void displayBSP ( BSP_tree *T )
{
if ( T != NULL) {
if ( viewer is in front of T->root ) { // display backChild first
displayBSP ( T->backChild );
displayPolygon ( T->root );
displayBSP ( T->frontChild );
}
else { // display
frontChild first
displayBSP ( T->frontChild );
displayPolygon ( T->root );
displayBSP ( T->backChild );
}
}