returns an edge to face map for a THREE geometry. make sure the vertices are merged before hand

getWingedEdges.js

'use strict'

var getWingedEdges = function( g ){

  var edgeMap = {};

  // find the edges
  function getEdgeKey( i0, i1 ){
    return Math.min(i0, i1) + '_' + Math.max(i0, i1);
  }

  function addEdge( i0, i1, faceIndex ){
  
    var edgeKey = getEdgeKey( i0, i1 );

    if(edgeMap[ edgeKey ] !== undefined){
    
      edgeMap[ edgeKey ].f.push( faceIndex );
    
    } else {
    
      edgeMap[ edgeKey ] = {
        v: [i0, i1],
        f: [faceIndex],
        normal: new THREE.Vector3()
      }

    }

    return edgeKey;    
  }

  g.faces.forEach( function( f, i ){
  
    addEdge(f.a, f.b, i);
    addEdge(f.b, f.c, i);
    addEdge(f.c, f.a, i);
  
  });

  // compute edge normals
  g.computeFaceNormals();

  function normalFromTwoVector( v0, v1 ){
    return v0.cross( v1 ).normalize();
  }

  var v = g.vertices;
  for(var i in edgeMap){
    
    var e = edgeMap[i];

    if(e.f.length > 1){

      // average the face normals
      var n = e.normal.set(0,0,0);

      e.f.forEach( function( i ){
      
        n.add( g.faces[i].normal );
      
      });

      n.divideScalar( e.f.length );

    } else {
      
      var f = g.faces[ e.f[0] ];

      var v0 = f.normal.normalize().clone();
      
      var v1 = v[ e.v[0] ].clone().sub( v[ e.v[1] ] ).normalize();

      e.normal.copy( normalFromTwoVector( v0, v1 ) );
    }

  }

  return edgeMap
}


module.exports = getWingedEdges;