Target Acquisition in Multiscale Electronic Worlds

11

without varying its scale. If multiscale pointing follows Fitts’ law, we expect movement time
to vary with task difficulty but not with task scale.

Figure 8. Variation of task difficulty at a constant level of scale. Movement amplitude (D) is constant.
Relative movement amplitude (D/W_a, D/W_b, D/W_c) and hence the ID increases from a to b to c. W_b, and W_c
are not shown for readability. This is not a space-scale diagram; if it were, the three cases would appear
on a single horizontal line).

3.3. Extant Techniques for Multiscale Pointing

Several techniques have been introduced to help users reach their targets in large multiscale
documents (see Leung and Apperley, 1994, for a review). We describe three, the bi-focal,
fish-eye and pan-and-zoom techniques, using space-scale diagrams, and briefly discuss their
advantages and drawbacks in relation to the target-acquisition problem. Note that, in
principle, the various techniques can be combined.

3.3.1. The Bi-Focal (Context-plus-Detail) Technique

A bi-focal display (Apperly and Spence, 1982) is a display that simultaneously offers two
views: a small-scale view, which provides context by showing all or most of the document,
with little detail, and a large-scale view, which shows the details of a small focus area.

The two views can be either tiled or overlaid. Tiling may make it difficult to relate a detailed
view to its general context. With overlaying, however, the detailed view typically hides part
of the context view: DragMag (Ware & Lewis, 1995), for example, addresses this problem by
drawing a link with thin outlines and allowing the user to freely move either view relative to
the other (see Figure 9).

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