LOD_EdgeCollapser.cpp

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 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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 * The Original Code is Copyright (C) 2001-2002 by NaN Holding BV.
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#include "LOD_EdgeCollapser.h"

#include "LOD_ManMesh2.h"
#include "CTR_TaggedSetOps.h"
#include <algorithm>
#include <functional>


using namespace std;


    LOD_EdgeCollapser * 
LOD_EdgeCollapser::
New(
){
    return new LOD_EdgeCollapser();
}


    bool
LOD_EdgeCollapser::
TJunctionTest(
    LOD_ManMesh2 &mesh,
    vector<LOD_EdgeInd> &e_v0v1,
    LOD_EdgeInd collapse_edge
){

    // we need to copy the egdes in e_v0v1 from the mesh
    // into a new buffer -> we are going to modify them

    int original_size = e_v0v1.size();
    if (original_size == 0) return true;

    vector<LOD_Edge> &edge_set = mesh.EdgeSet();

    LOD_VertexInd c_v0 = edge_set[collapse_edge].m_verts[0];
    LOD_VertexInd c_v1 = edge_set[collapse_edge].m_verts[1];

    vector<LOD_Edge> temp_edges;
    temp_edges.reserve(e_v0v1.size());

    vector<LOD_EdgeInd>::iterator edge_it = e_v0v1.begin();
    vector<LOD_EdgeInd>::const_iterator edge_end = e_v0v1.end();

    for (;edge_it != edge_end; ++edge_it) {
        temp_edges.push_back(edge_set[*edge_it]);
    }

    // in the copied edges replace all instances of c_v0 with c_v1

    vector<LOD_Edge>::iterator e_it = temp_edges.begin();
    vector<LOD_Edge>::const_iterator e_it_end = temp_edges.end();
        
    for (; e_it != e_it_end; ++e_it) {

        if (e_it->m_verts[0] == c_v0) {
            e_it->m_verts[0] = c_v1;
        }
        if (e_it->m_verts[1] == c_v0) {
            e_it->m_verts[1] = c_v1;
        }

        // normalize the edge
        if (int(e_it->m_verts[0]) > int(e_it->m_verts[1])) {
            LOD_EdgeInd temp = e_it->m_verts[0];
            e_it->m_verts[0] = e_it->m_verts[1];
            e_it->m_verts[1] = temp;
        }
    }

    // sort the edges using the edge less functional 

    sort(temp_edges.begin(),temp_edges.end(),LOD_EdgeCollapser::less());
    // count the unique edges.

    e_it = temp_edges.begin();
    e_it_end = temp_edges.end();
    
    int coincedent_edges = 0;

    vector<LOD_Edge>::const_iterator last_edge = e_it;
    ++e_it;
        
    for (; e_it != e_it_end; ++e_it) {
    
        if ((e_it->m_verts[0] == last_edge->m_verts[0]) &&
            (e_it->m_verts[1] == last_edge->m_verts[1])
        ) {
            ++coincedent_edges;
        }
        last_edge = e_it;
    }

    // now if the collapse edge is a boundary edges 
    // then we are alloved at most one coincedent edge

    // otherwise at most 2 coincedent edges

    if (edge_set[collapse_edge].BoundaryEdge()) {
        return (coincedent_edges > 1);
    } else {
        return (coincedent_edges > 2);
    }


}


    
    bool
LOD_EdgeCollapser::
CollapseEdge(
    LOD_EdgeInd ei,
    LOD_ManMesh2 &mesh,
    vector<LOD_EdgeInd> & degenerate_edges,
    vector<LOD_FaceInd> & degenerate_faces,
    vector<LOD_VertexInd> & degenerate_vertices,
    vector<LOD_EdgeInd> & new_edges,
    vector<LOD_FaceInd> & update_faces,
    vector<LOD_VertexInd> & update_vertices
){

    vector<LOD_Vertex>  &verts = mesh.VertexSet();
    vector<LOD_Edge>    &edges = mesh.EdgeSet();
    vector<LOD_TriFace> &faces = mesh.FaceSet();

