GUI: implement canvas caching

changelog

@@ -47,6 +47,8 @@ Version 1.13.8+dev:
    * Fixed delay or clients gettings stuck when starting a mp game (Bug #1674)
  * Performance:
    * Rewrote the FPS cap implementation. This greatly improves smoothness ingame.
+   * Implemented GUI canvas caching. It speeds up multiple areas, but especially
+     the story screen.
  * Units:
    * Added new lvl0 Giant Scorpling, leveling into the Giant Scorpion.
  * User Interface:

players_changelog

@@ -32,6 +32,8 @@ Version 1.13.8+dev:
 
  * Performance:
    * Rewrote the FPS cap implementation. This greatly improves smoothness ingame.
+   * Implemented GUI canvas caching. It speeds up multiple areas, but especially
+     the story screen.
 
  * User Interface:
    * Unit recall dialog now sorts the units by both level and required XP for

src/gui/core/canvas.cpp

@@ -34,6 +34,8 @@
 #include "video.hpp"
 #include "wml_exception.hpp"
 
+#include <iterator>
+
 namespace gui2
 {
 
@@ -1391,9 +1393,13 @@ void canvas::draw(const bool force)
 		variables_.add("height", wfl::variant(h_));
 	}
 
-	// create surface
-	DBG_GUI_D << "Canvas: create new empty canvas.\n";
-	canvas_.assign(create_neutral_surface(w_, h_));
+	if(!canvas_.null()) {
+		DBG_GUI_D << "Canvas: use cached canvas.\n";
+	} else {
+		// create surface
+		DBG_GUI_D << "Canvas: create new empty canvas.\n";
+		canvas_.assign(create_neutral_surface(w_, h_));
+	}
 
 	SDL_DestroyRenderer(renderer_);
 
@@ -1407,6 +1413,10 @@ void canvas::draw(const bool force)
 		shape->draw(canvas_, renderer_, variables_);
 	}
 
+	// The shapes have been drawn and the draw result has been cached. Clear the list.
+	std::copy(shapes_.begin(), shapes_.end(), std::back_inserter(drawn_shapes_));
+	shapes_.clear();
+
 	SDL_RenderPresent(renderer_);
 
 	is_dirty_ = false;
@@ -1484,12 +1494,29 @@ void canvas::parse_cfg(const config& cfg)
 
 void canvas::clear_shapes(const bool force)
 {
-	const auto iter = std::remove_if(shapes_.begin(), shapes_.end(), [&force](const shape_ptr s)
+	auto iter = std::remove_if(shapes_.begin(), shapes_.end(), [force](const shape_ptr s)
 	{
 		return !s->immutable() && !force;
 	});
-
 	shapes_.erase(iter, shapes_.end());
+
+	iter = std::remove_if(drawn_shapes_.begin(), drawn_shapes_.end(), [force](const shape_ptr s)
+	{
+		return !s->immutable() && !force;
+	});
+	drawn_shapes_.erase(iter, drawn_shapes_.end());
+}
+
+void canvas::invalidate_cache()
+{
+	canvas_.assign(nullptr);
+
+	if(shapes_.empty()) {
+		shapes_.swap(drawn_shapes_);
+	} else {
+		std::copy(drawn_shapes_.begin(), drawn_shapes_.end(), std::inserter(shapes_, shapes_.begin()));
+		drawn_shapes_.clear();
+	}
 }
 
 /***** ***** ***** ***** ***** SHAPE ***** ***** ***** ***** *****/

src/gui/core/canvas.hpp

@@ -35,9 +35,6 @@ namespace gui2
  *
  * The class has a config which contains what to draw.
  *
- * NOTE we might add some caching in a later state, for now every draw cycle
- * does a full redraw.
- *
  * The copy constructor does a shallow copy of the shapes to draw.
  * a clone() will be implemented if really needed.
  */
@@ -121,6 +118,7 @@ class canvas
 	void set_cfg(const config& cfg, const bool force = false)
 	{
 		clear_shapes(force);
+		invalidate_cache();
 		parse_cfg(cfg);
 	}
 
@@ -142,6 +140,7 @@ class canvas
 	{
 		w_ = width;
 		set_is_dirty(true);
+		invalidate_cache();
 	}
 
 	unsigned get_width() const
@@ -153,6 +152,7 @@ class canvas
 	{
 		h_ = height;
 		set_is_dirty(true);
+		invalidate_cache();
 	}
 
 	unsigned get_height() const
@@ -169,6 +169,7 @@ class canvas
 	{
 		variables_.add(key, value);
 		set_is_dirty(true);
+		invalidate_cache();
 	}
 
 	void set_is_dirty(const bool is_dirty)
@@ -180,6 +181,10 @@ class canvas
 	/** Vector with the shapes to draw. */
 	std::vector<shape_ptr> shapes_;
 
+	/** All shapes which have been already drawn. Kept around in case
+	 * the cache needs to be invalidated. */
+	std::vector<shape_ptr> drawn_shapes_;
+
 	/**
 	 * The depth of the blur to use in the pre committing.
 	 *
@@ -223,6 +228,7 @@ class canvas
 	void parse_cfg(const config& cfg);
 
 	void clear_shapes(const bool force);
+	void invalidate_cache();
 };
 
 } // namespace gui2