Started pixel aligning hairlines (#166351)

fixes https://github.com/flutter/flutter/issues/162378
builds from: https://github.com/flutter/flutter/pull/165079
gets rid of the line artifacts in
https://github.com/flutter/flutter/issues/165689

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

engine/src/flutter/display_list/geometry/dl_path.cc

@@ -224,6 +224,16 @@ bool DlPath::IsOval(DlRect* bounds) const {
   return GetSkPath().isOval(ToSkRect(bounds));
 }
 
+bool DlPath::IsLine(DlPoint* start, DlPoint* end) const {
+  SkPoint sk_points[2];
+  if (GetSkPath().isLine(sk_points)) {
+    *start = ToDlPoint(sk_points[0]);
+    *end = ToDlPoint(sk_points[1]);
+    return true;
+  }
+  return false;
+}
+
 bool DlPath::IsRoundRect(DlRoundRect* rrect) const {
   SkRRect sk_rrect;
   bool ret = GetSkPath().isRRect(rrect ? &sk_rrect : nullptr);

engine/src/flutter/display_list/geometry/dl_path.h

@@ -118,6 +118,7 @@ class DlPath {
 
   bool IsRect(DlRect* rect = nullptr, bool* is_closed = nullptr) const;
   bool IsOval(DlRect* bounds = nullptr) const;
+  bool IsLine(DlPoint* start = nullptr, DlPoint* end = nullptr) const;
   bool IsRoundRect(DlRoundRect* rrect = nullptr) const;
 
   bool IsSkRect(SkRect* rect, bool* is_closed = nullptr) const;

engine/src/flutter/display_list/geometry/dl_path_unittests.cc

@@ -555,6 +555,45 @@ TEST(DisplayListPath, ConstructFromImpellerEqualsConstructFromSkia) {
   EXPECT_EQ(DlPath(path_builder, DlPathFillType::kNonZero), DlPath(sk_path));
 }
 
+TEST(DisplayListPath, IsLineFromSkPath) {
+  SkPath sk_path;
+  sk_path.moveTo(SkPoint::Make(0, 0));
+  sk_path.lineTo(SkPoint::Make(100, 100));
+
+  DlPath path = DlPath(sk_path);
+
+  DlPoint start;
+  DlPoint end;
+  EXPECT_TRUE(path.IsLine(&start, &end));
+  EXPECT_EQ(start, DlPoint::MakeXY(0, 0));
+  EXPECT_EQ(end, DlPoint::MakeXY(100, 100));
+
+  EXPECT_FALSE(DlPath(SkPath::Rect(SkRect::MakeLTRB(0, 0, 100, 100))).IsLine());
+}
+
+TEST(DisplayListPath, IsLineFromImpellerPath) {
+  DlPathBuilder path_builder;
+  path_builder.MoveTo({0, 0});
+  path_builder.LineTo({100, 0});
+  DlPath path = DlPath(path_builder, DlPathFillType::kNonZero);
+
+  DlPoint start;
+  DlPoint end;
+  EXPECT_TRUE(path.IsLine(&start, &end));
+  EXPECT_EQ(start, DlPoint::MakeXY(0, 0));
+  EXPECT_EQ(end, DlPoint::MakeXY(100, 0));
+
+  {
+    DlPathBuilder path_builder;
+    path_builder.MoveTo({0, 0});
+    path_builder.LineTo({100, 0});
+    path_builder.LineTo({100, 100});
+
+    DlPath path = DlPath(path_builder, DlPathFillType::kNonZero);
+    EXPECT_FALSE(path.IsLine());
+  }
+}
+
 namespace {
 class DlPathReceiverMock : public DlPathReceiver {
  public:

