ion There are times when the amount of data produced by a program overwhelms the user. When this happens, an animation confuses more than it educates. If the algorithm is complex, and uses several different data structures and sub-algorithms, the user may get lost in the details and not see the over-all picture. In such cases, the programmer should condense complicated parts of the scene into simpler items, like boxes. This is the approach taken in several videos we have reviewed. Time can also be abstracted, if several phases of an algorithm are omitted and only the final result of several program steps is presented. An ideal system should include facilities that help the programmer implement this sort of abstraction. Ideally, all the detail should be accessible to the user if he/she needs to see it. This is called semantic zooming. Sometimes, of course, it may be desirable to present the viewer with large amounts of information. This occurs when several sorts are simultaneously c...