Sub-option MOVE

This gives the user the choice of two different operations, MOVE or MERGE, without resort to a further sub-menu. The MOVE operation consists simply of moving a designated vertex to a new location, which should lie strictly within the polygon formed by the neighbours of the vertex, otherwise some line segments will cross. The vertex and its new location are selected with the mouse and cursor. The display shows the revised positions of the line segments linking the moved vertex to its neighbours, so that the move can be confirmed or cancelled. In the latter case, the vertex and its connections are redisplayed in their original configuration. Figure 5.3 gives examples of permissible and impermissible moves. The Editor does not check for impermissible moves such as those shown in Figs. 5.3c and 5.3d, but they are very obvious when displayed.

MERGE (associated with MOVE in {TOP.EDIT}

If a vertex is moved so that it coincides or nearly coincides with one of its neighbours, the assumption is made that the user wishes to merge the two vertices into a single vertex connected to all the vertices to which the two merging vertices were singly or jointly connected. An example is shown in Figure 5.4. Figure 5.4b shows the result of moving vertex A to coincide with B. The Editor alerts the user as to whether it recognizes a proposed move as a MOVE or a MERGE request by displaying the messages "MOVE O.K.?" or "MERGE O.K.?" respectively. As it is important to know when the Editor does not recognize an intended MERGE operation, the message "This is a move, NOT a merge" is displayed when the new location lies near a neighbour but not quite near enough to cause a MERGE.

Normally, the Editor examines only the neighbours of the point being moved when checking whether a move or merge should take place. See option GRIDMERGE in {TOP.CONFIG} for details on use of MERGE for joining grids, in which case a wider search is necessary.

Values of depth and computational code are changed automatically when vertices are merged. If the vertex being moved is labelled A, and B designates the stationary vertex with which A is merged, as in Fig. 5.4, then the depth and code for the merged vertex at B are set as follows:

• if A and B are both interior points of the grid, B retains its original code (0) and depth.
• if A is an interior point and B is a boundary point, B retains its original code and depth.
• if A is a boundary point and B is an interior point, B assumes the code and depth of A.
• if A and B are both boundary points, B retains its original code and depth. If the points originally lay on boundaries of different types, the user should check whether the code and depth at B should equal the original code and depth at B or A, and reset them with option {TOP.INFO}CHANGE if necessary.

Depths and codes at individual nodes can be changed if necessary by means of option CHANGE in {TOP.INFO} .