//****************************************************************************80

bool triangle_mask ( int dim_num, int triangle_order, int nodes[], 
  double coord[] )

//****************************************************************************80
//
//  Purpose:
//
//    TRIANGLE_MASK is a user routine which masks triangles.
//
//  Discussion:
//
//    The region to be considered is the [0,4]x[0,4] square.
//
//    We want to remove the lower left triangular corner,
//    and part of the upper right triangular corner.
//
//    The following diagram of the 25 nodes indicates by "O" the
//    nodes that should end up being deleted, although the deletion
//    is actually done by triangles.
//
//    Before masking:
//
//      X - X - X - X - X
//      | \ | \ | \ | \ |
//      X - X - X - X - X
//      | \ | \ | \ | \ |
//      X - X - X - X - X
//      | \ | \ | \ | \ |
//      X - X - X - X - X
//      | \ | \ | \ | \ |
//      X - X - X - X - X
//
//    After masking:
//
//      X - X   O   O   O
//      | \ | \          
//      X - X - X   O   O
//      | \ | \ | \      
//      X - X - X - X - X
//        \ | \ | \ | \ |
//      O   X - X - X - X
//            \ | \ | \ |
//      O   O   X - X - X
//
//  Licensing:
//
//    This code is distributed under the GNU LGPL license. 
//
//  Modified:
//
//    14 January 2007
//
//  Author:
//
//    John Burkardt
//
//  Parameters:
//
//    Input, int DIM_NUM, the spatial dimension.
//
//    Input, int TRIANGLE_ORDER, the number of nodes in the triangle.
//
//    Input, int NODES[TRIANGLE_ORDER], the indices of the nodes.
//
//    Input, double COORD[DIM_NUM*TRIANGLE_ORDER], the coordinates
//    of the nodes.
//
//    Output, bool TRIANGLE_MASK, is TRUE if the triangle should be discarded,
//    and FALSE if the triangle should be retained.
//
{
  double centroid[2];
  int dim;
  bool mask;
  int order;
//
//  Compute the centroid.
//
  for ( dim = 0; dim < 2; dim++ )
  {
    centroid[dim] = 0.0;
    for ( order = 0; order < triangle_order; order++ )
    {
      centroid[dim] = centroid[dim] + coord[dim+order*dim_num];
    }
    centroid[dim] = centroid[dim] / ( double ) ( triangle_order );
  }
//
//  Remove the lower left corner
//
  if ( centroid[0] + centroid[1] < 2.0 )
  {
    mask = true;
  }
//
//  Remove the upper right section.
//
  else if ( 5.0 < centroid[0] + centroid[1] &&
            2.0 < centroid[1] )
  {
    mask = true;
  }
//
//  Keep everything else.
//
  else
  {
    mask = false;
  }

  return mask;
}