mechanism to define priority fallback fonts through lua AddFallbackFont
and ClearFallbackFonts.
* First search game defined fonts and then system fonts.

cont/LuaUI/callins.lua

@@ -94,6 +94,8 @@ CallInsList = {
 	"DrawScreen",
 	"DrawInMiniMap",
 
+	"FontsChanged",
+
 	"SunChanged",
 
 	"Explosion",

cont/LuaUI/widgets.lua

@@ -169,6 +169,7 @@ local flexCallIns = {
   'DrawScreenEffects',
   'DrawScreenPost',
   'DrawInMiniMap',
+  'FontsChanged',
   'SunChanged',
   'RecvSkirmishAIMessage',
 }
@@ -2167,6 +2168,18 @@ function widgetHandler:DownloadProgress(id, downloaded, total)
   end
 end
 
+
+--------------------------------------------------------------------------------
+--
+--  Font call-ins
+--
+
+function widgetHandler:FontsChaged()
+  for _,w in ripairs(self.FontsChangedList) do
+    w:FontsChanged()
+  end
+end
+
 --------------------------------------------------------------------------------
 --------------------------------------------------------------------------------
 

cont/base/springcontent/LuaGadgets/callins.lua

@@ -167,6 +167,8 @@ CALLIN_LIST = {
 	"DrawProjectile",
 	"DrawMaterial",
 
+	"FontsChanged",
+
 	"SunChanged",
 
 	-- unsynced message callins

cont/base/springcontent/LuaGadgets/gadgets.lua

@@ -2198,6 +2198,15 @@ end
 --------------------------------------------------------------------------------
 --------------------------------------------------------------------------------
 
+function gadgetHandler:FontsChanged()
+  for _,g in r_ipairs(self.FontsChangedList) do
+    g:FontsChanged()
+  end
+end
+
+--------------------------------------------------------------------------------
+--------------------------------------------------------------------------------
+
 gadgetHandler:Initialize()
 
 --------------------------------------------------------------------------------

rts/Lua/LuaFonts.cpp

@@ -25,6 +25,8 @@ bool LuaFonts::PushEntries(lua_State* L)
 
 	REGISTER_LUA_CFUNC(LoadFont);
 	REGISTER_LUA_CFUNC(DeleteFont);
+	REGISTER_LUA_CFUNC(AddFallbackFont);
+	REGISTER_LUA_CFUNC(ClearFallbackFonts);
 
 	return true;
 }
@@ -205,6 +207,49 @@ int LuaFonts::DeleteFont(lua_State* L)
 	return meta_gc(L);
 }
 
+/*** Adds a fallback font for the font rendering engine.
+ *
+ * Fonts added first will have higher priority.
+ * When a glyph isn't found when rendering a font, the fallback fonts
+ * will be searched first, otherwise os fonts will be used.
+ *
+ * The application should listen for the unsynced 'FontsChanged' callin so
+ * modules can clear any already reserved display lists or other relevant
+ * caches.
+ *
+ * Note the callin won't be executed at the time of calling this method,
+ * but later, on the Update cycle (before other Update and Draw callins).
+ *
+ * @function gl.AddFallbackFont
+ * @string filePath VFS path to the file, for example "fonts/myfont.ttf". Uses VFS.RAW_FIRST access mode.
+ * @treturn bool success
+ */
+int LuaFonts::AddFallbackFont(lua_State* L)
+{
+	RECOIL_DETAILED_TRACY_ZONE;
+
+	const auto font = luaL_checkstring(L, 1);
+
+	const bool res = CFontTexture::AddFallbackFont(font);
+	lua_pushboolean(L, res);
+	return 1;
+}
+
+/*** Clears all fallback fonts.
+ *
+ * See the note at 'AddFallbackFont' about the 'FontsChanged' callin,
+ * it also applies when calling this method.
+ *
+ * @function gl.ClearFallbackFonts
+ * @treturn nil
+ */
+int LuaFonts::ClearFallbackFonts(lua_State* L)
+{
+	RECOIL_DETAILED_TRACY_ZONE;
+
+	CFontTexture::ClearFallbackFonts();
+	return 0;
+}
 
 /******************************************************************************/
 /******************************************************************************/

rts/Lua/LuaFonts.h

@@ -22,6 +22,8 @@ class LuaFonts {
 	private: // call-outs
 		static int LoadFont(lua_State* L);
 		static int DeleteFont(lua_State* L);
+		static int AddFallbackFont(lua_State* L);
+		static int ClearFallbackFonts(lua_State* L);
 
 	private: // userdata call-outs
 		static int Print(lua_State* L);

rts/Lua/LuaHandle.cpp

@@ -2429,6 +2429,27 @@ void CLuaHandle::ViewResize()
 	RunCallIn(L, cmdStr, 1, 0);
 }
 
+/*** Called whenever fonts are updated. Signals the game display lists
+ *   and other caches should be discarded.
+ *
+ * Gets called before other Update and Draw callins.
+ *
+ * @function FontsChanged
+ */
+void CLuaHandle::FontsChanged()
+{
+	RECOIL_DETAILED_TRACY_ZONE;
+	LUA_CALL_IN_CHECK(L);
+	luaL_checkstack(L, 2, __func__);
+
+	static const LuaHashString cmdStr(__func__);
+
+	if (!cmdStr.GetGlobalFunc(L))
+		return;
+
+	RunCallIn(L, cmdStr, 0, 0);
+}
+
 /***
  * @function SunChanged
  */

rts/Lua/LuaHandle.h

@@ -237,6 +237,8 @@ class CLuaHandle : public CEventClient
 
 		void ViewResize() override;
 
+		void FontsChanged() override;
+
 		void SunChanged() override;
 
 		void DrawGenesis() override;