stroke_path_geometry.cc

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// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#include "impeller/entity/geometry/stroke_path_geometry.h"
 
#include "impeller/core/buffer_view.h"
#include "impeller/core/formats.h"
#include "impeller/core/host_buffer.h"
#include "impeller/entity/contents/pipelines.h"
#include "impeller/entity/geometry/geometry.h"
#include "impeller/geometry/constants.h"
#include "impeller/geometry/path_builder.h"
#include "impeller/geometry/path_component.h"
#include "impeller/geometry/separated_vector.h"
#include "impeller/geometry/wangs_formula.h"
 
namespace impeller {
 
namespace {
 
class PositionWriter {
 public:
  explicit PositionWriter(std::vector<Point>& points)
      : points_(points), oversized_() {
    FML_DCHECK(points_.size() == kPointArenaSize);
  }
 
  void AppendVertex(const Point& point) {
    if (offset_ >= kPointArenaSize) {
      oversized_.push_back(point);
    } else {
      points_[offset_++] = point;
    }
  }
 
  /// @brief Return the number of points used in the arena, followed by
  ///        the number of points allocated in the overized buffer.
  std::pair<size_t, size_t> GetUsedSize() const {
    return std::make_pair(offset_, oversized_.size());
  }
 
  bool HasOversizedBuffer() const { return !oversized_.empty(); }
 
  const std::vector<Point>& GetOversizedBuffer() const { return oversized_; }
 
 private:
  std::vector<Point>& points_;
  std::vector<Point> oversized_;
  size_t offset_ = 0u;
};
 
using CapProc = std::function<void(PositionWriter& vtx_builder,
                                   const Point& position,
                                   const Point& offset,
                                   Scalar scale,
                                   bool reverse)>;
 
using JoinProc = std::function<void(PositionWriter& vtx_builder,
                                    const Point& position,
                                    const Point& start_offset,
                                    const Point& end_offset,
                                    Scalar miter_limit,
                                    Scalar scale)>;
 
class StrokeGenerator {
 public:
  StrokeGenerator(const Path::Polyline& p_polyline,
                  const Scalar p_stroke_width,
                  const Scalar p_scaled_miter_limit,
                  const JoinProc& p_join_proc,
                  const CapProc& p_cap_proc,
                  const Scalar p_scale)
      : polyline(p_polyline),
        stroke_width(p_stroke_width),
        scaled_miter_limit(p_scaled_miter_limit),
        join_proc(p_join_proc),
        cap_proc(p_cap_proc),
        scale(p_scale) {}
 
  void Generate(PositionWriter& vtx_builder) {
    for (size_t contour_i = 0; contour_i < polyline.contours.size();
         contour_i++) {
      const Path::PolylineContour& contour = polyline.contours[contour_i];
      size_t contour_start_point_i, contour_end_point_i;
      std::tie(contour_start_point_i, contour_end_point_i) =
          polyline.GetContourPointBounds(contour_i);
 
      size_t contour_delta = contour_end_point_i - contour_start_point_i;
      if (contour_delta == 0) {
        continue;  // This contour has no renderable content.
      }
 
      if (contour_i > 0) {
        // This branch only executes when we've just finished drawing a contour
        // and are switching to a new one.
        // We're drawing a triangle strip, so we need to "pick up the pen" by
        // appending two vertices at the end of the previous contour and two
        // vertices at the start of the new contour (thus connecting the two
        // contours with two zero volume triangles, which will be discarded by
        // the rasterizer).
        vtx.position = polyline.GetPoint(contour_start_point_i - 1);
        // Append two vertices when "picking up" the pen so that the triangle
        // drawn when moving to the beginning of the new contour will have zero
        // volume.
        vtx_builder.AppendVertex(vtx.position);
        vtx_builder.AppendVertex(vtx.position);
 
        vtx.position = polyline.GetPoint(contour_start_point_i);
        // Append two vertices at the beginning of the new contour, which
        // appends  two triangles of zero area.
        vtx_builder.AppendVertex(vtx.position);
        vtx_builder.AppendVertex(vtx.position);
      }
 
      if (contour_delta == 1) {
        Point p = polyline.GetPoint(contour_start_point_i);
        cap_proc(vtx_builder, p, {-stroke_width * 0.5f, 0}, scale,
                 /*reverse=*/false);
        cap_proc(vtx_builder, p, {stroke_width * 0.5f, 0}, scale,
                 /*reverse=*/false);
        continue;
      }
 
      previous_offset = offset;
      offset = ComputeOffset(contour_start_point_i, contour_start_point_i,
                             contour_end_point_i, contour);
      const Point contour_first_offset = offset.GetVector();
 
