tessellator.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/tessellator/tessellator.h"
 
#include "third_party/libtess2/Include/tesselator.h"
 
namespace impeller {
 
static void* HeapAlloc(void* userData, unsigned int size) {
  return malloc(size);
}
 
static void* HeapRealloc(void* userData, void* ptr, unsigned int size) {
  return realloc(ptr, size);
}
 
static void HeapFree(void* userData, void* ptr) {
  free(ptr);
}
 
// Note: these units are "number of entities" for bucket size and not in KB.
static const TESSalloc kAlloc = {
    HeapAlloc, HeapRealloc, HeapFree, 0, /* =userData */
    16,                                  /* =meshEdgeBucketSize */
    16,                                  /* =meshVertexBucketSize */
    16,                                  /* =meshFaceBucketSize */
    16,                                  /* =dictNodeBucketSize */
    16,                                  /* =regionBucketSize */
    0                                    /* =extraVertices */
};
 
Tessellator::Tessellator() : 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);
  }
}
 
Tessellator::~Tessellator() = default;
 
static int ToTessWindingRule(FillType fill_type) {
  switch (fill_type) {
    case FillType::kOdd:
      return TESS_WINDING_ODD;
    case FillType::kNonZero:
      return TESS_WINDING_NONZERO;
    case FillType::kPositive:
      return TESS_WINDING_POSITIVE;
    case FillType::kNegative:
      return TESS_WINDING_NEGATIVE;
    case FillType::kAbsGeqTwo:
      return TESS_WINDING_ABS_GEQ_TWO;
  }
  return TESS_WINDING_ODD;
}
 
Tessellator::Result Tessellator::Tessellate(
    FillType fill_type,
    const Path::Polyline& polyline,
    const BuilderCallback& callback) const {
  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;
}
 
void DestroyTessellator(TESStesselator* tessellator) {
  if (tessellator != nullptr) {
    ::tessDeleteTess(tessellator);
  }
}
 
}  // namespace impeller