3. Define interior interfaces
Interfaces are defined with the surface command. In this case the boundary type is intrface. If the command defines a volume (e.g. box, cylinder) then the interface is the surface of the volume defined. If the command defines a plane or sheet then the interface is the plane or sheet. It is important to remember that planes are infinite and that the order of points specifying the plane determines a normal to the plane in the usual right-hand-rule sense (see Section II.a.9). This direction is important in determining regions. In order to divide the unit cube defined above in half vertically, define a plane by:
The normal to this plane points in the positive z direction.
Interfaces must not be coincident with reflective boundaries. For example to embed a rectangle inside a cube, it is necessary to extend the ends of the rectangle beyond the cube to avoid coincident reflective and interface surfaces: