impeller/tessellator/tessellator.cc

// 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()
    : point_buffer_(std::make_unique<std::vector<Point>>()),
      c_tessellator_(nullptr, &DestroyTessellator) {
  point_buffer_->reserve(2048);
  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(const Path& path,
                                            Scalar tolerance,
                                            const BuilderCallback& callback) {
  if (!callback) {
    return Result::kInputError;
  }

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

  auto fill_type = path.GetFillType();

  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;
}

std::vector<Point> Tessellator::TessellateConvex(const Path& path,
                                                 Scalar tolerance) {
  std::vector<Point> output;

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

  output.reserve(polyline.points->size() +
                 (4 * (polyline.contours.size() - 1)));
  for (auto j = 0u; j < polyline.contours.size(); j++) {
    auto [start, end] = polyline.GetContourPointBounds(j);
    auto first_point = polyline.GetPoint(start);

    // Some polygons will not self close and an additional triangle
    // must be inserted, others will self close and we need to avoid
    // inserting an extra triangle.
    if (polyline.GetPoint(end - 1) == first_point) {
      end--;
    }

    if (j > 0) {
      // Triangle strip break.
      output.emplace_back(output.back());
      output.emplace_back(first_point);
      output.emplace_back(first_point);
    } else {
      output.emplace_back(first_point);
    }

    size_t a = start + 1;
    size_t b = end - 1;
    while (a < b) {
      output.emplace_back(polyline.GetPoint(a));
      output.emplace_back(polyline.GetPoint(b));
      a++;
      b--;
    }
    if (a == b) {
      output.emplace_back(polyline.GetPoint(a));
    }
  }
  return output;
}

void DestroyTessellator(TESStesselator* tessellator) {
  if (tessellator != nullptr) {
    ::tessDeleteTess(tessellator);
  }
}

}  // namespace impeller