[impeller] adds test for catching shimmer in gaussian blur (flutter#54116)

fixes flutter#152195

Part of a series of gaussian blur changes:
1) flutter/engine#54148
1) flutter/engine#54116
1) flutter/engine#54150
1) flutter/engine#54149

Note: this test won't pass until
flutter/engine#54148 lands.

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Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
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Author: gaaclarke

impeller/display_list/BUILD.gn

@@ -91,6 +91,7 @@ template("display_list_unittests_component") {
     deps += [
       ":display_list",
       "../playground:playground_test",
+      "//flutter/impeller/golden_tests:screenshot",
       "//flutter/impeller/scene",
       "//flutter/impeller/typographer/backends/stb:typographer_stb_backend",
       "//flutter/third_party/txt",

impeller/display_list/dl_golden_blur_unittests.cc

@@ -6,6 +6,7 @@
 
 #include "flutter/display_list/dl_builder.h"
 #include "flutter/display_list/effects/dl_mask_filter.h"
+#include "flutter/impeller/golden_tests/screenshot.h"
 #include "flutter/testing/testing.h"
 #include "gtest/gtest.h"
 #include "impeller/typographer/backends/skia/text_frame_skia.h"
@@ -110,5 +111,120 @@ TEST_P(DlGoldenTest, TextBlurMaskFilterDisrespectCTM) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+namespace {
+double CalculateDistance(const uint8_t* left, const uint8_t* right) {
+  double diff[4] = {
+      static_cast<double>(left[0]) - right[0],  //
+      static_cast<double>(left[1]) - right[1],  //
+      static_cast<double>(left[2]) - right[2],  //
+      static_cast<double>(left[3]) - right[3]   //
+  };
+  return sqrt((diff[0] * diff[0]) +  //
+              (diff[1] * diff[1]) +  //
+              (diff[2] * diff[2]) +  //
+              (diff[3] * diff[3]));
+}
+
+double RMSE(const impeller::testing::Screenshot* left,
+            const impeller::testing::Screenshot* right) {
+  FML_CHECK(left);
+  FML_CHECK(right);
+  FML_CHECK(left->GetWidth() == right->GetWidth());
+  FML_CHECK(left->GetHeight() == right->GetHeight());
+
+  int64_t samples = left->GetWidth() * left->GetHeight();
+  double tally = 0;
+
+  const uint8_t* left_ptr = left->GetBytes();
+  const uint8_t* right_ptr = right->GetBytes();
+  for (int64_t i = 0; i < samples; ++i, left_ptr += 4, right_ptr += 4) {
+    double distance = CalculateDistance(left_ptr, right_ptr);
+    tally += distance * distance;
+  }
+
+  return sqrt(tally / static_cast<double>(samples));
+}
+}  // namespace
+
+// This is a test to make sure that we don't regress "shimmering" in the
+// gaussian blur. Shimmering is abrupt changes in signal when making tiny
+// changes to the blur parameters.
+//
+// See also:
+//   - https://github.com/flutter/flutter/issues/152195
+TEST_P(DlGoldenTest, ShimmerTest) {
+  impeller::Point content_scale = GetContentScale();
+  auto draw = [&](DlCanvas* canvas, const std::vector<sk_sp<DlImage>>& images,
+                  float sigma) {
+    canvas->DrawColor(DlColor(0xff111111));
+    canvas->Scale(content_scale.x, content_scale.y);
+
+    DlPaint paint;
+    canvas->DrawImage(images[0], SkPoint::Make(10.135, 10.36334),
+                      DlImageSampling::kLinear, &paint);
+
+    SkRect save_layer_bounds = SkRect::MakeLTRB(0, 0, 1024, 768);
+    DlBlurImageFilter blur(sigma, sigma, DlTileMode::kDecal);
+    canvas->ClipRect(SkRect::MakeLTRB(11.125, 10.3737, 911.25, 755.3333));
+    canvas->SaveLayer(&save_layer_bounds, /*paint=*/nullptr, &blur);
+    canvas->Restore();
+  };
+
+  std::vector<sk_sp<DlImage>> images;
+  images.emplace_back(CreateDlImageForFixture("boston.jpg"));
+
+  auto make_screenshot = [&](float sigma) {
+    DisplayListBuilder builder;
+    draw(&builder, images, sigma);
+
+    std::unique_ptr<impeller::testing::Screenshot> screenshot =
+        MakeScreenshot(builder.Build());
+    return screenshot;
+  };
+
+  float start_sigma = 10.0f;
+  std::unique_ptr<impeller::testing::Screenshot> left =
+      make_screenshot(start_sigma);
+  if (!left) {
+    GTEST_SKIP() << "making screenshots not supported.";
+  }
+
+  double average_rmse = 0.0;
+  const int32_t sample_count = 200;
+  for (int i = 1; i <= sample_count; ++i) {
+    float sigma = start_sigma + (i / 2.f);
+    std::unique_ptr<impeller::testing::Screenshot> right =
+        make_screenshot(sigma);
+    double rmse = RMSE(left.get(), right.get());
+    average_rmse += rmse;
+
+    // To debug this output the frames can be written out to disk then
+    // transformed to a video with ffmpeg.
+    //
+    // ## save images command
+    // std::stringstream ss;
+    // ss << "_" << std::setw(3) << std::setfill('0') << (i - 1);
+    // SaveScreenshot(std::move(left), ss.str());
+    //
+    // ## ffmpeg command
+    // ```
+    // ffmpeg -framerate 30 -pattern_type glob -i '*.png' \
+    //   -c:v libx264 -pix_fmt yuv420p out.mp4
+    // ```
+    left = std::move(right);
+  }
+
+  average_rmse = average_rmse / sample_count;
+
+  // This is a somewhat arbitrary threshold. It could be increased if we wanted.
+  // In the problematic cases previously we should values like 28. Before
+  // increasing this you should manually inspect the behavior in
+  // `AiksTest.GaussianBlurAnimatedBackdrop`. Average RMSE is a able to catch
+  // shimmer but it isn't perfect.
+  EXPECT_TRUE(average_rmse < 1.0) << "average_rmse: " << average_rmse;
+  // An average rmse of 0 would mean that the blur isn't blurring.
+  EXPECT_TRUE(average_rmse >= 0.0) << "average_rmse: " << average_rmse;
+}
+
 }  // namespace testing
 }  // namespace flutter

