Figure 2 provides an overview of the editing operation for changing the depth of a vertex. Figure 2(a) shows that the vertex is moved from its initial (depth) position to the next (depth) position in the cross-sectional plane by dragging the mouse. The cross-sectional plane is called the “depth control plane.”. Next, by moving the cross-sectional plane back and forth, the user can move the vertex in the cross-sectional plane to adjust the depth position. This means that the depth of the vertex can be changed by varying the position of the cross-sectional plane.