HalfEdgeTriangleMesh.h

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#pragma once

#include <Eigen/Core>
#include <unordered_map>

#include "Open3D/Geometry/TriangleMesh.h"
#include "Open3D/Geometry/Geometry3D.h"

namespace open3d {
namespace geometry {

class HalfEdgeTriangleMesh : public TriangleMesh {
public:
    class HalfEdge {
    public:
        HalfEdge() {}
        HalfEdge(const Eigen::Vector2i &vertex_indices,
                 int triangle_index,
                 int next,
                 int twin);
        bool IsBoundary() const { return twin_ == -1; }

    public:
        // Index of the next HalfEdge
        int next_ = -1;
        // Index of the twin HalfEdge
        int twin_ = -1;
        // Index of the ordered vertices forming this half edge
        Eigen::Vector2i vertex_indices_ = Eigen::Vector2i(-1, -1);
        // Index of the triangle containing this half edge
        int triangle_index_ = -1;
    };

public:
    HalfEdgeTriangleMesh()
        : TriangleMesh(Geometry::GeometryType::HalfEdgeTriangleMesh){};

    bool ComputeHalfEdges();

    bool HasHalfEdges() const;

    std::vector<int> BoundaryHalfEdgesFromVertex(int vertex_index) const;

    std::vector<int> BoundaryVerticesFromVertex(int vertex_index) const;

    std::vector<std::vector<int>> GetBoundaries() const;

    void Clear() override;

    HalfEdgeTriangleMesh &operator+=(const HalfEdgeTriangleMesh &mesh);

    HalfEdgeTriangleMesh operator+(const HalfEdgeTriangleMesh &mesh) const;

protected:
    HalfEdgeTriangleMesh(Geometry::GeometryType type) : TriangleMesh(type){};
    void RemoveDuplicatedVertices() override;
    void RemoveDuplicatedTriangles() override;
    void RemoveNonManifoldVertices() override;
    void RemoveNonManifoldTriangles() override;

public:
    std::vector<HalfEdge> half_edges_;

    std::vector<std::vector<int>> ordered_half_edge_from_vertex_;

protected:
    int NextHalfEdgeFromVertex(int init_half_edge_index) const;
    int NextHalfEdgeOnBoundary(int curr_half_edge_index) const;
};

std::shared_ptr<HalfEdgeTriangleMesh> CreateHalfEdgeMeshFromMesh(
        const TriangleMesh &mesh);

}  // namespace geometry
}  // namespace open3d