//---------------------------------------------------------------------------
bool TriangulatorClass::LineSegmentIntersect(unsigned int Vertex1, unsigned int Vertex2, unsigned int Vertex3, unsigned int Vertex4, unsigned int *IntersectionPoint)
    {
    VertexStruct Direction1;
    VertexStruct Direction2;
    VertexStruct Cross;
    VertexStruct Cross1;
    VertexStruct Point1;
    VertexStruct Point2;
    double Dot1;
    double Length;
    double t1;
    double t2;

    //convert the four vertexes into two line segments (parametric equation)
    //  (we just need the direction, V1 and V3 will serve as the start points)
    Direction1.Position = InputVertex[Vertex2].Position - InputVertex[Vertex1].Position;
    Direction2.Position = InputVertex[Vertex4].Position - InputVertex[Vertex3].Position;

    D3DXVec3Cross(&Cross.Position, &Direction1.Position, &Direction2.Position);
    Length = D3DXVec3Length(&Cross.Position);
    if (IsEqual(Length, 0.0) == true)
        {
        //the lines are either parallel or coincident. 
        ....omitted code, this seems to work okay...
        }

    //the lines either intersect or they are skew. Determine the closest approaching points
    // first point
    D3DXVec3Cross(&Cross1.Position, &(InputVertex[Vertex3].Position - InputVertex[Vertex1].Position), &Direction2.Position);
    Dot1 = D3DXVec3Dot(&Cross1.Position, &Cross.Position);
    t1 = Dot1/(Length * Length);
    //if we are at either end point, return true with the endpoint as the intersection point
    if (IsEqual(t1, 0.0f))
        {
        *IntersectionPoint = Vertex1;
        return true;
        }
    if (IsEqual(t1, 1.0f))
        {
        *IntersectionPoint = Vertex2;
        return true;
        }
    //boundary check to make sure the calculated point is still on our line segement
    if (t1 < 0.0f || t1 > 1.0f)
        return false;
    Point1.Position = InputVertex[Vertex1].Position + t1 * Direction1.Position;

    //second point
    D3DXVec3Cross(&Cross1.Position, &(InputVertex[Vertex3].Position - InputVertex[Vertex1].Position), &Direction1.Position);
    Dot1 = D3DXVec3Dot(&Cross1.Position, &Cross.Position);
    t2 = Dot1/(Length * Length);
    if (IsEqual(t2, 0.0f))
        {
        *IntersectionPoint = Vertex3;
        return true;
        }
    if (IsEqual(t2, 1.0f))
        {
        *IntersectionPoint = Vertex4;
        return true;
        }
    //boundary check to make sure the calculated point is still on our line segement
    if (t2 < 0.0f || t2 > 1.0f)
        return false;
    Point2.Position = InputVertex[Vertex3].Position + t2 * Direction2.Position;

    //check the distance between the two points
    Length = D3DXVec3Length(&(Point1.Position - Point2.Position));
    if (IsEqual(Length, 0.0f) == true)
        {
        AppendVertex(Point1.Position.x, Point1.Position.y, Point1.Position.z);
        *IntersectionPoint = InputVertex.size()-1;
        return true;
        }

    //distance >0. The two lines must be skew
    return false;
    }