Reset the state of the previous frame if overlay processing is skipped.

cc/output/overlay_processor.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/output/overlay_processor.h"
 
 #include "cc/output/dc_layer_overlay.h"
 #include "cc/output/output_surface.h"
 #include "cc/output/overlay_strategy_single_on_top.h"
 #include "cc/output/overlay_strategy_underlay.h"
@@ skipping 110 matching lines @@
     DCLayerOverlayList* dc_layer_overlays,
     gfx::Rect* damage_rect,
     std::vector<gfx::Rect>* content_bounds) {
 #if defined(OS_ANDROID)
   // Be sure to send out notifications, regardless of whether we get to
   // processing for overlays or not.  If we don't, then we should notify that
   // they are not promotable.
   SendPromotionHintsBeforeReturning notifier(resource_provider, candidates);
 #endif
 
+  // If overlay processing is skipped, reset |previous_frame_underlay_rect_|.
+  // Otherwise, |previous_frame_underlay_rect_| will be set to current frame
+  // underlay rect.
+  const gfx::Rect previous_frame_underlay_rect = previous_frame_underlay_rect_;
+  previous_frame_underlay_rect_ = gfx::Rect();

[danakj, 2017/04/27 15:32:00]

Doesn't this just mean every frame previous_frame_underlay_rect_ = an empty
rect, so we never subtract damage?

The comment says "if overlay processing is skipped" but this is running when
ProcessFor*Layers() is called so, I'm not understanding what it means to say.

[AndyWu, 2017/04/27 15:53:01]

The thing we care about is UpdateDamageRect() being called or not. I am not
familiar with these terms. Maybe I should say "if hardware overlay processing is
skipped"? Or maybe I should directly say "if UpdateDamageRect() is not called"?

[danakj, 2017/04/27 16:26:19]

I see, UpdateDamageRect() is setting this to something not empty. OK Thanks.

+
   // If we have any copy requests, we can't remove any quads for overlays or
   // CALayers because the framebuffer would be missing the removed quads'
   // contents.
   if (!render_pass->copy_requests.empty()) {
     dc_processor_.ClearOverlayState();
-    // If overlay processing was skipped for a frame there's no way to be sure
-    // of the state of the previous frame, so reset.
-    previous_frame_underlay_rect_ = gfx::Rect();
     return;
   }
 
   // First attempt to process for CALayers.
   if (ProcessForCALayers(resource_provider, render_pass, render_pass_filters,
                          render_pass_background_filters, candidates,
                          ca_layer_overlays, damage_rect)) {
     return;
   }
 
   if (ProcessForDCLayers(resource_provider, render_pass, render_pass_filters,
                          render_pass_background_filters, candidates,
                          dc_layer_overlays, damage_rect)) {
     return;
   }
 
   // Only if that fails, attempt hardware overlay strategies.
   for (const auto& strategy : strategies_) {
     if (!strategy->Attempt(resource_provider, render_pass, candidates,
                            content_bounds))
       continue;
 
-    UpdateDamageRect(candidates, damage_rect);
+    UpdateDamageRect(candidates, previous_frame_underlay_rect, damage_rect);
     return;
   }
 }
 
 // Subtract on-top overlays from the damage rect, unless the overlays use
 // the backbuffer as their content (in which case, add their combined rect
 // back to the damage at the end).
 // Also subtract unoccluded underlays from the damage rect if we know that the
 // same underlay was scheduled on the previous frame. If the renderer decides
 // not to swap the framebuffer there will still be a transparent hole in the
 // previous frame. This only handles the common case of a single underlay quad
 // for fullscreen video.
-void OverlayProcessor::UpdateDamageRect(OverlayCandidateList* candidates,
-                                        gfx::Rect* damage_rect) {
+void OverlayProcessor::UpdateDamageRect(
+    OverlayCandidateList* candidates,
+    const gfx::Rect& previous_frame_underlay_rect,
+    gfx::Rect* damage_rect) {
   gfx::Rect output_surface_overlay_damage_rect;
   gfx::Rect this_frame_underlay_rect;
   for (const OverlayCandidate& overlay : *candidates) {
     if (overlay.plane_z_order > 0) {
       const gfx::Rect overlay_display_rect =
           ToEnclosedRect(overlay.display_rect);
       overlay_damage_rect_.Union(overlay_display_rect);
       damage_rect->Subtract(overlay_display_rect);
       if (overlay.use_output_surface_for_resource)
         output_surface_overlay_damage_rect.Union(overlay_display_rect);
     } else if (overlay.plane_z_order < 0 && overlay.is_unoccluded &&
                this_frame_underlay_rect.IsEmpty()) {
       this_frame_underlay_rect = ToEnclosedRect(overlay.display_rect);
     }
   }
 
-  if (this_frame_underlay_rect == previous_frame_underlay_rect_)
+  if (this_frame_underlay_rect == previous_frame_underlay_rect)
     damage_rect->Subtract(this_frame_underlay_rect);
   previous_frame_underlay_rect_ = this_frame_underlay_rect;
 
   damage_rect->Union(output_surface_overlay_damage_rect);
 }
 
 }  // namespace cc