impeller/display_list/dl_playground.cc

@@ -71,6 +71,11 @@ bool DlPlayground::OpenPlaygroundHere(DisplayListPlaygroundCallback callback) {
       });
 }
 
+std::unique_ptr<testing::Screenshot> DlPlayground::MakeScreenshot(
+    const sk_sp<flutter::DisplayList>& list) {
+  return nullptr;
+}
+
 SkFont DlPlayground::CreateTestFontOfSize(SkScalar scalar) {
   static constexpr const char* kTestFontFixture = "Roboto-Regular.ttf";
   auto mapping = flutter::testing::OpenFixtureAsSkData(kTestFontFixture);

impeller/display_list/dl_playground.h

@@ -7,6 +7,7 @@
 
 #include "flutter/display_list/display_list.h"
 #include "flutter/display_list/dl_builder.h"
+#include "flutter/impeller/golden_tests/screenshot.h"
 #include "impeller/playground/playground_test.h"
 #include "third_party/skia/include/core/SkFont.h"
 
@@ -27,6 +28,9 @@ class DlPlayground : public PlaygroundTest {
 
   bool OpenPlaygroundHere(DisplayListPlaygroundCallback callback);
 
+  std::unique_ptr<testing::Screenshot> MakeScreenshot(
+      const sk_sp<flutter::DisplayList>& list);
+
   SkFont CreateTestFontOfSize(SkScalar scalar);
 
