[Impeller] new blur: clamped downscale to 1/16

Added test for reproducing blur shimmering when translating content under a blur

impeller/aiks/aiks_unittests.cc

@@ -3968,5 +3968,44 @@ TEST_P(AiksTest, CorrectClipDepthAssignedToEntities) {
   }
 }
 
+TEST_P(AiksTest, GaussianBlurAnimatedBackdrop) {
+  // This test is for checking out how stable rendering is when content is
+  // translated underneath a blur.  Animating under a blur can cause
+  // *shimmering* to happen as a result of pixel alignment.
+  // See also: https://github.com/flutter/flutter/issues/140193
+  auto boston = std::make_shared<Image>(
+      CreateTextureForFixture("boston.jpg", /*enable_mipmapping=*/true));
+  ASSERT_TRUE(boston);
+  int64_t count = 0;
+  Scalar sigma = 20.0;
+  Scalar freq = 0.1;
+  Scalar amp = 50.0;
+  auto callback = [&](AiksContext& renderer) -> std::optional<Picture> {
+    ImGui::Begin("Controls", nullptr, ImGuiWindowFlags_AlwaysAutoResize);
+    {
+      ImGui::SliderFloat("Sigma", &sigma, 0, 200);
+      ImGui::SliderFloat("Frequency", &freq, 0.01, 2.0);
+      ImGui::SliderFloat("Amplitude", &amp, 1, 100);
+    }
+    ImGui::End();
+
+    Canvas canvas;
+    canvas.Scale(GetContentScale());
+    Scalar y = amp * sin(freq * 2.0 * M_PI * count / 60);
+    canvas.DrawImage(boston,
+                     Point(1024 / 2 - boston->GetSize().width / 2,
+                           (768 / 2 - boston->GetSize().height / 2) + y),
+                     {});
+    canvas.ClipRect(Rect::MakeLTRB(100, 100, 900, 700));
+    canvas.SaveLayer({.blend_mode = BlendMode::kSource}, std::nullopt,
+                     ImageFilter::MakeBlur(Sigma(sigma), Sigma(sigma),
+                                           FilterContents::BlurStyle::kNormal,
+                                           Entity::TileMode::kClamp));
+    count += 1;
+    return canvas.EndRecordingAsPicture();
+  };
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
+}
+
 }  // namespace testing
 }  // namespace impeller