For an interactive system such as this to work well, the user interface must
be well thought out and intuitive. When designing the user interface, there
are many choices which must be made about how the system should behave.
When making these choices, the designer should keep in mind what the user
is trying to do when she
encounters that part of the interface, and which
alternative would be the most convenient to the user.
For example, while coding the facial evolver, I had to decide whether to preserve the selection at the end of operations or to unselect all selected faces. The behaviour which I chose is context-dependent; after a breed operation, the previously selected faces are unselected, as all the other facial expressions have changed to offspring of the two selected ``parents'' and it is unlikely that the user will want to repeat the procedure. After any other operator is applied, however, the selection is preserved; the user will often want to repeat the operation ( towards, mutate, randomise or exaggerate) more than once, keeping the same expressions selected. I believe that this behaviour keeps the need to set or unset the selection to a minimum.