We establish that two-part models of pointing performance (Welford’s model) describe pointing on a computer display significantly better than traditional one-part models (Fitts’s Law). We explore the space of pointing models and describe how independent contributions of movement amplitude and target width to pointing time can be captured in a parameter k . Through a reanalysis of data from related work we demonstrate that one-part formulations are fragile in describing pointing performance, and that this fragility is present for various devices and techniques. We show that this same data can be significantly better described using two-part models. Finally, we demonstrate through further analysis of previous work and new experimental data that k increases linearly with gain. Our primary contribution is the demonstration that Fitts’s Law is more limited in applicability than previously appreciated, and that more robust models, such as Welford’s formulation, should be adopted in many cases of practical interest.

Multi-point interaction tasks involve the manipulation of several mutually-dependent control points in a visual workspace -- for example, adjusting a selection rectangle in a drawing application. Multi-point interactions place conflicting requirements on the interface: the system must display objects at sufficient scale for detailed manipulation, but it must also provide an efficient means of navigating from one control point to another. Current interfaces lack any explicit support for tasks that combine these two requirements, forcing users to carry out sequences of zoom and pan actions. In this paper, we describe three novel mechanisms for view control that explicitly support multi-point interactions with a single mouse, and preserve both visibility and scale for multiple regions of interest. We carried out a study to compare two of the designs against standard zoom and pan techniques, and found that task completion time was significantly reduced with the new approaches. The study shows the potential of interfaces that combine support for both scale and navigation.

We introduce Shadow Reaching, an interaction technique that makes use of a perspective projection applied to a shadow representation of a user. The technique was designed to facilitate manipulation over large distances and enhance understanding in collaborative settings. We describe three prototype implementations that illustrate the technique, examining the advantages of using shadows as an interaction metaphor to support single users and groups of collaborating users. Using these prototypes as a design probe, we discuss how the three components of the technique (sensing, modeling, and rendering) can be accomplished with real (physical) or computed (virtual) shadows, and the benefits and drawbacks of each approach.