glVertex3f(x1, 0, 0); glVertex3f(x2, 0, 0);
glVertex3f(0 , y1, 0); glVertex3f(0, y2, 0);
glVertex3f(0, 0, z1); glVertex3f(0, 0, z2);
glVertex3f(x1, 0, 0); glVertex3f(x2, 0, 0);
glVertex3f(0 , y1, 0); glVertex3f(0, y2, 0);
glVertex3f(0, 0, z1); glVertex3f(0, 0, z2);