impeller::Tessellator Class Reference

A utility that generates triangles of the specified fill type given a polyline. This happens on the CPU. More...

#include <tessellator.h>

Classes
class	EllipticalVertexGenerator
	The |VertexGenerator| implementation common to all shapes that are based on a polygonal representation of an ellipse. More...
class	VertexGenerator
	An object which produces a list of vertices as |Point|s that tessellate a previously provided shape and delivers the vertices through a |TessellatedVertexProc| callback. More...

Public Types
enum	Result {   Result::kSuccess,   Result::kInputError,   Result::kTessellationError }
using	TessellatedVertexProc = std::function< void(const Point &p)>
	A callback function for a |VertexGenerator| to deliver the vertices it computes as |Point| objects. More...

Public Member Functions
	Tessellator ()
virtual	~Tessellator ()
VertexBuffer	TessellateConvex (const Path &path, HostBuffer &host_buffer, Scalar tolerance)
	Given a convex path, create a triangle fan structure. More...
Path::Polyline	CreateTempPolyline (const Path &path, Scalar tolerance)
	Create a temporary polyline. Only one per-process can exist at a time. More...
EllipticalVertexGenerator	FilledCircle (const Matrix &view_transform, const Point &center, Scalar radius)
	Create a |VertexGenerator| that can produce vertices for a filled circle of the given radius around the given center with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform. More...
EllipticalVertexGenerator	StrokedCircle (const Matrix &view_transform, const Point &center, Scalar radius, Scalar half_width)
	Create a |VertexGenerator| that can produce vertices for a stroked circle of the given radius and half_width around the given shared center with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform. The outer edge of the stroked circle is generated at (radius + half_width) and the inner edge is generated at (radius - half_width). More...
EllipticalVertexGenerator	RoundCapLine (const Matrix &view_transform, const Point &p0, const Point &p1, Scalar radius)
	Create a |VertexGenerator| that can produce vertices for a line with round end caps of the given radius with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform. More...
EllipticalVertexGenerator	FilledEllipse (const Matrix &view_transform, const Rect &bounds)
	Create a |VertexGenerator| that can produce vertices for a filled ellipse inscribed within the given bounds with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform. More...
EllipticalVertexGenerator	FilledRoundRect (const Matrix &view_transform, const Rect &bounds, const Size &radii)
	Create a |VertexGenerator| that can produce vertices for a filled round rect within the given bounds and corner radii with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform. More...

Static Public Member Functions
static void	TessellateConvexInternal (const Path &path, std::vector< Point > &point_buffer, std::vector< uint16_t > &index_buffer, Scalar tolerance)

Static Public Attributes
static constexpr Scalar	kCircleTolerance = 0.1f
	The pixel tolerance used by the algorighm to determine how many divisions to create for a circle. More...

Protected Attributes
std::unique_ptr< std::vector< Point > >	point_buffer_
	Used for polyline generation. More...
std::unique_ptr< std::vector< uint16_t > >	index_buffer_

Detailed Description

A utility that generates triangles of the specified fill type given a polyline. This happens on the CPU.

Also contains functionality for optimized generation of circles and ellipses.

This object is not thread safe, and its methods must not be called from multiple threads.

Definition at line 31 of file tessellator.h.

Member Typedef Documentation

TessellatedVertexProc

using impeller::Tessellator::TessellatedVertexProc = std::function<void(const Point& p)>

A callback function for a |VertexGenerator| to deliver the vertices it computes as |Point| objects.

Definition at line 78 of file tessellator.h.

Member Enumeration Documentation

Result

enum impeller::Tessellator::Result

strong

Enumerator
kSuccess
kInputError
kTessellationError

Definition at line 70 of file tessellator.h.

                    {
    kSuccess,
    kInputError,
    kTessellationError,
  };

Constructor & Destructor Documentation

Tessellator()

impeller::Tessellator::Tessellator

(

)

Definition at line 9 of file tessellator.cc.

