edge_collapse.h

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/****************************************************************************
* VCGLib                                                            o o     *
* Visual and Computer Graphics Library                            o     o   *
*                                                                _   O  _   *
* Copyright(C) 2004                                                \/)\/    *
* Visual Computing Lab                                            /\/|      *
* ISTI - Italian National Research Council                           |      *
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* This program is distributed in the hope that it will be useful,           *
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* for more details.                                                         *
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#ifndef __VCG_TETRA_TRI_COLLAPSE
#define __VCG_TETRA_TRI_COLLAPSE


#include<vcg/simplex/face/pos.h>
#include<vcg/simplex/face/topology.h>

namespace vcg{
namespace tri{

template < class VERTEX_TYPE>
class BasicVertexPair {
public:
  inline BasicVertexPair() {}
  inline BasicVertexPair( VERTEX_TYPE * v0, VERTEX_TYPE * v1){V(0) = v0; V(1) = v1; }
  void Sort() {if(V(0)<V(0)) std::swap(V(0),V(0)); }
  VERTEX_TYPE *&V(int i) { return v[i]; }
  VERTEX_TYPE *cV(int i) const { return v[i]; }
private:
  VERTEX_TYPE *v[2]; // remember that v[0] will be deleted and v[1] will survive (eventually with a new position)
};


template <class TRI_MESH_TYPE, class VertexPair>
class EdgeCollapser
{
    public:
  typedef        TRI_MESH_TYPE TriMeshType;
  typedef       typename TriMeshType::FaceType FaceType;
    typedef     typename FaceType::VertexType VertexType;
  typedef       typename FaceType::VertexPointer VertexPointer;
  typedef       typename TriMeshType::VertexIterator VertexIterator;
  typedef       typename TriMeshType::FaceIterator FaceIterator;
    typedef     typename FaceType::VertexType::CoordType CoordType;
  typedef       typename TriMeshType::VertexType::ScalarType ScalarType;
  typedef typename TriMeshType::FaceContainer FaceContainer;
  typedef typename TriMeshType::VertContainer VertContainer;
  //typedef typename TriMeshType::FaceType::EdgeType EdgeType;
//      typedef typename std::vector<EdgeType> EdgeVec;
    typedef typename vcg::face::VFIterator<FaceType>  VFI;
    typedef typename std::vector<vcg::face::VFIterator<FaceType> > VFIVec;
private:
  struct EdgeSet
  {
     VFIVec av0,av1,av01;
     VFIVec & AV0() { return av0;}
     VFIVec & AV1() { return av1;}
     VFIVec & AV01(){ return av01;}
  };

  static void FindSets(VertexPair &p, EdgeSet &es)
    {
        VertexType * v0 = p.V(0);
        VertexType * v1 = p.V(1);

    es.AV0().clear();  // Facce incidenti in v0
    es.AV1().clear();  // Facce incidenti in v1
    es.AV01().clear(); // Facce incidenti in v0 e v1

        VFI x;

        for( x.f = v0->VFp(), x.z = v0->VFi(); x.f!=0; ++x)
        {
            int zv1 = -1;

            for(int j=0;j<3;++j)
                if( x.f->V(j)==&*v1 )   {
                    zv1 = j;
                    break;
                }
      if(zv1==-1)       es.AV0().push_back( x ); // la faccia x.f non ha il vertice v1 => e' incidente solo in v0
      else                      es.AV01().push_back( x );
        }

        for( x.f = v1->VFp(), x.z = v1->VFi(); x.f!=0; ++x )
        {
            int zv0 = -1;

            for(int j=0;j<3;++j)
                if( x.f->V(j)==&*v0 )   {
                    zv0 = j;
                    break;
                }
      if(zv0==-1)       es.AV1().push_back( x ); // la faccia x.f non ha il vertice v1 => e' incidente solo in v0
        }
}
/*
    Link Conditions test, as described in

    Topology Preserving Edge Contraction
    T. Dey, H. Edelsbrunner,
    Pub. Inst. Math. 1999

    Lk (sigma) is the set of all the faces of the cofaces of sigma that are disjoint from sigma

    Lk(v0) inters Lk(v1) == Lk(v0-v1)

    To perform these tests using only the VF adjacency we resort to some virtual counters over
    the vertices and the edges, we implement them as std::maps, and we increase these counters
    by running over all the faces around each vertex of the collapsing edge.

    At the end (after adding dummy stuff) we should have
       2 for vertices not shared
       4 for vertices shared
       2 for edges shared
       1 for edges not shared.


