triangleArea ( Vector3 vectorA, Vector3 vectorB, Vector3 vectorC ) {

 var tmp = (vectorB - vectorA).cross( vectorC - vectorA );

 return 0.5 * tmp.length;
}

clone( Geometry geometry ) {

   var cloneGeo = new Geometry();

   var i, il;

   var vertices = geometry.vertices,
     faces = geometry.faces,
     uvs = geometry.faceVertexUvs[ 0 ];

   // materials

   if ( geometry.materials != null) {

     cloneGeo.materials = new List.from(geometry.materials);

   }

   // vertices
   cloneGeo.vertices = vertices.map((vertex) => vertex.clone()).toList();

   // faces
   cloneGeo.faces = faces.map((face) => face.clone()).toList();

   // uvs
   il = uvs.length;
   for ( i = 0; i < il; i ++ ) {

     var uv = uvs[ i ], uvCopy = [];

     var jl = uv.length;
     for ( var j = 0; j < jl; j ++ ) {

       uvCopy.add( new UV( uv[ j ].u, uv[ j ].v ) );

     }

     cloneGeo.faceVertexUvs[ 0 ].add( uvCopy );

   }

   return cloneGeo;

}