    : point_buffer_(std::make_unique<std::vector<Point>>()),
      index_buffer_(std::make_unique<std::vector<uint16_t>>()) {
  point_buffer_->reserve(2048);
  index_buffer_->reserve(2048);
}

References index_buffer_, and point_buffer_.

~Tessellator()

impeller::Tessellator::~Tessellator ( )

virtualdefault

Member Function Documentation

CreateTempPolyline()

Path::Polyline impeller::Tessellator::CreateTempPolyline	(	const Path &	path,
		Scalar	tolerance
	)

Create a temporary polyline. Only one per-process can exist at a time.

The tessellator itself is not a thread safe class and should only be used from the raster thread.

Definition at line 18 of file tessellator.cc.

                                                                 {
  FML_DCHECK(point_buffer_);
  point_buffer_->clear();
  auto polyline =
      path.CreatePolyline(tolerance, std::move(point_buffer_),
                          [this](Path::Polyline::PointBufferPtr point_buffer) {
                            point_buffer_ = std::move(point_buffer);
                          });
  return polyline;
}

References impeller::Path::CreatePolyline(), point_buffer_, and polyline.

FilledCircle()

EllipticalVertexGenerator impeller::Tessellator::FilledCircle	(	const Matrix &	view_transform,
		const Point &	center,
		Scalar	radius
	)

Create a |VertexGenerator| that can produce vertices for a filled circle of the given radius around the given center with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform.

Note that the view transform is only used to choose the number of sample points to use per quarter circle and the returned points are not transformed by it, instead they are relative to the coordinate space of the center point.

Definition at line 237 of file tessellator.cc.

                   {
  auto divisions =
      ComputeQuadrantDivisions(view_transform.GetMaxBasisLength() * radius);
  return EllipticalVertexGenerator(Tessellator::GenerateFilledCircle,
                                   GetTrigsForDivisions(divisions),
                                   PrimitiveType::kTriangleStrip, 4,
                                   {
                                       .reference_centers = {center, center},
                                       .radii = {radius, radius},
                                       .half_width = -1.0f,
                                   });
}

References impeller::ComputeQuadrantDivisions(), and impeller::Matrix::GetMaxBasisLength().

FilledEllipse()

EllipticalVertexGenerator impeller::Tessellator::FilledEllipse	(	const Matrix &	view_transform,
		const Rect &	bounds
	)

Create a |VertexGenerator| that can produce vertices for a filled ellipse inscribed within the given bounds with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform.

Note that the view transform is only used to choose the number of sample points to use per quarter circle and the returned points are not transformed by it, instead they are relative to the coordinate space of the bounds.

Definition at line 297 of file tessellator.cc.

                        {
  if (bounds.IsSquare()) {
    return FilledCircle(view_transform, bounds.GetCenter(),
                        bounds.GetWidth() * 0.5f);
  }
  auto max_radius = bounds.GetSize().MaxDimension();
  auto divisions =
      ComputeQuadrantDivisions(view_transform.GetMaxBasisLength() * max_radius);
  auto center = bounds.GetCenter();
  return EllipticalVertexGenerator(Tessellator::GenerateFilledEllipse,
                                   GetTrigsForDivisions(divisions),
                                   PrimitiveType::kTriangleStrip, 4,
                                   {
                                       .reference_centers = {center, center},
                                       .radii = bounds.GetSize() * 0.5f,
                                       .half_width = -1.0f,
                                   });
}

References impeller::ComputeQuadrantDivisions(), impeller::TRect< T >::GetCenter(), impeller::Matrix::GetMaxBasisLength(), impeller::TRect< T >::GetSize(), impeller::TRect< T >::GetWidth(), and impeller::TRect< T >::IsSquare().

FilledRoundRect()

EllipticalVertexGenerator impeller::Tessellator::FilledRoundRect	(	const Matrix &	view_transform,
		const Rect &	bounds,
		const Size &	radii
	)

Create a |VertexGenerator| that can produce vertices for a filled round rect within the given bounds and corner radii with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform.