      // Generate start cap.
      if (!polyline.contours[contour_i].is_closed) {
        Point cap_offset =
            Vector2(-contour.start_direction.y, contour.start_direction.x) *
            stroke_width * 0.5f;  // Counterclockwise normal
        cap_proc(vtx_builder, polyline.GetPoint(contour_start_point_i),
                 cap_offset, scale, /*reverse=*/true);
      }
 
      for (size_t contour_component_i = 0;
           contour_component_i < contour.components.size();
           contour_component_i++) {
        const Path::PolylineContour::Component& component =
            contour.components[contour_component_i];
        bool is_last_component =
            contour_component_i == contour.components.size() - 1;
 
        size_t component_start_index = component.component_start_index;
        size_t component_end_index =
            is_last_component ? contour_end_point_i - 1
                              : contour.components[contour_component_i + 1]
                                    .component_start_index;
        if (component.is_curve) {
          AddVerticesForCurveComponent(
              vtx_builder, component_start_index, component_end_index,
              contour_start_point_i, contour_end_point_i, contour);
        } else {
          AddVerticesForLinearComponent(
              vtx_builder, component_start_index, component_end_index,
              contour_start_point_i, contour_end_point_i, contour);
        }
      }
 
      // Generate end cap or join.
      if (!contour.is_closed) {
        auto cap_offset =
            Vector2(-contour.end_direction.y, contour.end_direction.x) *
            stroke_width * 0.5f;  // Clockwise normal
        cap_proc(vtx_builder, polyline.GetPoint(contour_end_point_i - 1),
                 cap_offset, scale, /*reverse=*/false);
      } else {
        join_proc(vtx_builder, polyline.GetPoint(contour_start_point_i),
                  offset.GetVector(), contour_first_offset, scaled_miter_limit,
                  scale);
      }
    }
  }
 
  /// Computes offset by calculating the direction from point_i - 1 to point_i
  /// if point_i is within `contour_start_point_i` and `contour_end_point_i`;
  /// Otherwise, it uses direction from contour.
  SeparatedVector2 ComputeOffset(const size_t point_i,
                                 const size_t contour_start_point_i,
                                 const size_t contour_end_point_i,
                                 const Path::PolylineContour& contour) const {
    Point direction;
    if (point_i >= contour_end_point_i) {
      direction = contour.end_direction;
    } else if (point_i <= contour_start_point_i) {
      direction = -contour.start_direction;
    } else {
      direction = (polyline.GetPoint(point_i) - polyline.GetPoint(point_i - 1))
                      .Normalize();
    }
    return SeparatedVector2(Vector2{-direction.y, direction.x},
                            stroke_width * 0.5f);
  }
 
  void AddVerticesForLinearComponent(PositionWriter& vtx_builder,
                                     const size_t component_start_index,
                                     const size_t component_end_index,
                                     const size_t contour_start_point_i,
                                     const size_t contour_end_point_i,
                                     const Path::PolylineContour& contour) {
    bool is_last_component = component_start_index ==
                             contour.components.back().component_start_index;
 
    for (size_t point_i = component_start_index; point_i < component_end_index;
         point_i++) {
      bool is_end_of_component = point_i == component_end_index - 1;
 
      Point offset_vector = offset.GetVector();
 
      vtx.position = polyline.GetPoint(point_i) + offset_vector;
      vtx_builder.AppendVertex(vtx.position);
      vtx.position = polyline.GetPoint(point_i) - offset_vector;
      vtx_builder.AppendVertex(vtx.position);
 