    // shouldn't do this (use mesh interface instead!)
    LOD_VertexInd v0_ind = edges[ei].m_verts[0];
    LOD_VertexInd v1_ind = edges[ei].m_verts[1];
#if 0
    LOD_Vertex &v0 = verts[v0_ind];
    LOD_Vertex &v1 = verts[v1_ind];
#endif
    vector<vector<LOD_EdgeInd> > e_v01(2);
    e_v01[0].reserve(32);
    e_v01[1].reserve(32);
    
    mesh.VertexEdges(v0_ind,e_v01[0]);
    mesh.VertexEdges(v1_ind,e_v01[1]);


    // compute the union of e_v0 and e_v1 -> this is the degenerate edges of the collapse
    // we remove old edges and replace edges inside the collapse zone with new ones 

    CTR_TaggedSetOps<LOD_EdgeInd,LOD_Edge>::Union(e_v01,edges,degenerate_edges);

    vector< vector<LOD_FaceInd> > p_v01(2);
    p_v01[0].reserve(32);
    p_v01[1].reserve(32);

    mesh.VertexFaces(v0_ind,p_v01[0]);
    mesh.VertexFaces(v1_ind,p_v01[1]);

    // compute the union of p_v0 anf p_v1
    vector<LOD_FaceInd> p_v0v1;
    p_v0v1.reserve(32);

    CTR_TaggedSetOps<LOD_FaceInd,LOD_TriFace>::Union(p_v01,faces,p_v0v1);

    // compute the union of all the edges in p_v0v1 this is the collapse zone

    vector<vector<LOD_EdgeInd> > e_input_vectors(p_v0v1.size());

    vector<LOD_FaceInd>::iterator p_v0v1_end = p_v0v1.end();
    vector<LOD_FaceInd>::iterator p_v0v1_start = p_v0v1.begin();

    vector<vector<LOD_FaceInd> >::iterator vector_insert_it = e_input_vectors.begin();
    
    for (;p_v0v1_start != p_v0v1_end; ++p_v0v1_start , ++vector_insert_it) {
        mesh.FaceEdges(*p_v0v1_start,*vector_insert_it);
    }

    vector<LOD_EdgeInd> collapse_zone;
    collapse_zone.reserve(32);

    CTR_TaggedSetOps<LOD_EdgeInd,LOD_Edge>::Union(e_input_vectors,edges,collapse_zone);

    // compute the ring edges = collpase_zone - e_v0v1

    vector<LOD_EdgeInd> edge_ring;
    edge_ring.reserve(32);

    CTR_TaggedSetOps<LOD_EdgeInd,LOD_Edge>::Difference(collapse_zone,degenerate_edges,edges,edge_ring);

    // T Junction test
    // At this point we check to see if any of the polygons
    // in p_v0v1 are coninceddent - this leads
    // to errors later on if we try and insert a polygon
    // into the mesh to an edge which already has 2 polygons.

    // not that t junctions occur naturally from edge collapses
    // and are not just the result of coincedent polygons
    // for example consider collapsing an edge that forms part
    // of a triangular bottle neck.

    // Really we need to make sure that we don't create t-junctions.

    // I think that a sufficient test is to check the number of
    // coincedent edge pairs after a collapse. If it is more than 2
    // then collapsing the edge may result in an undeleted edge 
    // sharing more than 2 polygons. This test probably is too 
    // restictive though.
    
    // To perform this test we need to make a copy of the edges
    // in e_v0v1. We then apply the contraction to these edge
    // copies. Sort them using a function that places coincedent 
    // edges next to each other. And then count the number
    // of coincedent pairs. 

    // Of course we have to do this test before we change any of the
    // mesh -> so we can back out safely.

    if (TJunctionTest(mesh,degenerate_edges,ei)) return false; 

    // Compute the set of possibly degenerate vertices
    // this is the union of all the vertices of polygons
    // of v0 and v1

    vector<LOD_FaceInd>::iterator face_it = p_v0v1.begin();
    vector<LOD_FaceInd>::const_iterator face_end = p_v0v1.end();


    vector<vector<LOD_VertexInd> > p_v0v1_vertices(p_v0v1.size());
    
    for (int i = 0; face_it != face_end; ++face_it, ++i) {
        mesh.FaceVertices(*face_it,p_v0v1_vertices[i]);
    }
    
    vector<LOD_VertexInd> vertex_ring;
    vertex_ring.reserve(32);

    CTR_TaggedSetOps<LOD_VertexInd,LOD_Vertex>::Union(p_v0v1_vertices,verts,vertex_ring);

    // remove all the internal edges e_v0v1 from the mesh.
    // for each edge remove the egde from it's vertices edge lists.