   SkFont CreateTestFont();

impeller/golden_tests/BUILD.gn

@@ -12,6 +12,7 @@ impeller_component("golden_playground_test") {
 
   deps = [
     ":digest",
+    ":screenshot",
     "//flutter/fml",
     "//flutter/impeller/aiks",
     "//flutter/impeller/display_list:display_list",
@@ -44,6 +45,11 @@ impeller_component("digest") {
   deps = [ "//flutter/fml" ]
 }
 
+impeller_component("screenshot") {
+  testonly = true
+  sources = [ "screenshot.h" ]
+}
+
 if (is_mac) {
   impeller_component("metal_screenshot") {
     testonly = true
@@ -53,13 +59,13 @@ if (is_mac) {
       "metal_screenshot.mm",
       "metal_screenshotter.h",
       "metal_screenshotter.mm",
-      "screenshot.h",
       "screenshotter.h",
       "vulkan_screenshotter.h",
       "vulkan_screenshotter.mm",
     ]
 
     deps = [
+      ":screenshot",
       "//flutter/impeller/aiks",
       "//flutter/impeller/display_list",
       "//flutter/impeller/playground",

impeller/golden_tests/golden_playground_test.h

@@ -10,6 +10,7 @@
 #include "flutter/display_list/display_list.h"
 #include "flutter/display_list/image/dl_image.h"
 #include "flutter/impeller/aiks/aiks_context.h"
+#include "flutter/impeller/golden_tests/screenshot.h"
 #include "flutter/impeller/playground/playground.h"
 #include "flutter/impeller/renderer/render_target.h"
 #include "flutter/testing/testing.h"
@@ -51,6 +52,12 @@ class GoldenPlaygroundTest
 
   bool OpenPlaygroundHere(const sk_sp<flutter::DisplayList>& list);
 
+  std::unique_ptr<testing::Screenshot> MakeScreenshot(
+      const sk_sp<flutter::DisplayList>& list);
+
+  static bool SaveScreenshot(std::unique_ptr<testing::Screenshot> screenshot,
+                             const std::string& postfix = "");
+
   static bool ImGuiBegin(const char* name,
                          bool* p_open,
                          ImGuiWindowFlags flags);

impeller/golden_tests/golden_playground_test_mac.cc

@@ -140,17 +140,20 @@ std::string GetTestName() {
   return result;
 }
 
-std::string GetGoldenFilename() {
-  return GetTestName() + ".png";
+std::string GetGoldenFilename(const std::string& postfix) {
+  return GetTestName() + postfix + ".png";
 }
+}  // namespace
 
-bool SaveScreenshot(std::unique_ptr<testing::Screenshot> screenshot) {
+bool GoldenPlaygroundTest::SaveScreenshot(
+    std::unique_ptr<testing::Screenshot> screenshot,
+    const std::string& postfix) {
   if (!screenshot || !screenshot->GetBytes()) {
     FML_LOG(ERROR) << "Failed to collect screenshot for test " << GetTestName();
     return false;
   }
   std::string test_name = GetTestName();
-  std::string filename = GetGoldenFilename();
+  std::string filename = GetGoldenFilename(postfix);
   testing::GoldenDigest::Instance()->AddImage(
       test_name, filename, screenshot->GetWidth(), screenshot->GetHeight());
   if (!screenshot->WriteToPNG(
@@ -161,8 +164,6 @@ bool SaveScreenshot(std::unique_ptr<testing::Screenshot> screenshot) {
   return true;
 }
 
-}  // namespace
-
 struct GoldenPlaygroundTest::GoldenPlaygroundTestImpl {
   std::unique_ptr<PlaygroundImpl> test_vulkan_playground;
   std::unique_ptr<PlaygroundImpl> test_opengl_playground;
@@ -396,4 +397,19 @@ fml::Status GoldenPlaygroundTest::SetCapabilities(
   return pimpl_->screenshotter->GetPlayground().SetCapabilities(capabilities);
 }
 