      // For line components, two additional points need to be appended
      // prior to appending a join connecting the next component.
      vtx.position = polyline.GetPoint(point_i + 1) + offset_vector;
      vtx_builder.AppendVertex(vtx.position);
      vtx.position = polyline.GetPoint(point_i + 1) - offset_vector;
      vtx_builder.AppendVertex(vtx.position);
 
      previous_offset = offset;
      offset = ComputeOffset(point_i + 2, contour_start_point_i,
                             contour_end_point_i, contour);
      if (!is_last_component && is_end_of_component) {
        // Generate join from the current line to the next line.
        join_proc(vtx_builder, polyline.GetPoint(point_i + 1),
                  previous_offset.GetVector(), offset.GetVector(),
                  scaled_miter_limit, scale);
      }
    }
  }
 
  void AddVerticesForCurveComponent(PositionWriter& vtx_builder,
                                    const size_t component_start_index,
                                    const size_t component_end_index,
                                    const size_t contour_start_point_i,
                                    const size_t contour_end_point_i,
                                    const Path::PolylineContour& contour) {
    bool is_last_component = component_start_index ==
                             contour.components.back().component_start_index;
 
    for (size_t point_i = component_start_index; point_i < component_end_index;
         point_i++) {
      bool is_end_of_component = point_i == component_end_index - 1;
 
      vtx.position = polyline.GetPoint(point_i) + offset.GetVector();
      vtx_builder.AppendVertex(vtx.position);
      vtx.position = polyline.GetPoint(point_i) - offset.GetVector();
      vtx_builder.AppendVertex(vtx.position);
 
      previous_offset = offset;
      offset = ComputeOffset(point_i + 2, contour_start_point_i,
                             contour_end_point_i, contour);
 
      // If the angle to the next segment is too sharp, round out the join.
      if (!is_end_of_component) {
        constexpr Scalar kAngleThreshold = 10 * kPi / 180;
        // `std::cosf` is not constexpr-able, unfortunately, so we have to bake
        // the alignment constant.
        constexpr Scalar kAlignmentThreshold =
            0.984807753012208;  // std::cosf(kThresholdAngle) -- 10 degrees
 
        // Use a cheap dot product to determine whether the angle is too sharp.
        if (previous_offset.GetAlignment(offset) < kAlignmentThreshold) {
          Scalar angle_total = previous_offset.AngleTo(offset).radians;
          Scalar angle = kAngleThreshold;
 
          // Bridge the large angle with additional geometry at
          // `kAngleThreshold` interval.
          while (angle < std::abs(angle_total)) {
            Scalar signed_angle = angle_total < 0 ? -angle : angle;
            Point offset =
                previous_offset.GetVector().Rotate(Radians(signed_angle));
            vtx.position = polyline.GetPoint(point_i) + offset;
            vtx_builder.AppendVertex(vtx.position);
            vtx.position = polyline.GetPoint(point_i) - offset;
            vtx_builder.AppendVertex(vtx.position);
 
            angle += kAngleThreshold;
          }
        }
      }
 
      // For curve components, the polyline is detailed enough such that
      // it can avoid worrying about joins altogether.
      if (is_end_of_component) {
        // Append two additional vertices to close off the component. If we're
        // on the _last_ component of the contour then we need to use the
        // contour's end direction.
        // `ComputeOffset` returns the contour's end direction when attempting
        // to grab offsets past `contour_end_point_i`, so just use `offset` when
        // we're on the last component.
        Point last_component_offset = is_last_component
                                          ? offset.GetVector()
                                          : previous_offset.GetVector();
        vtx.position = polyline.GetPoint(point_i + 1) + last_component_offset;
        vtx_builder.AppendVertex(vtx.position);
        vtx.position = polyline.GetPoint(point_i + 1) - last_component_offset;
        vtx_builder.AppendVertex(vtx.position);
        // Generate join from the current line to the next line.
        if (!is_last_component) {
          join_proc(vtx_builder, polyline.GetPoint(point_i + 1),
                    previous_offset.GetVector(), offset.GetVector(),
                    scaled_miter_limit, scale);
        }
      }
    }
  }
 
  const Path::Polyline& polyline;
  const Scalar stroke_width;
  const Scalar scaled_miter_limit;
  const JoinProc& join_proc;
  const CapProc& cap_proc;
  const Scalar scale;
 
  SeparatedVector2 previous_offset;
  SeparatedVector2 offset;
  SolidFillVertexShader::PerVertexData vtx;
};
 
void CreateButtCap(PositionWriter& vtx_builder,
                   const Point& position,
                   const Point& offset,
                   Scalar scale,
                   bool reverse) {
  Point orientation = offset * (reverse ? -1 : 1);
  vtx_builder.AppendVertex(position + orientation);
  vtx_builder.AppendVertex(position - orientation);
}
 
void CreateRoundCap(PositionWriter& vtx_builder,
                    const Point& position,
                    const Point& offset,
                    Scalar scale,
                    bool reverse) {
  Point orientation = offset * (reverse ? -1 : 1);
  Point forward(offset.y, -offset.x);
  Point forward_normal = forward.Normalize();
 
