From 925cbeaadffddbb42dbedc1a61230a4db97c8a44 Mon Sep 17 00:00:00 2001
From: Benjamin Otte <otte@redhat.com>
Date: Wed, 28 Mar 2018 15:43:43 +0200
Subject: [PATCH] renderer: Track the previous node

... and diff the previous node with the current one to determine the
clip region.

This doubles the work necessary to track clip regions, but the following
commits will clean that up.
---
 gsk/gskrenderer.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/gsk/gskrenderer.c b/gsk/gskrenderer.c
index c011fb6cf2..e4945feb5e 100644
--- a/gsk/gskrenderer.c
+++ b/gsk/gskrenderer.c
@@ -66,6 +66,7 @@ typedef struct
   GObject parent_instance;
 
   GdkSurface *surface;
+  GskRenderNode *prev_node;
   GskRenderNode *root_node;
   GdkDisplay *display;
 
@@ -375,6 +376,8 @@ gsk_renderer_unrealize (GskRenderer *renderer)
 
   GSK_RENDERER_GET_CLASS (renderer)->unrealize (renderer);
 
+  g_clear_pointer (&priv->prev_node, gsk_render_node_unref);
+
   priv->is_realized = FALSE;
 }
 
@@ -465,32 +468,37 @@ gsk_renderer_render (GskRenderer          *renderer,
                      const cairo_region_t *region)
 {
   GskRendererPrivate *priv = gsk_renderer_get_instance_private (renderer);
-  cairo_region_t *real_region;
+  cairo_region_t *clip;
 
   g_return_if_fail (GSK_IS_RENDERER (renderer));
   g_return_if_fail (priv->is_realized);
   g_return_if_fail (GSK_IS_RENDER_NODE (root));
   g_return_if_fail (priv->root_node == NULL);
 
-  priv->root_node = gsk_render_node_ref (root);
-
-  if (region == NULL || GSK_RENDERER_DEBUG_CHECK (renderer, FULL_REDRAW))
+  if (region == NULL || priv->prev_node == NULL || GSK_RENDERER_DEBUG_CHECK (renderer, FULL_REDRAW))
     {
-      real_region = cairo_region_create_rectangle (&(GdkRectangle) {
-                                                       0, 0,
-                                                       gdk_surface_get_width (priv->surface),
-                                                       gdk_surface_get_height (priv->surface)
-                                                   });
-
-      GSK_RENDERER_GET_CLASS (renderer)->render (renderer, root, real_region);
-
-      cairo_region_destroy (real_region);
+      clip = cairo_region_create_rectangle (&(GdkRectangle) {
+                                                0, 0,
+                                                gdk_surface_get_width (priv->surface),
+                                                gdk_surface_get_height (priv->surface)
+                                            });
     }
   else
     {
-      GSK_RENDERER_GET_CLASS (renderer)->render (renderer, root, region);
+      clip = cairo_region_copy (region);
+      gsk_render_node_diff (priv->prev_node, root, clip);
+
+      if (cairo_region_is_empty (clip))
+        {
+          cairo_region_destroy (clip);
+          return;
+        }
     }
 
+  priv->root_node = gsk_render_node_ref (root);
+
+  GSK_RENDERER_GET_CLASS (renderer)->render (renderer, root, clip);
+
 #ifdef G_ENABLE_DEBUG
   if (GSK_RENDERER_DEBUG_CHECK (renderer, RENDERER))
     {
@@ -508,7 +516,9 @@ gsk_renderer_render (GskRenderer          *renderer,
     }
 #endif
 
-  g_clear_pointer (&priv->root_node, gsk_render_node_unref);
+  g_clear_pointer (&priv->prev_node, gsk_render_node_unref);
+  priv->prev_node = priv->root_node;
+  priv->root_node = NULL;
 }
 
 /*< private >
-- 
2.30.2