From 979fc44c483173e95aebfaa8bdde949217ae8abf Mon Sep 17 00:00:00 2001
From: Jim Graham <flar@google.com>
Date: Tue, 23 Aug 2022 16:28:14 -0700
Subject: [PATCH] [Impeller] Enforce a minimum stroke width
 (flutter/engine#35576)

---
 .../display_list/display_list_unittests.cc    | 34 +++++++++++++++++++
 .../entity/contents/solid_stroke_contents.cc  | 16 +++++++--
 2 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/engine/src/flutter/impeller/display_list/display_list_unittests.cc b/engine/src/flutter/impeller/display_list/display_list_unittests.cc
index b887c22e49b..c29a999cc6c 100644
--- a/engine/src/flutter/impeller/display_list/display_list_unittests.cc
+++ b/engine/src/flutter/impeller/display_list/display_list_unittests.cc
@@ -521,5 +521,39 @@ TEST_P(DisplayListTest, CanDrawShadow) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+TEST_P(DisplayListTest, CanDrawZeroWidthLine) {
+  flutter::DisplayListBuilder builder;
+  std::vector<flutter::DlStrokeCap> caps = {
+      flutter::DlStrokeCap::kButt,
+      flutter::DlStrokeCap::kRound,
+      flutter::DlStrokeCap::kSquare,
+  };
+  flutter::DlPaint paint =                              //
+      flutter::DlPaint()                                //
+          .setColor(flutter::DlColor::kWhite())         //
+          .setDrawStyle(flutter::DlDrawStyle::kStroke)  //
+          .setStrokeWidth(0);
+  flutter::DlPaint outline_paint =                      //
+      flutter::DlPaint()                                //
+          .setColor(flutter::DlColor::kYellow())        //
+          .setDrawStyle(flutter::DlDrawStyle::kStroke)  //
+          .setStrokeCap(flutter::DlStrokeCap::kSquare)  //
+          .setStrokeWidth(1);
+  SkPath path = SkPath().addPoly({{150, 50}, {160, 50}}, false);
+  for (auto cap : caps) {
+    paint.setStrokeCap(cap);
+    builder.drawLine({50, 50}, {60, 50}, paint);
+    builder.drawRect({45, 45, 65, 55}, outline_paint);
+    builder.drawLine({100, 50}, {100, 50}, paint);
+    if (cap != flutter::DlStrokeCap::kButt) {
+      builder.drawRect({95, 45, 105, 55}, outline_paint);
+    }
+    builder.drawPath(path, paint);
+    builder.drawRect(path.getBounds().makeOutset(5, 5), outline_paint);
+    builder.translate(0, 150);
+  }
+  ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
+}
+
 }  // namespace testing
 }  // namespace impeller
diff --git a/engine/src/flutter/impeller/entity/contents/solid_stroke_contents.cc b/engine/src/flutter/impeller/entity/contents/solid_stroke_contents.cc
index 326b90f3428..357498ad18c 100644
--- a/engine/src/flutter/impeller/entity/contents/solid_stroke_contents.cc
+++ b/engine/src/flutter/impeller/entity/contents/solid_stroke_contents.cc
@@ -52,8 +52,13 @@ std::optional<Rect> SolidStrokeContents::GetCoverage(
   if (join_ == Join::kMiter) {
     max_radius = std::max(max_radius, miter_limit_ * 0.5f);
   }
+  Scalar determinant = entity.GetTransformation().GetDeterminant();
+  if (determinant == 0) {
+    return std::nullopt;
+  }
+  Scalar min_size = 1.0f / sqrt(std::abs(determinant));
   Vector2 max_radius_xy = entity.GetTransformation().TransformDirection(
-      Vector2(max_radius, max_radius) * stroke_size_);
+      Vector2(max_radius, max_radius) * std::max(stroke_size_, min_size));
 
   return Rect(path_coverage.origin - max_radius_xy,
               Size(path_coverage.size.width + max_radius_xy.x * 2,
@@ -182,7 +187,7 @@ static VertexBuffer CreateSolidStrokeVertices(
 bool SolidStrokeContents::Render(const ContentContext& renderer,
                                  const Entity& entity,
                                  RenderPass& pass) const {
-  if (stroke_size_ <= 0.0) {
+  if (stroke_size_ < 0.0) {
     return true;
   }
 
@@ -192,7 +197,12 @@ bool SolidStrokeContents::Render(const ContentContext& renderer,
   VS::VertInfo vert_info;
   vert_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *
                   entity.GetTransformation();
-  vert_info.size = stroke_size_;
+  Scalar determinant = entity.GetTransformation().GetDeterminant();
+  if (determinant == 0) {
+    return true;
+  }
+  Scalar min_size = 1.0f / sqrt(std::abs(determinant));
+  vert_info.size = std::max(stroke_size_, min_size);
 
   FS::FragInfo frag_info;
   frag_info.color = color_.Premultiply();