  CubicPathComponent arc;
  if (reverse) {
    arc = CubicPathComponent(
        forward, forward + orientation * PathBuilder::kArcApproximationMagic,
        orientation + forward * PathBuilder::kArcApproximationMagic,
        orientation);
  } else {
    arc = CubicPathComponent(
        orientation,
        orientation + forward * PathBuilder::kArcApproximationMagic,
        forward + orientation * PathBuilder::kArcApproximationMagic, forward);
  }
 
  Point vtx = position + orientation;
  vtx_builder.AppendVertex(vtx);
  vtx = position - orientation;
  vtx_builder.AppendVertex(vtx);
 
  Scalar line_count = std::ceilf(ComputeCubicSubdivisions(scale, arc));
  for (size_t i = 1; i < line_count; i++) {
    Point point = arc.Solve(i / line_count);
    vtx = position + point;
    vtx_builder.AppendVertex(vtx);
    vtx = position + (-point).Reflect(forward_normal);
    vtx_builder.AppendVertex(vtx);
  }
 
  Point point = arc.p2;
  vtx = position + point;
  vtx_builder.AppendVertex(position + point);
  vtx = position + (-point).Reflect(forward_normal);
  vtx_builder.AppendVertex(vtx);
}
 
void CreateSquareCap(PositionWriter& vtx_builder,
                     const Point& position,
                     const Point& offset,
                     Scalar scale,
                     bool reverse) {
  Point orientation = offset * (reverse ? -1 : 1);
  Point forward(offset.y, -offset.x);
 
  Point vtx = position + orientation;
  vtx_builder.AppendVertex(vtx);
  vtx = position - orientation;
  vtx_builder.AppendVertex(vtx);
  vtx = position + orientation + forward;
  vtx_builder.AppendVertex(vtx);
  vtx = position - orientation + forward;
  vtx_builder.AppendVertex(vtx);
}
 
Scalar CreateBevelAndGetDirection(PositionWriter& vtx_builder,
                                  const Point& position,
                                  const Point& start_offset,
                                  const Point& end_offset) {
  Point vtx = position;
  vtx_builder.AppendVertex(vtx);
 
  Scalar dir = start_offset.Cross(end_offset) > 0 ? -1 : 1;
  vtx = position + start_offset * dir;
  vtx_builder.AppendVertex(vtx);
  vtx = position + end_offset * dir;
  vtx_builder.AppendVertex(vtx);
 
  return dir;
}
 
void CreateMiterJoin(PositionWriter& vtx_builder,
                     const Point& position,
                     const Point& start_offset,
                     const Point& end_offset,
                     Scalar miter_limit,
                     Scalar scale) {
  Point start_normal = start_offset.Normalize();
  Point end_normal = end_offset.Normalize();
 
  // 1 for no joint (straight line), 0 for max joint (180 degrees).
  Scalar alignment = (start_normal.Dot(end_normal) + 1) / 2;
  if (ScalarNearlyEqual(alignment, 1)) {
    return;
  }
 
  Scalar direction = CreateBevelAndGetDirection(vtx_builder, position,
                                                start_offset, end_offset);
 
  Point miter_point = (((start_offset + end_offset) / 2) / alignment);
  if (miter_point.GetDistanceSquared({0, 0}) > miter_limit * miter_limit) {
    return;  // Convert to bevel when we exceed the miter limit.
  }
 
  // Outer miter point.
  vtx_builder.AppendVertex(position + miter_point * direction);
}
 
void CreateRoundJoin(PositionWriter& vtx_builder,
                     const Point& position,
                     const Point& start_offset,
                     const Point& end_offset,
                     Scalar miter_limit,
                     Scalar scale) {
  Point start_normal = start_offset.Normalize();
  Point end_normal = end_offset.Normalize();
 
  // 0 for no joint (straight line), 1 for max joint (180 degrees).
  Scalar alignment = 1 - (start_normal.Dot(end_normal) + 1) / 2;
  if (ScalarNearlyEqual(alignment, 0)) {
    return;
  }
 