    vector<LOD_EdgeInd>::iterator edge_it = degenerate_edges.begin();
    vector<LOD_EdgeInd>::const_iterator edge_end = degenerate_edges.end();

    for (; !(edge_it == edge_end); ++edge_it) {
            
        LOD_EdgeInd ed = (*edge_it);
        LOD_Edge & edge = edges[ed];//*edge_it];
    
        verts[edge.m_verts[0]].RemoveEdge(ed);
        verts[edge.m_verts[1]].RemoveEdge(ed);
    }

    // we postpone deletion of the internal edges untill the end
    // this is because deleting edges invalidates all of the 
    // EdgeInd vectors above.

        
    // now untie all the polygons in p_v0v1 from the edge ring

    // select all polygons in p_v0v1

    face_it = p_v0v1.begin();
    face_end = p_v0v1.end();

    for (;face_it != face_end; ++face_it) {
        faces[*face_it].SetSelectTag(true);
    }

    edge_it = edge_ring.begin();
    edge_end = edge_ring.end();

    for (;edge_it != edge_end; ++edge_it) {
        LOD_Edge & edge = edges[*edge_it];

        // presumably all edges in edge_ring point to at least
        // one polygon from p_v0v1
        
        if (!edge.m_faces[0].IsEmpty() && faces[edge.m_faces[0]].SelectTag()) {
            edge.m_faces[0].Invalidate();
        }

        if (!edge.m_faces[1].IsEmpty() && faces[edge.m_faces[1]].SelectTag()) {
            edge.m_faces[1].Invalidate();
        }
    }
    
    // deselect the faces

    face_it = p_v0v1.begin();
    face_end = p_v0v1.end();

    for (;face_it != face_end; ++face_it) {
        faces[*face_it].SetSelectTag(false);
    }

    // perform the edge collapse

    // iterate through the polygons of p_v0 and replace the vertex
    // index v0 with v1

    face_it = p_v01[0].begin();
    face_end = p_v01[0].end();
    
    for (;face_it != face_end; ++face_it) {
        faces[*face_it].SwapVertex(v0_ind,v1_ind);
    }

    face_it = p_v0v1.begin();
    face_end = p_v0v1.end();
    
    for (;face_it != face_end; ++face_it) {
        if (faces[*face_it].Degenerate()) {
            degenerate_faces.push_back(*face_it);
        } else {
            update_faces.push_back(*face_it);
        }
    }
    
    // Add all the non-degenerate faces back into the 
    // mesh. Get a record of the new edges created in
    // this process.

    face_it = update_faces.begin();
    face_end = update_faces.end();

    for (;face_it != face_end; ++face_it) {
        mesh.ConnectTriangle(*face_it,new_edges);
    }

    // degenerate ring primitives

    // we now need to examine each of the edges on the ring
    // and work out if they are degenerate - if so we attempt
    // to delete them -> add them to the other edges to delete
    // in e_v0v1

    edge_it = edge_ring.begin();
    edge_end = edge_ring.end();

    for (;edge_it != edge_end; ++edge_it) {
        if (edges[*edge_it].Degenerate()) {
            degenerate_edges.push_back(*edge_it);
        }
    }

    // do the same for the ring vertices.
        
    vector<LOD_VertexInd>::iterator vertex_it = vertex_ring.begin();
    vector<LOD_VertexInd>::const_iterator vertex_end = vertex_ring.end();

    for (;vertex_it != vertex_end; ++vertex_it) {
        if (verts[*vertex_it].Degenerate()) {
            degenerate_vertices.push_back(*vertex_it);
        } else {
            update_vertices.push_back(*vertex_it);
        }
    }

    // we now know all the degenerate primitives
    // and the new primitives we have inserted into the mesh

    // We now delete the mesh primitives, mesh.DeleteXXXXXX() methods
    // assume that the index vectors are sorted into descending order.
    // we do that now.

    sort(degenerate_edges.begin(),degenerate_edges.end(),LOD_EdgeInd::greater());
    sort(degenerate_faces.begin(),degenerate_faces.end(),LOD_FaceInd::greater());
    sort(degenerate_vertices.begin(),degenerate_vertices.end(),LOD_VertexInd::greater());

    
    return true;
    
}

LOD_EdgeCollapser::
LOD_EdgeCollapser(
){
    // nothing to do
}