[Impeller] compute path bounds once, use Skia computed bounds where possible. (flutter/engine#45456)

Now that Paths are externally immutable, we can compute the bounds once at construction. This also allows the Skia conversions to set the already computed bounds.

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

@@ -122,6 +122,8 @@ Path ToPath(const SkPath& path, Point shift) {
   builder.SetConvexity(path.isConvex() ? Convexity::kConvex
                                        : Convexity::kUnknown);
   builder.Shift(shift);
+  auto sk_bounds = path.getBounds().makeOutset(shift.x, shift.y);
+  builder.SetBounds(ToRect(sk_bounds));
   return builder.TakePath(fill_type);
 }
 
@@ -129,6 +131,7 @@ Path ToPath(const SkRRect& rrect) {
   return PathBuilder{}
       .AddRoundedRect(ToRect(rrect.getBounds()), ToRoundingRadii(rrect))
       .SetConvexity(Convexity::kConvex)
+      .SetBounds(ToRect(rrect.getBounds()))
       .TakePath();
 }
 

engine/src/flutter/impeller/geometry/geometry_unittests.cc

@@ -2422,6 +2422,25 @@ TEST(GeometryTest, PathShifting) {
   ASSERT_EQ(cubic.p2, Point(31, 31));
 }
 
+TEST(GeometryTest, PathBuilderWillComputeBounds) {
+  PathBuilder builder;
+  auto path_1 = builder.AddLine({0, 0}, {1, 1}).TakePath();
+
+  ASSERT_EQ(path_1.GetBoundingBox().value(), Rect::MakeLTRB(0, 0, 1, 1));
+
+  auto path_2 = builder.AddLine({-1, -1}, {1, 1}).TakePath();
+
+  // Verify that PathBuilder recomputes the bounds.
+  ASSERT_EQ(path_2.GetBoundingBox().value(), Rect::MakeLTRB(-1, -1, 1, 1));
+
+  // PathBuilder can set the bounds to whatever it wants
+  auto path_3 = builder.AddLine({0, 0}, {1, 1})
+                    .SetBounds(Rect::MakeLTRB(0, 0, 100, 100))
+                    .TakePath();
+
+  ASSERT_EQ(path_3.GetBoundingBox().value(), Rect::MakeLTRB(0, 0, 100, 100));
+}
+
 }  // namespace testing
 }  // namespace impeller
 

engine/src/flutter/impeller/geometry/path.cc

@@ -397,14 +397,19 @@ Path::Polyline Path::CreatePolyline(Scalar scale) const {
 }
 
 std::optional<Rect> Path::GetBoundingBox() const {
+  return computed_bounds_;
+}
+
+void Path::ComputeBounds() {
   auto min_max = GetMinMaxCoveragePoints();
   if (!min_max.has_value()) {
-    return std::nullopt;
+    computed_bounds_ = std::nullopt;
+    return;
   }
   auto min = min_max->first;
   auto max = min_max->second;
   const auto difference = max - min;
-  return Rect{min.x, min.y, difference.x, difference.y};
+  computed_bounds_ = Rect{min.x, min.y, difference.x, difference.y};
 }
 
 std::optional<Rect> Path::GetTransformedBoundingBox(
@@ -461,4 +466,8 @@ std::optional<std::pair<Point, Point>> Path::GetMinMaxCoveragePoints() const {
   return std::make_pair(min.value(), max.value());
 }
 
+void Path::SetBounds(Rect rect) {
+  computed_bounds_ = rect;
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/geometry/path.h

@@ -138,6 +138,8 @@ class Path {
 
   void SetFillType(FillType fill);
 
+  void SetBounds(Rect rect);
+
   Path& AddLinearComponent(Point p1, Point p2);
 
   Path& AddQuadraticComponent(Point p1, Point cp, Point p2);
@@ -146,6 +148,12 @@ class Path {
 
   Path& AddContourComponent(Point destination, bool is_closed = false);
 
+  /// @brief Called by `PathBuilder` to compute the bounds for certain paths.
+  ///
+  /// `PathBuilder` may set the bounds directly, in case they come from a source
+  /// with already computed bounds, such as an SkPath.
+  void ComputeBounds();
+
   void SetContourClosed(bool is_closed);
 
   void Shift(Point shift);
@@ -178,6 +186,8 @@ class Path {
   std::vector<QuadraticPathComponent> quads_;
   std::vector<CubicPathComponent> cubics_;
   std::vector<ContourComponent> contours_;
+
+  std::optional<Rect> computed_bounds_;
 };
 
 }  // namespace impeller

engine/src/flutter/impeller/geometry/path_builder.cc

@@ -22,6 +22,10 @@ Path PathBuilder::TakePath(FillType fill) {
   auto path = prototype_;
   path.SetFillType(fill);
   path.SetConvexity(convexity_);
+  if (!did_compute_bounds_) {
+    path.ComputeBounds();
+  }
+  did_compute_bounds_ = false;
   return path;
 }
 
@@ -458,4 +462,10 @@ PathBuilder& PathBuilder::Shift(Point offset) {
   return *this;
 }
 
+PathBuilder& PathBuilder::SetBounds(Rect bounds) {
+  prototype_.SetBounds(bounds);
+  did_compute_bounds_ = true;
+  return *this;
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/geometry/path_builder.h

@@ -97,6 +97,14 @@ class PathBuilder {
   /// `offset`.
   PathBuilder& Shift(Point offset);
 
+  /// @brief Set the bounding box that will be used by `Path.GetBoundingBox` in
+  /// place of performing the computation.
+  ///
+  ///        When Impeller recieves Skia Path objects, many of these already
+  ///        have computed bounds. This method is used to avoid needlessly
+  ///        recomputing these bounds.
+  PathBuilder& SetBounds(Rect bounds);
+
   struct RoundingRadii {
     Point top_left;
     Point bottom_left;
@@ -133,6 +141,7 @@ class PathBuilder {
   Point current_;
   Path prototype_;
   Convexity convexity_;
+  bool did_compute_bounds_ = false;
 
   PathBuilder& AddRoundedRectTopLeft(Rect rect, RoundingRadii radii);