  Scalar direction = CreateBevelAndGetDirection(vtx_builder, position,
                                                start_offset, end_offset);
 
  Point middle =
      (start_offset + end_offset).Normalize() * start_offset.GetLength();
  Point middle_normal = middle.Normalize();
 
  Point middle_handle = middle + Point(-middle.y, middle.x) *
                                     PathBuilder::kArcApproximationMagic *
                                     alignment * direction;
  Point start_handle = start_offset + Point(start_offset.y, -start_offset.x) *
                                          PathBuilder::kArcApproximationMagic *
                                          alignment * direction;
 
  CubicPathComponent arc(start_offset, start_handle, middle_handle, middle);
  Scalar line_count = std::ceilf(ComputeCubicSubdivisions(scale, arc));
  for (size_t i = 1; i < line_count; i++) {
    Point point = arc.Solve(i / line_count);
    vtx_builder.AppendVertex(position + point * direction);
    vtx_builder.AppendVertex(position +
                             (-point * direction).Reflect(middle_normal));
  }
  vtx_builder.AppendVertex(position + arc.p2 * direction);
  vtx_builder.AppendVertex(position +
                           (-arc.p2 * direction).Reflect(middle_normal));
}
 
void CreateBevelJoin(PositionWriter& vtx_builder,
                     const Point& position,
                     const Point& start_offset,
                     const Point& end_offset,
                     Scalar miter_limit,
                     Scalar scale) {
  CreateBevelAndGetDirection(vtx_builder, position, start_offset, end_offset);
}
 
void CreateSolidStrokeVertices(PositionWriter& vtx_builder,
                               const Path::Polyline& polyline,
                               Scalar stroke_width,
                               Scalar scaled_miter_limit,
                               const JoinProc& join_proc,
                               const CapProc& cap_proc,
                               Scalar scale) {
  StrokeGenerator stroke_generator(polyline, stroke_width, scaled_miter_limit,
                                   join_proc, cap_proc, scale);
  stroke_generator.Generate(vtx_builder);
}
 
// static
 
JoinProc GetJoinProc(Join stroke_join) {
  switch (stroke_join) {
    case Join::kBevel:
      return &CreateBevelJoin;
    case Join::kMiter:
      return &CreateMiterJoin;
    case Join::kRound:
      return &CreateRoundJoin;
  }
}
 
CapProc GetCapProc(Cap stroke_cap) {
  switch (stroke_cap) {
    case Cap::kButt:
      return &CreateButtCap;
    case Cap::kRound:
      return &CreateRoundCap;
    case Cap::kSquare:
      return &CreateSquareCap;
  }
}
}  // namespace
 
std::vector<Point> StrokePathGeometry::GenerateSolidStrokeVertices(
    const Path::Polyline& polyline,
    Scalar stroke_width,
    Scalar miter_limit,
    Join stroke_join,
    Cap stroke_cap,
    Scalar scale) {
  auto scaled_miter_limit = stroke_width * miter_limit * 0.5f;
  JoinProc join_proc = GetJoinProc(stroke_join);
  CapProc cap_proc = GetCapProc(stroke_cap);
  StrokeGenerator stroke_generator(polyline, stroke_width, scaled_miter_limit,
                                   join_proc, cap_proc, scale);
  std::vector<Point> points(4096);
  PositionWriter vtx_builder(points);
  stroke_generator.Generate(vtx_builder);
  return points;
}
 
StrokePathGeometry::StrokePathGeometry(const Path& path,
                                       Scalar stroke_width,
                                       Scalar miter_limit,
                                       Cap stroke_cap,
                                       Join stroke_join)
    : path_(path),
      stroke_width_(stroke_width),
      miter_limit_(miter_limit),
      stroke_cap_(stroke_cap),
      stroke_join_(stroke_join) {}
 
StrokePathGeometry::~StrokePathGeometry() = default;
 
Scalar StrokePathGeometry::GetStrokeWidth() const {
  return stroke_width_;
}
 
Scalar StrokePathGeometry::GetMiterLimit() const {
  return miter_limit_;
}
 
Cap StrokePathGeometry::GetStrokeCap() const {
  return stroke_cap_;
}
 
Join StrokePathGeometry::GetStrokeJoin() const {
  return stroke_join_;
}
 
Scalar StrokePathGeometry::ComputeAlphaCoverage(const Matrix& transform) const {
  return Geometry::ComputeStrokeAlphaCoverage(transform, stroke_width_);
}
 