Note that the view transform is only used to choose the number of sample points to use per quarter circle and the returned points are not transformed by it, instead they are relative to the coordinate space of the bounds.

Definition at line 318 of file tessellator.cc.

                       {
  if (radii.width * 2 < bounds.GetWidth() ||
      radii.height * 2 < bounds.GetHeight()) {
    auto max_radius = radii.MaxDimension();
    auto divisions = ComputeQuadrantDivisions(
        view_transform.GetMaxBasisLength() * max_radius);
    auto upper_left = bounds.GetLeftTop() + radii;
    auto lower_right = bounds.GetRightBottom() - radii;
    return EllipticalVertexGenerator(Tessellator::GenerateFilledRoundRect,
                                     GetTrigsForDivisions(divisions),
                                     PrimitiveType::kTriangleStrip, 4,
                                     {
                                         .reference_centers =
                                             {
                                                 upper_left,
                                                 lower_right,
                                             },
                                         .radii = radii,
                                         .half_width = -1.0f,
                                     });
  } else {
    return FilledEllipse(view_transform, bounds);
  }
}

References impeller::ComputeQuadrantDivisions(), impeller::TRect< T >::GetHeight(), impeller::TRect< T >::GetLeftTop(), impeller::Matrix::GetMaxBasisLength(), impeller::TRect< T >::GetRightBottom(), impeller::TRect< T >::GetWidth(), impeller::TSize< T >::height, impeller::TSize< T >::MaxDimension(), and impeller::TSize< T >::width.

RoundCapLine()

EllipticalVertexGenerator impeller::Tessellator::RoundCapLine	(	const Matrix &	view_transform,
		const Point &	p0,
		const Point &	p1,
		Scalar	radius
	)

Create a |VertexGenerator| that can produce vertices for a line with round end caps of the given radius with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform.

Note that the view transform is only used to choose the number of sample points to use per quarter circle and the returned points are not transformed by it, instead they are relative to the coordinate space of the two points.

Definition at line 274 of file tessellator.cc.

                   {
  auto along = p1 - p0;
  auto length = along.GetLength();
  if (length > kEhCloseEnough) {
    auto divisions =
        ComputeQuadrantDivisions(view_transform.GetMaxBasisLength() * radius);
    return EllipticalVertexGenerator(Tessellator::GenerateRoundCapLine,
                                     GetTrigsForDivisions(divisions),
                                     PrimitiveType::kTriangleStrip, 4,
                                     {
                                         .reference_centers = {p0, p1},
                                         .radii = {radius, radius},
                                         .half_width = -1.0f,
                                     });
  } else {
    return FilledCircle(view_transform, p0, radius);
  }
}

References impeller::ComputeQuadrantDivisions(), impeller::TPoint< T >::GetLength(), impeller::Matrix::GetMaxBasisLength(), and impeller::kEhCloseEnough.

StrokedCircle()

EllipticalVertexGenerator impeller::Tessellator::StrokedCircle	(	const Matrix &	view_transform,
		const Point &	center,
		Scalar	radius,
		Scalar	half_width
	)

Create a |VertexGenerator| that can produce vertices for a stroked circle of the given radius and half_width around the given shared center with enough polygon sub-divisions to provide reasonable fidelity when viewed under the given view transform. The outer edge of the stroked circle is generated at (radius + half_width) and the inner edge is generated at (radius - half_width).

Note that the view transform is only used to choose the number of sample points to use per quarter circle and the returned points are not transformed by it, instead they are relative to the coordinate space of the center point.

Definition at line 253 of file tessellator.cc.

                       {
  if (half_width > 0) {
    auto divisions = ComputeQuadrantDivisions(
        view_transform.GetMaxBasisLength() * radius + half_width);
    return EllipticalVertexGenerator(Tessellator::GenerateStrokedCircle,
                                     GetTrigsForDivisions(divisions),
                                     PrimitiveType::kTriangleStrip, 8,
                                     {
                                         .reference_centers = {center, center},
                                         .radii = {radius, radius},
                                         .half_width = half_width,
                                     });
  } else {
    return FilledCircle(view_transform, center, radius);
  }
}

References impeller::ComputeQuadrantDivisions(), and impeller::Matrix::GetMaxBasisLength().

