double slcDetermineClockwiseness( double *vertexlist, size_t vertexcount )
{
  size_t i, rmin;
  double minx, miny, px, py;
  double *prev, *next, *corner;
  rmin = 0;
  minx = DBL_MAX;
  miny = DBL_MAX;
  for( i = 0 ; i < vertexcount ; i++ )
  {
    py = vertexlist[(i<<1)+1];
    if( py > miny )
      continue;
    px = vertexlist[(i<<1)+0];
    if( py == miny )
    {
      if( px < minx )
        continue;
    }
    rmin = i;
    minx = px;
    miny = py;
  }
  corner = &vertexlist[ rmin * 2 ];
  prev = &vertexlist[ ( ( rmin + vertexcount - 1 ) % vertexcount ) * 2 ];
  next = &vertexlist[ ( ( rmin + 1 ) % vertexcount ) * 2 ];
  return ( ( next[0] - prev[0] ) * ( corner[1] - prev[1] ) ) - ( ( corner[0] - prev[0] ) * ( next[1] - prev[1] ) );
}