GeometryResult StrokePathGeometry::GetPositionBuffer(
    const ContentContext& renderer,
    const Entity& entity,
    RenderPass& pass) const {
  if (stroke_width_ < 0.0) {
    return {};
  }
  Scalar max_basis = entity.GetTransform().GetMaxBasisLengthXY();
  if (max_basis == 0) {
    return {};
  }
 
  Scalar min_size = kMinStrokeSize / max_basis;
  Scalar stroke_width = std::max(stroke_width_, min_size);
 
  auto& host_buffer = renderer.GetTransientsBuffer();
  auto scale = entity.GetTransform().GetMaxBasisLengthXY();
 
  PositionWriter position_writer(
      renderer.GetTessellator().GetStrokePointCache());
  Path::Polyline polyline =
      renderer.GetTessellator().CreateTempPolyline(path_, scale);
 
  CreateSolidStrokeVertices(position_writer, polyline, stroke_width,
                            miter_limit_ * stroke_width_ * 0.5f,
                            GetJoinProc(stroke_join_), GetCapProc(stroke_cap_),
                            scale);
 
  const auto [arena_length, oversized_length] = position_writer.GetUsedSize();
  if (!position_writer.HasOversizedBuffer()) {
    BufferView buffer_view = host_buffer.Emplace(
        renderer.GetTessellator().GetStrokePointCache().data(),
        arena_length * sizeof(Point), alignof(Point));
 
    return GeometryResult{.type = PrimitiveType::kTriangleStrip,
                          .vertex_buffer =
                              {
                                  .vertex_buffer = buffer_view,
                                  .vertex_count = arena_length,
                                  .index_type = IndexType::kNone,
                              },
                          .transform = entity.GetShaderTransform(pass),
                          .mode = GeometryResult::Mode::kPreventOverdraw};
  }
  const std::vector<Point>& oversized_data =
      position_writer.GetOversizedBuffer();
  BufferView buffer_view = host_buffer.Emplace(
      /*buffer=*/nullptr,                                 //
      (arena_length + oversized_length) * sizeof(Point),  //
      alignof(Point)                                      //
  );
  memcpy(buffer_view.GetBuffer()->OnGetContents() +
             buffer_view.GetRange().offset,                       //
         renderer.GetTessellator().GetStrokePointCache().data(),  //
         arena_length * sizeof(Point)                             //
  );
  memcpy(buffer_view.GetBuffer()->OnGetContents() +
             buffer_view.GetRange().offset + arena_length * sizeof(Point),  //
         oversized_data.data(),                                             //
         oversized_data.size() * sizeof(Point)                              //
  );
  buffer_view.GetBuffer()->Flush(buffer_view.GetRange());
 
  return GeometryResult{.type = PrimitiveType::kTriangleStrip,
                        .vertex_buffer =
                            {
                                .vertex_buffer = buffer_view,
                                .vertex_count = arena_length + oversized_length,
                                .index_type = IndexType::kNone,
                            },
                        .transform = entity.GetShaderTransform(pass),
                        .mode = GeometryResult::Mode::kPreventOverdraw};
}
 
GeometryResult::Mode StrokePathGeometry::GetResultMode() const {
  return GeometryResult::Mode::kPreventOverdraw;
}
 
std::optional<Rect> StrokePathGeometry::GetCoverage(
    const Matrix& transform) const {
  auto path_bounds = path_.GetBoundingBox();
  if (!path_bounds.has_value()) {
    return std::nullopt;
  }
 
  Scalar max_radius = 0.5;
  if (stroke_cap_ == Cap::kSquare) {
    max_radius = max_radius * kSqrt2;
  }
  if (stroke_join_ == Join::kMiter) {
    max_radius = std::max(max_radius, miter_limit_ * 0.5f);
  }
  Scalar max_basis = transform.GetMaxBasisLengthXY();
  if (max_basis == 0) {
    return {};
  }
  // Use the most conervative coverage setting.
  Scalar min_size = kMinStrokeSize / max_basis;
  max_radius *= std::max(stroke_width_, min_size);
  return path_bounds->Expand(max_radius).TransformBounds(transform);
}
 
}  // namespace impeller