mtllib plane.mtl

o  cube
v  -20.0 0.0 -20.0
v  -20.0 0.0  20.0
v   20.0 0.0 -20.0
v   20.0 0.0  20.0

vn  0 1 0

vt  0 0 0
vt  1 0 0
vt  1 1 0
vt  0 1 0


f  1/1/1  2/2/1  3/3/1  
f  2/2/1  4/4/1  3/3/1