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>

Public Types
enum	Result {   Result::kSuccess,   Result::kInputError,   Result::kTessellationError }
using	BuilderCallback = std::function< bool(const float *vertices, size_t vertices_count, const uint16_t *indices, size_t indices_count)>
	A callback that returns the results of the tessellation. More...

Public Member Functions
	Tessellator ()
	~Tessellator ()
Tessellator::Result	Tessellate (FillType fill_type, const Path::Polyline &polyline, const BuilderCallback &callback) const
	Generates filled triangles from the polyline. A callback is invoked once for the entire tessellation. More...

Detailed Description

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

Definition at line 35 of file tessellator.h.

Member Typedef Documentation

BuilderCallback

using impeller::Tessellator::BuilderCallback = std::function<bool(const float* vertices, size_t vertices_count, const uint16_t* indices, size_t indices_count)>

A callback that returns the results of the tessellation.

   The index buffer may not be populated, in which case [indices] will
   be nullptr and indices_count will be 0. 

Definition at line 54 of file tessellator.h.

Member Enumeration Documentation

Result

enum impeller::Tessellator::Result

strong

Enumerator
kSuccess
kInputError
kTessellationError

Definition at line 37 of file tessellator.h.

                    {
    kSuccess,
    kInputError,
    kTessellationError,
  };

Constructor & Destructor Documentation

Tessellator()

impeller::Tessellator::Tessellator

(

)

Definition at line 34 of file tessellator.cc.

                         : c_tessellator_(nullptr, &DestroyTessellator) {
  TESSalloc alloc = kAlloc;
  {
    // libTess2 copies the TESSalloc despite the non-const argument.
    CTessellator tessellator(::tessNewTess(&alloc), &DestroyTessellator);
    c_tessellator_ = std::move(tessellator);
  }
}

References impeller::DestroyTessellator(), and impeller::kAlloc.

~Tessellator()

impeller::Tessellator::~Tessellator ( )

default

Member Function Documentation

Tessellate()

Tessellator::Result impeller::Tessellator::Tessellate	(	FillType	fill_type,
		const Path::Polyline &	polyline,
		const BuilderCallback &	callback
	)		const

Generates filled triangles from the polyline. A callback is invoked once for the entire tessellation.

Parameters

[in]	fill_type	The fill rule to use when filling.
[in]	polyline	The polyline
[in]	callback	The callback, return false to indicate failure.

Returns

The result status of the tessellation.

Feed contour information to the tessellator.

Let's tessellate.

Definition at line 61 of file tessellator.cc.

                                           {
  if (!callback) {
    return Result::kInputError;
  }
 
  if (polyline.points.empty()) {
    return Result::kInputError;
  }
 
  auto tessellator = c_tessellator_.get();
  if (!tessellator) {
    return Result::kTessellationError;
  }
 
  constexpr int kVertexSize = 2;
  constexpr int kPolygonSize = 3;
 
  //----------------------------------------------------------------------------
  /// Feed contour information to the tessellator.
  ///
  static_assert(sizeof(Point) == 2 * sizeof(float));
  for (size_t contour_i = 0; contour_i < polyline.contours.size();
       contour_i++) {
    size_t start_point_index, end_point_index;
    std::tie(start_point_index, end_point_index) =
        polyline.GetContourPointBounds(contour_i);
 
    ::tessAddContour(tessellator,  // the C tessellator
                     kVertexSize,  //
                     polyline.points.data() + start_point_index,  //
                     sizeof(Point),                               //
                     end_point_index - start_point_index          //
    );
  }
 
  //----------------------------------------------------------------------------
  /// Let's tessellate.
  ///
  auto result = ::tessTesselate(tessellator,                   // tessellator
                                ToTessWindingRule(fill_type),  // winding
                                TESS_POLYGONS,                 // element type
                                kPolygonSize,                  // polygon size
                                kVertexSize,                   // vertex size
                                nullptr  // normal (null is automatic)
  );
 
  if (result != 1) {
    return Result::kTessellationError;
  }
 
  int element_item_count = tessGetElementCount(tessellator) * kPolygonSize;
 
  // We default to using a 16bit index buffer, but in cases where we generate
  // more tessellated data than this can contain we need to fall back to
  // dropping the index buffer entirely. Instead code could instead switch to
  // a uint32 index buffer, but this is done for simplicity with the other
  // fast path above.
  if (element_item_count < USHRT_MAX) {
    int vertex_item_count = tessGetVertexCount(tessellator);
    auto vertices = tessGetVertices(tessellator);
    auto elements = tessGetElements(tessellator);
 
    // libtess uses an int index internally due to usage of -1 as a sentinel
    // value.
    std::vector<uint16_t> indices(element_item_count);
    for (int i = 0; i < element_item_count; i++) {
      indices[i] = static_cast<uint16_t>(elements[i]);
    }
    if (!callback(vertices, vertex_item_count, indices.data(),
                  element_item_count)) {
      return Result::kInputError;
    }
  } else {
    std::vector<Point> points;
    std::vector<float> data;
 
    int vertex_item_count = tessGetVertexCount(tessellator) * kVertexSize;
    auto vertices = tessGetVertices(tessellator);
    points.reserve(vertex_item_count);
    for (int i = 0; i < vertex_item_count; i += 2) {
      points.emplace_back(vertices[i], vertices[i + 1]);
    }
 
    int element_item_count = tessGetElementCount(tessellator) * kPolygonSize;
    auto elements = tessGetElements(tessellator);
    data.reserve(element_item_count);
    for (int i = 0; i < element_item_count; i++) {
      data.emplace_back(points[elements[i]].x);
      data.emplace_back(points[elements[i]].y);
    }
    if (!callback(data.data(), element_item_count, nullptr, 0u)) {
      return Result::kInputError;
    }
  }
 
  return Result::kSuccess;
}

References impeller::Path::Polyline::contours, impeller::Path::Polyline::GetContourPointBounds(), kInputError, kSuccess, kTessellationError, impeller::Path::Polyline::points, and impeller::ToTessWindingRule().

Referenced by impeller::BM_Polyline(), impeller::Tessellate(), impeller::testing::TEST(), and impeller::testing::TEST_P().

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

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