*/

public:
  static bool LinkConditions(VertexPair &pos)
  {
    typedef typename vcg::face::VFIterator<FaceType> VFIterator;
    // at the end of the loop each vertex must be counted twice
    // except for boundary vertex.
    std::map<VertexPointer,int> VertCnt;
    std::map<std::pair<VertexPointer,VertexPointer>,int> EdgeCnt;

    // the list of the boundary vertexes for the two endpoints
    std::vector<VertexPointer> BoundaryVertexVec[2];

    // Collect vertexes and edges of V0 and V1
    VFIterator vfi;
    for(int i=0;i<2;++i)
    {
      vfi = VFIterator(pos.V(i));
      for( ;!vfi.End();++vfi)
      {
        ++ VertCnt[vfi.V1()];
        ++ VertCnt[vfi.V2()];
        if(vfi.V1()<vfi.V2()) ++EdgeCnt[std::make_pair(vfi.V1(),vfi.V2())];
                         else ++EdgeCnt[std::make_pair(vfi.V2(),vfi.V1())];
      }
      // Now a loop to add dummy stuff: add the dummy vertex and two dummy edges
      // (and remember to increase the counters for the two boundary vertexes involved)
      typename std::map<VertexPointer,int>::iterator vcmit;
      for(vcmit=VertCnt.begin();vcmit!=VertCnt.end();++vcmit)
      {
        if((*vcmit).second==1) // boundary vertexes are counted only once
          BoundaryVertexVec[i].push_back((*vcmit).first);
      }
      if(BoundaryVertexVec[i].size()==2)
      { // aha! one of the two vertex of the collapse is on the boundary
        // so add dummy vertex and two dummy edges
        VertCnt[0]+=2;
        ++ EdgeCnt[std::make_pair(VertexPointer(0),BoundaryVertexVec[i][0]) ] ;
        ++ EdgeCnt[std::make_pair(VertexPointer(0),BoundaryVertexVec[i][1]) ] ;
        // remember to hide the boundaryness of the two boundary vertexes
        ++VertCnt[BoundaryVertexVec[i][0]];
        ++VertCnt[BoundaryVertexVec[i][1]];
      }
    }

    // Final loop to find cardinality of Lk( V0-V1 )
    // Note that Lk(edge) is only a set of vertices.
    std::vector<VertexPointer> LkEdge;

    for( vfi = VFIterator(pos.V(0)); !vfi.End(); ++vfi)
    {
      if(vfi.V1() == pos.V(1) ) LkEdge.push_back(vfi.V2());
      if(vfi.V2() == pos.V(1) ) LkEdge.push_back(vfi.V1());
    }

    // if the collapsing edge was a boundary edge, we must add the dummy vertex.
    // Note that this implies that Lk(edge) >=2;
    if(LkEdge.size()==1)
    {
      LkEdge.push_back(0);
    }

    // NOW COUNT!!!
    size_t SharedEdgeCnt=0;
    typename std::map<std::pair<VertexPointer,VertexPointer>, int>::iterator eci;
    for(eci=EdgeCnt.begin();eci!=EdgeCnt.end();++eci)
      if((*eci).second == 2) SharedEdgeCnt ++;

    if(SharedEdgeCnt>0) return false;
    size_t SharedVertCnt=0;
    typename std::map<VertexPointer,int>::iterator vci;
    for(vci=VertCnt.begin();vci!=VertCnt.end();++vci)
      if((*vci).second == 4) SharedVertCnt++;

    if(SharedVertCnt != LkEdge.size() ) return false;

    return true;
  }

  // Main function; the one that actually make the collapse
  // remember that v[0] will be deleted and v[1] will survive (eventually with a new position)
  // hint to do a 'collapse onto a vertex simply pass p as the position of the surviving vertex
  static int Do(TriMeshType &m, VertexPair & c, const Point3<ScalarType> &p)
    {
    EdgeSet es;
    FindSets(c,es);
        typename VFIVec::iterator i;
    int n_face_del =0 ;

    for(i=es.AV01().begin();i!=es.AV01().end();++i)
        {
            FaceType  & f = *((*i).f);
            assert(f.V((*i).z) == c.V(0));
            vcg::face::VFDetach(f,((*i).z+1)%3);
            vcg::face::VFDetach(f,((*i).z+2)%3);
            Allocator<TriMeshType>::DeleteFace(m,f);
      n_face_del++;
    }

        //set Vertex Face topology
    for(i=es.AV0().begin();i!=es.AV0().end();++i)
        {
            (*i).f->V((*i).z) = c.V(1);                                                                  // In tutte le facce incidenti in v0, si sostituisce v0 con v1
            (*i).f->VFp((*i).z) = (*i).f->V((*i).z)->VFp(); // e appendo la lista di facce incidenti in v1 a questa faccia
            (*i).f->VFi((*i).z) = (*i).f->V((*i).z)->VFi();
            (*i).f->V((*i).z)->VFp() = (*i).f;
            (*i).f->V((*i).z)->VFi() = (*i).z;
        }

        Allocator<TriMeshType>::DeleteVertex(m,*(c.V(0)));
        c.V(1)->P()=p;
        return n_face_del;
    }

};

}
}
#endif