+std::unique_ptr<testing::Screenshot> GoldenPlaygroundTest::MakeScreenshot(
+    const sk_sp<flutter::DisplayList>& list) {
+  AiksContext renderer(GetContext(), typographer_context_);
+
+  DlDispatcher dispatcher;
+  list->Dispatch(dispatcher);
+  Picture picture = dispatcher.EndRecordingAsPicture();
+
+  std::unique_ptr<testing::Screenshot> screenshot =
+      pimpl_->screenshotter->MakeScreenshot(renderer, picture,
+                                            pimpl_->window_size);
+
+  return screenshot;
+}
+
 }  // namespace impeller

impeller/golden_tests/golden_playground_test_stub.cc

@@ -89,4 +89,9 @@ fml::Status GoldenPlaygroundTest::SetCapabilities(
       "GoldenPlaygroundTest-Stub doesn't support SetCapabilities.");
 }
 
+std::unique_ptr<testing::Screenshot> GoldenPlaygroundTest::MakeScreenshot(
+    const sk_sp<flutter::DisplayList>& list) {
+  return nullptr;
+}
+
 }  // namespace impeller

impeller/golden_tests/metal_screenshotter.mm

@@ -37,30 +37,32 @@
 std::unique_ptr<Screenshot> MetalScreenshotter::MakeScreenshot(
     AiksContext& aiks_context,
     const std::shared_ptr<Texture> texture) {
-  id<MTLTexture> metal_texture =
-      std::static_pointer_cast<TextureMTL>(texture)->GetMTLTexture();
+  @autoreleasepool {
+    id<MTLTexture> metal_texture =
+        std::static_pointer_cast<TextureMTL>(texture)->GetMTLTexture();
 
-  CGColorSpaceRef color_space = CGColorSpaceCreateDeviceRGB();
-  CIImage* ciImage = [[CIImage alloc]
-      initWithMTLTexture:metal_texture
-                 options:@{kCIImageColorSpace : (__bridge id)color_space}];
-  CGColorSpaceRelease(color_space);
-  FML_CHECK(ciImage);
+    CGColorSpaceRef color_space = CGColorSpaceCreateDeviceRGB();
+    CIImage* ciImage = [[CIImage alloc]
+        initWithMTLTexture:metal_texture
+                   options:@{kCIImageColorSpace : (__bridge id)color_space}];
+    CGColorSpaceRelease(color_space);
+    FML_CHECK(ciImage);
 
-  std::shared_ptr<Context> context = playground_->GetContext();
-  std::shared_ptr<ContextMTL> context_mtl =
-      std::static_pointer_cast<ContextMTL>(context);
-  CIContext* cicontext =
-      [CIContext contextWithMTLDevice:context_mtl->GetMTLDevice()];
-  FML_CHECK(context);
+    std::shared_ptr<Context> context = playground_->GetContext();
+    std::shared_ptr<ContextMTL> context_mtl =
+        std::static_pointer_cast<ContextMTL>(context);
+    CIContext* cicontext =
+        [CIContext contextWithMTLDevice:context_mtl->GetMTLDevice()];
+    FML_CHECK(context);
 
-  CIImage* flipped = [ciImage
-      imageByApplyingOrientation:kCGImagePropertyOrientationDownMirrored];
+    CIImage* flipped = [ciImage
+        imageByApplyingOrientation:kCGImagePropertyOrientationDownMirrored];
 
-  CGImageRef cgImage = [cicontext createCGImage:flipped
-                                       fromRect:[ciImage extent]];
+    CGImageRef cgImage = [cicontext createCGImage:flipped
+                                         fromRect:[ciImage extent]];
 
-  return std::unique_ptr<MetalScreenshot>(new MetalScreenshot(cgImage));
+    return std::unique_ptr<MetalScreenshot>(new MetalScreenshot(cgImage));
+  }
 }
 
 }  // namespace testing