TessellateConvex()

VertexBuffer impeller::Tessellator::TessellateConvex	(	const Path &	path,
		HostBuffer &	host_buffer,
		Scalar	tolerance
	)

Given a convex path, create a triangle fan structure.

Parameters

[in]	path	The path to tessellate.
[in]	tolerance	The tolerance value for conversion of the path to a polyline. This value is often derived from the Matrix::GetMaxBasisLength of the CTM applied to the path for rendering.

Returns

A point vector containing the vertices in triangle strip format.

Parameters

[in]

host_buffer

The host buffer for allocation of vertices/index data.

Returns

A vertex buffer containing all data from the provided curve.

Definition at line 30 of file tessellator.cc.

                                                             {
  FML_DCHECK(point_buffer_);
  FML_DCHECK(index_buffer_);
  TessellateConvexInternal(path, *point_buffer_, *index_buffer_, tolerance);
 
  if (point_buffer_->empty()) {
    return VertexBuffer{
        .vertex_buffer = {},
        .index_buffer = {},
        .vertex_count = 0u,
        .index_type = IndexType::k16bit,
    };
  }
 
  BufferView vertex_buffer = host_buffer.Emplace(
      point_buffer_->data(), sizeof(Point) * point_buffer_->size(),
      alignof(Point));
 
  BufferView index_buffer = host_buffer.Emplace(
      index_buffer_->data(), sizeof(uint16_t) * index_buffer_->size(),
      alignof(uint16_t));
 
  return VertexBuffer{
      .vertex_buffer = std::move(vertex_buffer),
      .index_buffer = std::move(index_buffer),
      .vertex_count = index_buffer_->size(),
      .index_type = IndexType::k16bit,
  };
}

References impeller::HostBuffer::Emplace(), index_buffer_, impeller::k16bit, point_buffer_, TessellateConvexInternal(), and impeller::VertexBuffer::vertex_buffer.

TessellateConvexInternal()

void impeller::Tessellator::TessellateConvexInternal ( const Path &  path, std::vector< Point > &  point_buffer, std::vector< uint16_t > &  index_buffer, Scalar  tolerance  )

static

Visible for testing.

This method only exists for the ease of benchmarking without using the real allocator needed by the [host_buffer].

Definition at line 62 of file tessellator.cc.

                                                             {
  point_buffer.clear();
  index_buffer.clear();
 
  VertexWriter writer(point_buffer, index_buffer);
 
  path.WritePolyline(tolerance, writer);
}

References impeller::Path::WritePolyline().

Referenced by impeller::BM_Convex(), TessellateConvex(), and impeller::testing::TEST().

Member Data Documentation

index_buffer_

std::unique_ptr<std::vector<uint16_t> > impeller::Tessellator::index_buffer_

protected

Definition at line 288 of file tessellator.h.

Referenced by TessellateConvex(), and Tessellator().

kCircleTolerance

constexpr Scalar impeller::Tessellator::kCircleTolerance = 0.1f

staticconstexpr

The pixel tolerance used by the algorighm to determine how many divisions to create for a circle.

No point on the polygon of vertices should deviate from the true circle by more than this tolerance.

Definition at line 214 of file tessellator.h.

Referenced by impeller::ComputeQuadrantDivisions(), and impeller::testing::TEST().

point_buffer_

std::unique_ptr<std::vector<Point> > impeller::Tessellator::point_buffer_

protected

Used for polyline generation.

Definition at line 287 of file tessellator.h.

Referenced by CreateTempPolyline(), TessellateConvex(), and Tessellator().

---

The documentation for this class was generated from the following files:

• impeller/tessellator/tessellator.h
• impeller/tessellator/tessellator.cc