engine/src/flutter/impeller/display_list/aiks_dl_path_unittests.cc

@@ -258,6 +258,88 @@ TEST_P(AiksTest, CanRenderStrokedConicPaths) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+TEST_P(AiksTest, HairlinePath) {
+  Scalar scale = 1.f;
+  Scalar rotation = 0.f;
+  Scalar offset = 0.f;
+  auto callback = [&]() -> sk_sp<DisplayList> {
+    if (AiksTest::ImGuiBegin("Controls", nullptr,
+                             ImGuiWindowFlags_AlwaysAutoResize)) {
+      ImGui::SliderFloat("Scale", &scale, 0, 6);
+      ImGui::SliderFloat("Rotate", &rotation, 0, 90);
+      ImGui::SliderFloat("Offset", &offset, 0, 2);
+      ImGui::End();
+    }
+
+    DisplayListBuilder builder;
+    builder.Scale(GetContentScale().x, GetContentScale().y);
+    builder.DrawPaint(DlPaint(DlColor(0xff111111)));
+
+    DlPaint paint;
+    paint.setStrokeWidth(0.f);
+    paint.setColor(DlColor::kWhite());
+    paint.setStrokeCap(DlStrokeCap::kRound);
+    paint.setStrokeJoin(DlStrokeJoin::kRound);
+    paint.setDrawStyle(DlDrawStyle::kStroke);
+
+    builder.Translate(512, 384);
+    builder.Scale(scale, scale);
+    builder.Rotate(rotation);
+    builder.Translate(-512, -384 + offset);
+
+    for (int i = 0; i < 5; ++i) {
+      Scalar yoffset = i * 25.25f + 300.f;
+      DlPathBuilder path_builder;
+
+      path_builder.MoveTo(DlPoint(100, yoffset));
+      path_builder.LineTo(DlPoint(924, yoffset));
+      builder.DrawPath(DlPath(path_builder), paint);
+    }
+
+    return builder.Build();
+  };
+
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
+}
+
+TEST_P(AiksTest, HairlineDrawLine) {
+  Scalar scale = 1.f;
+  Scalar rotation = 0.f;
+  Scalar offset = 0.f;
+  auto callback = [&]() -> sk_sp<DisplayList> {
+    if (AiksTest::ImGuiBegin("Controls", nullptr,
+                             ImGuiWindowFlags_AlwaysAutoResize)) {
+      ImGui::SliderFloat("Scale", &scale, 0, 6);
+      ImGui::SliderFloat("Rotate", &rotation, 0, 90);
+      ImGui::SliderFloat("Offset", &offset, 0, 2);
+      ImGui::End();
+    }
+
+    DisplayListBuilder builder;
+    builder.Scale(GetContentScale().x, GetContentScale().y);
+    builder.DrawPaint(DlPaint(DlColor(0xff111111)));
+
+    DlPaint paint;
+    paint.setStrokeWidth(0.f);
+    paint.setColor(DlColor::kWhite());
+
+    builder.Translate(512, 384);
+    builder.Scale(scale, scale);
+    builder.Rotate(rotation);
+    builder.Translate(-512, -384 + offset);
+
+    for (int i = 0; i < 5; ++i) {
+      Scalar yoffset = i * 25.25f + 300.f;
+
+      builder.DrawLine(DlPoint(100, yoffset), DlPoint(924, yoffset), paint);
+    }
+
+    return builder.Build();
+  };
+
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
+}
+
 TEST_P(AiksTest, CanRenderTightConicPath) {
   DisplayListBuilder builder;
   builder.Scale(GetContentScale().x, GetContentScale().y);

engine/src/flutter/impeller/display_list/dl_dispatcher.cc

@@ -668,6 +668,13 @@ void DlDispatcherBase::SimplifyOrDrawPath(Canvas& canvas,
     return;
   }
 
+  DlPoint start;
+  DlPoint end;
+  if (path.IsLine(&start, &end)) {
+    canvas.DrawLine(start, end, paint);
+    return;
+  }
+
   canvas.DrawPath(path.GetPath(), paint);
 }
 

engine/src/flutter/impeller/entity/geometry/line_geometry.cc

@@ -78,22 +78,52 @@ Scalar LineGeometry::ComputeAlphaCoverage(const Matrix& entity) const {
   return Geometry::ComputeStrokeAlphaCoverage(entity, width_);
 }
 
+namespace {
+/// Minimizes the err when rounding to the closest 0.5 value.
+/// If we round up, it drops down a half.  If we round down it bumps up a half.
+Scalar RoundToHalf(Scalar x) {
+  Scalar whole;
+  std::modf(x, &whole);
+  return whole + 0.5;
+}
+}  // namespace
+
 GeometryResult LineGeometry::GetPositionBuffer(const ContentContext& renderer,
                                                const Entity& entity,
                                                RenderPass& pass) const {
   using VT = SolidFillVertexShader::PerVertexData;
 
-  auto& transform = entity.GetTransform();
+  Matrix transform = entity.GetTransform();
   auto radius = ComputePixelHalfWidth(transform, width_);
 
+  Point p0 = p0_;
+  Point p1 = p1_;
+
+  // Hairline pixel alignment.
+  if (width_ == 0.f && transform.IsTranslationScaleOnly()) {
+    p0 = transform * p0_;
+    p1 = transform * p1_;
+    transform = Matrix();
+    if (std::fabs(p0.x - p1.x) < kEhCloseEnough) {
+      p0.x = RoundToHalf(p0.x);
+      p1.x = p0.x;
+    } else if (std::fabs(p0.y - p1.y) < kEhCloseEnough) {
+      p0.y = RoundToHalf(p0.y);
+      p1.y = p0.y;
+    }
+  }
+
+  Entity fixed_transform = entity.Clone();
+  fixed_transform.SetTransform(transform);
+
   if (cap_ == Cap::kRound) {
     auto generator =
-        renderer.GetTessellator().RoundCapLine(transform, p0_, p1_, radius);
-    return ComputePositionGeometry(renderer, generator, entity, pass);
+        renderer.GetTessellator().RoundCapLine(transform, p0, p1, radius);
+    return ComputePositionGeometry(renderer, generator, fixed_transform, pass);
   }
 
   Point corners[4];
-  if (!ComputeCorners(corners, transform, cap_ == Cap::kSquare, p0_, p1_,
+  if (!ComputeCorners(corners, transform, cap_ == Cap::kSquare, p0, p1,
                       width_)) {
     return kEmptyResult;
   }
@@ -119,7 +149,7 @@ GeometryResult LineGeometry::GetPositionBuffer(const ContentContext& renderer,
               .vertex_count = count,
               .index_type = IndexType::kNone,
           },
-      .transform = entity.GetShaderTransform(pass),
+      .transform = fixed_transform.GetShaderTransform(pass),
   };
 }