path_tessellator.h

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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.
 
#ifndef FLUTTER_IMPELLER_TESSELLATOR_PATH_TESSELLATOR_H_
#define FLUTTER_IMPELLER_TESSELLATOR_PATH_TESSELLATOR_H_
 
#include <memory>
#include <tuple>
 
#include "flutter/impeller/geometry/path_source.h"
#include "flutter/impeller/geometry/scalar.h"
#include "flutter/impeller/geometry/wangs_formula.h"
 
namespace impeller {
 
class PathTessellator {
 public:
  /// @brief An interface for generating a multi contour polyline as a triangle
  ///        strip.
  class VertexWriter {
   public:
    virtual void Write(Point point) = 0;
    virtual void EndContour() = 0;
  };
 
  /// An interface for receiving pruned path segments.
  class SegmentReceiver {
   public:
    /// Every set of path segments will be surrounded by a Begin/EndContour
    /// pair with the same origin point.
    virtual void BeginContour(Point origin, bool will_be_closed) = 0;
 
    /// Guaranteed to be non-degenerate except in the single case of stroking
    /// where we have a MoveTo followed by any number of degenerate (single
    /// point, going nowhere) path segments.
    /// p1 will always be the last recorded point.
    virtual void RecordLine(Point p1, Point p2) = 0;
 
    /// Guaranteed to be non-degenerate (not a line).
    /// p1 will always be the last recorded point.
    virtual void RecordQuad(Point p1, Point cp, Point p2) = 0;
 
    /// Guaranteed to be non-degenerate (not a quad or line)
    /// p1 will always be the last recorded point.
    virtual void RecordConic(Point p1, Point cp, Point p2, Scalar weight) = 0;
 
    /// Guaranteed to be trivially non-degenerate (not all 4 points the same).
    /// p1 will always be the last recorded point.
    virtual void RecordCubic(Point p1, Point cp1, Point cp2, Point p2) = 0;
 
    /// Every set of path segments will be surrounded by a Begin/EndContour
    /// pair with the same origin point.
    /// The boolean indicates if the path was closed as the result of an
    /// explicit PathReceiver::Close invocation which tells a stroking
    /// sub-class whether to use end caps or a "join to first segment".
    /// Contours which are closed by a MoveTo will supply "false".
    virtual void EndContour(Point origin, bool with_close) = 0;
  };
 
  struct Quad {
    const Point p1;
    const Point cp;
    const Point p2;
 
    Point Last() const { return p2; }
 
    Point Solve(Scalar t) const {
      Scalar u = 1.0f - t;
      return p1 * u * u + 2 * cp * u * t + p2 * t * t;
    }
 
    Scalar SubdivisionCount(Scalar scale) const {
      return ComputeQuadradicSubdivisions(scale, p1, cp, p2);
    }
 
    std::optional<Vector2> GetStartDirection() const {
      if (p1 != cp) {
        return (p1 - cp).Normalize();
      }
      if (p1 != p2) {
        return (p1 - p2).Normalize();
      }
      return std::nullopt;
    }
 
    std::optional<Vector2> GetEndDirection() const {
      if (p2 != cp) {
        return (p2 - cp).Normalize();
      }
      if (p2 != p1) {
        return (p2 - p1).Normalize();
      }
      return std::nullopt;
    }
  };
 
  struct Conic {
    const Point p1;
    const Point cp;
    const Point p2;
    const Scalar weight;
 
    Point Last() const { return p2; }
 
    Point Solve(Scalar t) const {
      Scalar u = 1.0f - t;
      Scalar coeff_1 = u * u;
      Scalar coeff_c = 2 * u * t * weight;
      Scalar coeff_2 = t * t;
 
      return (p1 * coeff_1 + cp * coeff_c + p2 * coeff_2) /
             (coeff_1 + coeff_c + coeff_2);
    }
 
    Scalar SubdivisionCount(Scalar scale) const {
      return ComputeConicSubdivisions(scale, p1, cp, p2, weight);
    }
 
    std::optional<Vector2> GetStartDirection() const {
      if (p1 != cp) {
        return (p1 - cp).Normalize();
      }
      if (p1 != p2) {
        return (p1 - p2).Normalize();
      }
      return std::nullopt;
    }
 
    std::optional<Vector2> GetEndDirection() const {
      if (p2 != cp) {
        return (p2 - cp).Normalize();
      }
      if (p2 != p1) {
        return (p2 - p1).Normalize();
      }
      return std::nullopt;
    }
  };
 
  struct Cubic {
    const Point p1;
    const Point cp1;
    const Point cp2;
    const Point p2;
 
    Point Last() const { return p2; }
 
    Point Solve(Scalar t) const {
      Scalar u = 1.0f - t;
      return p1 * u * u * u +       //
             3 * cp1 * u * u * t +  //
             3 * cp2 * u * t * t +  //
             p2 * t * t * t;
    }
 
    Scalar SubdivisionCount(Scalar scale) const {
      return ComputeCubicSubdivisions(scale, p1, cp1, cp2, p2);
    }
 
    std::optional<Vector2> GetStartDirection() const {
      if (p1 != cp1) {
        return (p1 - cp1).Normalize();
      }
      if (p1 != cp2) {
        return (p1 - cp2).Normalize();
      }
      if (p1 != p2) {
        return (p1 - p2).Normalize();
      }
      return std::nullopt;
    }
 
    std::optional<Vector2> GetEndDirection() const {
      if (p2 != cp2) {
        return (p2 - cp2).Normalize();
      }
      if (p2 != cp1) {
        return (p2 - cp1).Normalize();
      }
      if (p2 != p1) {
        return (p2 - p1).Normalize();
      }
      return std::nullopt;
    }
  };
 
  static void PathToFilledSegments(const PathSource& source,
                                   SegmentReceiver& receiver);
 
  static void PathToStrokedSegments(const PathSource& source,
                                    SegmentReceiver& receiver);
 
  static std::pair<size_t, size_t> CountFillStorage(const PathSource& source,
                                                    Scalar scale);
 
  static void PathToFilledVertices(const PathSource& source,
                                   VertexWriter& writer,
                                   Scalar scale);
};
 
}  // namespace impeller
 
#endif  // FLUTTER_IMPELLER_TESSELLATOR_PATH_TESSELLATOR_H_