0 0 z a
0 0 z+1/2 a
0 1/2 z b
0 1/2 z+1/2 b
1/4 1/4 z c
1/4 3/4 z+1/2 c
x y z d
-x -y z d
x -y z+1/2 d
-x y z+1/2 d