From 6828abffee20f291b91fa166591dd06580e312e0 Mon Sep 17 00:00:00 2001
From: Brandon DeRosier <bdero@google.com>
Date: Tue, 23 May 2023 19:06:19 -0700
Subject: [PATCH] [Impeller] Set the coverage hint on everything
 (flutter/engine#42264)

Attaches a minimum coverage hint to all rendered entities. This makes ImageFilter blurs do less work when clipped. We can utilize this in a number of other filters as well.

![Screenshot 2023-05-23 at 3 16 49 PM](https://github.com/flutter/engine/assets/919017/5b7182c8-e704-4c4a-bb71-fdf226a60f4e)

The test I added also demonstrates a sampling issue in the gaussian blur that I'm trying to work out. Here it is without any clipping. This coverage hint change does not introduce the issue:

![Screenshot 2023-05-23 at 3 08 15 PM](https://github.com/flutter/engine/assets/919017/524c2fb1-3b58-4589-824b-0794c2bd5ea2)
---
 .../flutter/impeller/aiks/aiks_unittests.cc   | 21 +++++++++++++++++++
 .../flutter/impeller/entity/entity_pass.cc    | 17 ++++++---------
 2 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/engine/src/flutter/impeller/aiks/aiks_unittests.cc b/engine/src/flutter/impeller/aiks/aiks_unittests.cc
index 32e9bfc7f07..a34a9892387 100644
--- a/engine/src/flutter/impeller/aiks/aiks_unittests.cc
+++ b/engine/src/flutter/impeller/aiks/aiks_unittests.cc
@@ -2322,6 +2322,27 @@ TEST_P(AiksTest, CanRenderBackdropBlurHugeSigma) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, CanRenderClippedBlur) {
+  Canvas canvas;
+  canvas.ClipRect(Rect::MakeXYWH(100, 150, 400, 400));
+  canvas.DrawCircle(
+      {400, 400}, 200,
+      {
+          .color = Color::Green(),
+          .image_filter =
+              [](const FilterInput::Ref& input, const Matrix& effect_transform,
+                 bool is_subpass) {
+                return FilterContents::MakeGaussianBlur(
+                    input, Sigma(20), Sigma(20),
+                    FilterContents::BlurStyle::kNormal,
+                    Entity::TileMode::kClamp, effect_transform);
+              },
+      });
+  canvas.Restore();
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 // Regression test for https://github.com/flutter/flutter/issues/126701 .
 TEST_P(AiksTest, CanRenderClippedRuntimeEffects) {
   if (GetParam() != PlaygroundBackend::kMetal) {
diff --git a/engine/src/flutter/impeller/entity/entity_pass.cc b/engine/src/flutter/impeller/entity/entity_pass.cc
index 88622975f02..b929ab54d52 100644
--- a/engine/src/flutter/impeller/entity/entity_pass.cc
+++ b/engine/src/flutter/impeller/entity/entity_pass.cc
@@ -615,6 +615,12 @@ bool EntityPass::OnRender(
       stencil_coverage.coverage->origin += global_pass_position;
     }
 
+    // The coverage hint tells the rendered Contents which portion of the
+    // rendered output will actually be used, and so we set this to the current
+    // stencil coverage (which is the max clip bounds). The contents may
+    // optionally use this hint to avoid unnecessary rendering work.
+    element_entity.GetContents()->SetCoverageHint(current_stencil_coverage);
+
     switch (stencil_coverage.type) {
       case Contents::StencilCoverage::Type::kNoChange:
         break;
@@ -685,17 +691,6 @@ bool EntityPass::OnRender(
       return false;
     }
 
-    // Tell the backdrop contents which portion of the rendered output will
-    // actually be used. The contents may optionally use this hint to avoid
-    // unnecessary rendering work.
-    if (!stencil_coverage_stack.empty() &&
-        stencil_coverage_stack.back().coverage.has_value()) {
-      auto coverage_hint = Rect(
-          stencil_coverage_stack.back().coverage->origin - global_pass_position,
-          stencil_coverage_stack.back().coverage->size);
-      backdrop_filter_contents->SetCoverageHint(coverage_hint);
-    }
-
     Entity backdrop_entity;
     backdrop_entity.SetContents(std::move(backdrop_filter_contents));
     backdrop_entity.SetTransformation(