stage-impl: Do clipped redraws when drawing offscreen

This change allows clipped redraws for offscreen. The net
effect of this change is to preserve the original redraw clip when
possible (rather than overwriting it with the full view redraw) in
the paint context.

This eventually helps in retrieving the fine grained updated regions of
the frame since last redraw and sending it to the pipewire client
(as shown in a subsquent CL).

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2775>

1 files changed, 51 insertions, 21 deletions

diff --git a/src/backends/meta-stage-impl.c b/src/backends/meta-stage-impl.c
index 31e07d965..db94c7e40 100644
--- a/src/backends/meta-stage-impl.c
+++ b/src/backends/meta-stage-impl.c
@@ -454,6 +454,46 @@ transform_swap_region_to_onscreen (ClutterStageView *stage_view,
   return transformed_region;
 }
 
+static gboolean
+should_use_clipped_redraw (gboolean              is_full_redraw,
+                           gboolean              has_buffer_age,
+                           gboolean              buffer_has_valid_damage_history,
+                           ClutterDrawDebugFlag  paint_debug_flags,
+                           CoglFramebuffer      *framebuffer,
+                           ClutterStageWindow   *stage_window)
+{
+  gboolean can_blit_sub_buffer;
+  gboolean can_use_clipped_redraw;
+  gboolean is_warmed_up;
+
+  if (is_full_redraw)
+    return FALSE;
+
+  if (paint_debug_flags & CLUTTER_DEBUG_DISABLE_CLIPPED_REDRAWS)
+    return FALSE;
+
+  if (COGL_IS_OFFSCREEN (framebuffer))
+    return TRUE;
+
+  if (!buffer_has_valid_damage_history)
+    {
+      meta_topic (META_DEBUG_BACKEND,
+                  "Invalid back buffer age: forcing full redraw");
+      return FALSE;
+    }
+
+  can_blit_sub_buffer =
+    cogl_clutter_winsys_has_feature (COGL_WINSYS_FEATURE_SWAP_REGION);
+  can_use_clipped_redraw =
+    _clutter_stage_window_can_clip_redraws (stage_window) &&
+    (can_blit_sub_buffer || has_buffer_age);
+  /* Some drivers struggle to get going and produce some junk
+   * frames when starting up... */
+  is_warmed_up =
+    cogl_onscreen_get_frame_counter (COGL_ONSCREEN (framebuffer)) > 3;
+  return is_warmed_up && can_use_clipped_redraw;
+}
+
 static void
 meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
                                      ClutterStageView *stage_view,
@@ -465,8 +505,8 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
   CoglFramebuffer *onscreen = clutter_stage_view_get_onscreen (stage_view);
   cairo_rectangle_int_t view_rect;
   gboolean is_full_redraw;
-  gboolean use_clipped_redraw = TRUE;
-  gboolean can_blit_sub_buffer;
+  gboolean use_clipped_redraw;
+  gboolean buffer_has_valid_damage_history = FALSE;
   gboolean has_buffer_age;
   gboolean swap_with_damage;
   cairo_region_t *redraw_clip;
@@ -484,10 +524,6 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
   fb_width = cogl_framebuffer_get_width (fb);
   fb_height = cogl_framebuffer_get_height (fb);
 
-  can_blit_sub_buffer =
-    COGL_IS_ONSCREEN (onscreen) &&
-    cogl_clutter_winsys_has_feature (COGL_WINSYS_FEATURE_SWAP_REGION);
-
   has_buffer_age =
     COGL_IS_ONSCREEN (onscreen) &&
     cogl_clutter_winsys_has_feature (COGL_WINSYS_FEATURE_BUFFER_AGE);
@@ -505,26 +541,20 @@ meta_stage_impl_redraw_view_primary (MetaStageImpl    *stage_impl,
   if (has_buffer_age)
     {
       buffer_age = cogl_onscreen_get_buffer_age (COGL_ONSCREEN (onscreen));
-      if (!clutter_damage_history_is_age_valid (damage_history, buffer_age))
-        {
-          meta_topic (META_DEBUG_BACKEND,
-                      "Invalid back buffer(age=%d): forcing full redraw",
-                      buffer_age);
-          use_clipped_redraw = FALSE;
-        }
+      buffer_has_valid_damage_history =
+        clutter_damage_history_is_age_valid (damage_history,
+                                             buffer_age);
     }
 
   meta_get_clutter_debug_flags (NULL, &paint_debug_flags, NULL);
 
   use_clipped_redraw =
-    use_clipped_redraw &&
-    !(paint_debug_flags & CLUTTER_DEBUG_DISABLE_CLIPPED_REDRAWS) &&
-    _clutter_stage_window_can_clip_redraws (stage_window) &&
-    (can_blit_sub_buffer || has_buffer_age) &&
-    !is_full_redraw &&
-    /* some drivers struggle to get going and produce some junk
-     * frames when starting up... */
-    cogl_onscreen_get_frame_counter (COGL_ONSCREEN (onscreen)) > 3;
+    should_use_clipped_redraw (is_full_redraw,
+                               has_buffer_age,
+                               buffer_has_valid_damage_history,
+                               paint_debug_flags,
+                               onscreen,
+                               stage_window);
 
   if (use_clipped_redraw)
     {