From 9b3ee774f622c6ac1d4857d6ae6dbe78d475dfe0 Mon Sep 17 00:00:00 2001
From: Daniel van Vugt <daniel.van.vugt@canonical.com>
Date: Tue, 25 May 2021 17:33:01 +0800
Subject: [PATCH] st/theme-node-drawing: Conditionally paint the center shadow
 rectangle

Only when part of it is actually visible.

Because the central rectangle is generally the largest part, this
eliminates most of the shadow's render time. For example, animating
`.workspace-background` by tapping Super, the shell's overall render
time is reduced about 15%.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1915>
---
 src/st/st-theme-node-drawing.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/src/st/st-theme-node-drawing.c b/src/st/st-theme-node-drawing.c
index b32c76be2..72745ed66 100644
--- a/src/st/st-theme-node-drawing.c
+++ b/src/st/st-theme-node-drawing.c
@@ -2021,6 +2021,8 @@ st_theme_node_paint_sliced_shadow (StThemeNodePaintState *state,
   gfloat shadow_blur_radius, x_spread_factor, y_spread_factor;
   float rectangles[8 * 9];
   gint idx;
+  ClutterColor background_color;
+  static const ClutterColor invisible_occluded = {3, 2, 1, 0};
 
   if (paint_opacity == 0)
     return;
@@ -2178,16 +2180,23 @@ st_theme_node_paint_sliced_shadow (StThemeNodePaintState *state,
       rectangles[idx++] = s_bottom;
     }
 
-  /* Center middle */
-  rectangles[idx++] = left;
-  rectangles[idx++] = top;
-  rectangles[idx++] = right;
-  rectangles[idx++] = bottom;
+  /* Center middle is not definitely occluded? */
+  st_theme_node_get_background_color (node, &background_color);
+  if (!clutter_color_equal (&background_color, &invisible_occluded) ||
+      paint_opacity < 255 ||
+      xoffset > shadow_blur_radius || left < 0 ||
+      yoffset > shadow_blur_radius || top < 0)
+    {
+      rectangles[idx++] = left;
+      rectangles[idx++] = top;
+      rectangles[idx++] = right;
+      rectangles[idx++] = bottom;
 
-  rectangles[idx++] = s_left;
-  rectangles[idx++] = s_top;
-  rectangles[idx++] = s_right;
-  rectangles[idx++] = s_bottom;
+      rectangles[idx++] = s_left;
+      rectangles[idx++] = s_top;
+      rectangles[idx++] = s_right;
+      rectangles[idx++] = s_bottom;
+    }
 
   if (xend > right)
     {