Target Acquisition in Multiscale Electronic Worlds

15

Figure 14. Space-scale diagram in zoom-index space, showing the starting position and target of a pointing
task, as in Figure 7. Zoom-index value 0 (horizontal axis), corresponds to scale 1.

If we replace the vertical scale dimension by a zoom-index dimension in the space-scale
diagram, the great rays become exponential curves (Figure 14). Such a diagram is more true
to the user experience of navigating a multiscale world with zooming and panning than the
traditional one: constant-speed panning corresponds to constant-speed horizontal motion of
the view (as in a traditional space-scale diagram), and constant-speed zooming corresponds to
constant-speed vertical motion of the view (unlike a traditional space-scale diagram). Notice
that, since the space dimension has not been changed, the viewing window still has a constant
size across the diagram.

Target acquisition with a pan-and-zoom interface typically involves three consecutive phases
of action: (1) zooming-out, (2) zooming-in-and-panning, and (3) pointing (Figure 15). In the
first phase, the user zooms-out until the target area is visible in the view window; in the
second phase, the user combines zooming-in and panning until the target is large enough
within the view to be selected; in the third phase, the user selects the target with a traditional
pointing action. Either or both of the first two phases may be unnecessary: phase one is
unnecessary if the target is already visible, while phase two is unnecessary if the target is big
enough.

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