[Impeller] new blur: clamp downsample scalar to 1/16 (#50262)

fixes flutter/flutter#142753 The theory here is that once you start throwing away more than 15/16th of the signal it's pretty noticable. By looking at the blur under different circumstances that seems like a reasonable limit. There is no automated test for this. Doing so would be quite involved and would involve evaluating multiple rendered frames. There is a manual test. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
flutter · Feb 2, 2024 · 7c7631b · 7c7631b
1 parent fee0214
commit 7c7631b
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Showing 6 changed files with 53 additions and 5 deletions.
diff --git a/impeller/aiks/BUILD.gn b/impeller/aiks/BUILD.gn
@@ -98,7 +98,11 @@ template("aiks_unittests_component") {
     testonly = true
     if (defined(invoker.defines)) {
       defines = invoker.defines
+    } else {
+      defines = []
     }
+    defines += [ "_USE_MATH_DEFINES" ]
+
     sources = predefined_sources + additional_sources
     deps = [
       ":aiks",

diff --git a/impeller/aiks/aiks_unittests.cc b/impeller/aiks/aiks_unittests.cc
@@ -4024,5 +4024,44 @@ TEST_P(AiksTest, GaussianBlurBackdropTinyMipMap) {
   }
 }
 
+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
diff --git a/impeller/entity/contents/filters/gaussian_blur_filter_contents.cc b/impeller/entity/contents/filters/gaussian_blur_filter_contents.cc
@@ -209,7 +209,10 @@ Scalar GaussianBlurFilterContents::CalculateScale(Scalar sigma) {
   }
   Scalar result = 4.0 / sigma;
   // Round to the nearest 1/(2^n) to get the best quality down scaling.
-  Scalar rounded = pow(2.0f, round(log2(result)));
+  Scalar exponent = round(log2f(result));
+  // Don't scale down below 1/16th to preserve signal.
+  exponent = std::max(-4.0f, exponent);
+  Scalar rounded = powf(2.0f, exponent);
   return rounded;
 };
 
@@ -454,8 +457,8 @@ KernelPipeline::FragmentShader::KernelSamples GenerateBlurInfo(
   KernelPipeline::FragmentShader::KernelSamples result;
   result.sample_count =
       ((2 * parameters.blur_radius) / parameters.step_size) + 1;
-  // 32 comes from kernel.glsl.
-  FML_CHECK(result.sample_count < 32);
+  // 48 comes from kernel.glsl.
+  FML_CHECK(result.sample_count < 48);
 
   // Chop off the last samples if the radius >= 3 where they account for < 1.56%
   // of the result.

diff --git a/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc b/impeller/entity/contents/filters/gaussian_blur_filter_contents_unittests.cc
@@ -197,7 +197,8 @@ TEST(GaussianBlurFilterContentsTest, CalculateSigmaValues) {
   EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(3.0f), 1);
   EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(4.0f), 1);
   EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(16.0f), 0.25);
-  EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(1024.0f), 4.f / 1024.f);
+  // Downsample clamped to 1/16th.
+  EXPECT_EQ(GaussianBlurFilterContents::CalculateScale(1024.0f), 0.0625);
 }
 
 TEST_P(GaussianBlurFilterContentsTest, RenderCoverageMatchesGetCoverage) {

diff --git a/impeller/entity/shaders/gaussian_blur/kernel.glsl b/impeller/entity/shaders/gaussian_blur/kernel.glsl
@@ -16,7 +16,7 @@ struct KernelSample {
 
 uniform KernelSamples {
   int sample_count;
-  KernelSample samples[32];
+  KernelSample samples[48];
 }
 blur_info;
 

diff --git a/impeller/golden_tests/golden_playground_test_mac.cc b/impeller/golden_tests/golden_playground_test_mac.cc
@@ -76,6 +76,7 @@ static const std::vector<std::string> kSkipTests = {
     IMP_AIKSTEST(SceneColorSource),
     IMP_AIKSTEST(SolidStrokesRenderCorrectly),
     IMP_AIKSTEST(TextFrameSubpixelAlignment),
+    IMP_AIKSTEST(GaussianBlurAnimatedBackdrop),
     // TextRotated is flakey and we can't seem to get it to stabilize on Skia
     // Gold.
     IMP_AIKSTEST(TextRotated),