Prevent viewport_invalidate from causing covered viewports to redraw and cache window and viewport visibility.

src/drawing/engines/SoftwareDrawingEngine.cpp

@@ -365,6 +365,8 @@ public:
             _rainDrawer.SetDPI(&_bitsDPI);
             _rainDrawer.Restore();
 
+			ResetWindowVisbilities();
+
             // Redraw dirty regions before updating the viewports, otherwise
             // when viewports get panned, they copy dirty pixels
             DrawAllDirtyBlocks();
@@ -528,6 +530,16 @@ private:
         _dirtyGrid.Blocks = new uint8[_dirtyGrid.BlockColumns * _dirtyGrid.BlockRows];
     }
 
+	static void ResetWindowVisbilities()
+    {
+		// reset window visibilty status to unknown
+		for (rct_window *w = g_window_list; w < gWindowNextSlot; w++)
+		{
+			w->visibility = VC_UNKNOWN;
+			if (w->viewport != NULL) w->viewport->visibility = VC_UNKNOWN;
+		}
+    }
+
     void DrawAllDirtyBlocks()
     {
         uint32  dirtyBlockColumns = _dirtyGrid.BlockColumns;

src/interface/viewport.c

@@ -1396,6 +1396,23 @@ void get_map_coordinates_from_pos(int screenX, int screenY, int flags, sint16 *x
  */
 void viewport_invalidate(rct_viewport *viewport, int left, int top, int right, int bottom)
 {
+	// if unknown viewport visibility, use the containing window to discover the status
+	if (viewport->visibility == VC_UNKNOWN)
+	{
+		for (rct_window *w = g_window_list; w < gWindowNextSlot; w++)
+		{
+			if (w->classification != WC_MAIN_WINDOW && w->viewport != NULL && w->viewport == viewport)
+			{
+				// note, window_is_visible will update viewport->visibility, so this should have a low hit count
+				if (!window_is_visible(w)) {
+					return;
+				}
+			}
+		}
+	}
+
+	if (viewport->visibility == VC_COVERED) return;
+
 	int viewportLeft = viewport->view_x;
 	int viewportTop = viewport->view_y;
 	int viewportRight = viewport->view_x + viewport->view_width;

src/interface/window.c

@@ -1464,6 +1464,8 @@ void window_zoom_out(rct_window *w)
  */
 void window_draw(rct_drawpixelinfo *dpi, rct_window *w, int left, int top, int right, int bottom)
 {
+	if (!window_is_visible(w)) return;
+
 	// Split window into only the regions that require drawing
 	if (window_draw_split(dpi, w, left, top, right, bottom))
 		return;
@@ -1476,8 +1478,6 @@ void window_draw(rct_drawpixelinfo *dpi, rct_window *w, int left, int top, int r
 	if (left >= right) return;
 	if (top >= bottom) return;
 
-	if (!window_is_visible(w)) return;
-
 	// Draw the window in this region
 	for (rct_window *v = w; v < RCT2_NEW_WINDOW; v++) {
 		// Don't draw overlapping opaque windows, they won't have changed
@@ -2482,10 +2482,16 @@ void window_update_textbox()
 
 bool window_is_visible(rct_window* w)
 {
+	// w->visibility is used to prevent repeat calculations within an iteration by caching the result
+
+	if (w->visibility == VC_VISIBLE) return true;
+	if (w->visibility == VC_COVERED) return false;
+
 	// only consider viewports, consider the main window always visible
 	if (w == NULL || w->viewport == NULL || w->classification == WC_MAIN_WINDOW)
 	{
-		// default to previous behavior
+		// default to previous behaviour
+		w->visibility = VC_VISIBLE;
 		return true;
 	}
 
@@ -2498,10 +2504,14 @@ bool window_is_visible(rct_window* w)
 			&& w_other->x + w_other->width >= w->x + w->width
 			&& w_other->y + w_other->height >= w->y + w->height)
 		{
+			w->visibility = VC_COVERED;
+			w->viewport->visibility = VC_COVERED;
 			return false;
 		}
 	}
 
-	// default to previous behavior
+	// default to previous behaviour
+	w->visibility = VC_VISIBLE;
+	w->viewport->visibility = VC_VISIBLE;
 	return true;
 }

src/interface/window.h

@@ -85,6 +85,7 @@ typedef struct rct_viewport {
 	uint8 zoom;						// 0x10
 	uint8 var_11;
 	uint16 flags;					// 0x12
+	uint8 visibility;				// VISIBILITY_CACHE
 } rct_viewport;
 
 /**
@@ -288,6 +289,7 @@ typedef struct rct_window {
 	sint8 var_4B8;
 	sint8 var_4B9;
 	uint8 colours[6];			// 0x4BA
+	uint8 visibility;			// VISIBILITY_CACHE
 } rct_window;
 
 #define RCT_WINDOW_RIGHT(w) (w->x + w->width)
@@ -493,6 +495,13 @@ enum {
 	MODAL_RESULT_OK
 };
 
+enum VISIBILITY_CACHE
+{
+	VC_UNKNOWN = 0,
+	VC_VISIBLE = 1,
+	VC_COVERED = 2
+};
+
 typedef void (*modal_callback)(int result);
 typedef void (*scenarioselect_callback)(const utf8 *path);