F. Failure? Fix on a standard.

If after exploring all of the above design principles, no good natural design emerges, it may be time to fix on a standard. Even if the standard appears arbitrary, deciding on one and sticking to it will at least result in a clear and consistent model that can be learned.

Examples of such arbitrary but exceedingly useful standards abound in everyday life: the QWERTY keyboard, stop on red go on green, the arrangement of the digits on a calculator (which for some strange reason are different from their arrangement on a telephone), and so on.

In the above discussion, we have seen at least one completely arbitrary choice in the interface design of Quail, namely associating selection with a left mouse button click over the display and a pop-up menu with a middle-button click. Beyond associating the common gesture (left-button mouse) with the most common interest (selecting or highlighting points), the assignment of actions to gestures is arbitrary.

Largely because of design considerations (hiding complexity and constraining actions to be spatially associated with their target), it was clear early in the design of Quail (ca. 1988) that a three button mouse with two modifier keys (Shift and CTRL) be taken to be part of the standard design. This provides us with nine mouse `gestures' potentially applicable to any display.

To be at all useful, some rather arbitrary organization had to be applied ot these gestures. We took them to naturally be laid out in a grid as in

	Mouse button
Modifier	Left	Middle	Right
None	select	change style	change type
Shift	multiple	-	-
CTRL	viewed-object	viewed-object	viewed-object

This shows the present arrangement where the two cells with only a dash are unassigned and the last row does the same thing for all three mouse buttons. The hope is to provide roughly orthogonal behaviours associated with button choice that can be crossed in a meaningful way with modifier key choice. So, for example, no modifier key means that the target of all operations will be the selected graphic while the CTRL key modifier means that the target of all operations will be the viewed object of the graphic. Similarly, left should mean selection, middle changing features of the target, and right making fundamental changes in type to the target. While these behaviours have not yet been worked out for viewed-objects, they are held in reserve for that possibility.

At present, any CTRL mouse-button selection pops the menu of Figure

  

Figure: The ctrl middle button menu for a point-symbol.

at the current mouse position. This important menu allows the user to produce an interactive graphic display tailored to the particular viewed-object. For more discussion of these displays and the strategic importance of signposts see Oldford (1997).