University of Konstanz
Graduiertenkolleg / PhD Program
Computer and Information Science


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Author(s) B├╝ring, T.
Title Zoomable User Interfaces on Small Screens - Presentation and Interaction Design for Pen-operated Mobile Devices
Abstract Due to continuous and rapid advances in mobile hardware and wireless technology, devices such as smartphones and personal digital assistants (PDAs) are becoming a truly mobile alternative to bulky and heavy notebooks, allowing the users to access, search and explore remote data while on the road. Application fields that benefit from increased staff mobility are business consultants, mechanical engineers, and doctors in hospitals, for instance. However, a drawback that impedes this development is that the form factor of mobile devices requires a small-sized screen. Hence, given a large data set, only a fraction of it can be displayed. To identify important data the users are typically forced to linearly navigate the off-screen space via scrolling. For large information spaces, this is tedious, error-prone, and particularly slow. In contrast, the concept of zoomable user interfaces (ZUIs) has been found to improve the user performance for a variety of retrieval and exploration scenarios. While ZUIs have been investigated mainly in desktop environments, the objective of this work is to analyze the usability potentials of zooming and panning in a mobile context given the constraints of a small screen and pen-input.

Based on a comprehensive review of related work, the reported research is structured in two parts. First, we focus on the development of mobile starfield displays. Starfield displays are complex retrieval interfaces that encode and compress abstract data in a zoomable scatterplot visualization. To better adapt the interface to the requirements of a small screen, we merged the starfield approach with semantic zooming, providing a consistent and fluent transition from overview to detail information inside the scatterplot. While the participants in an informal study gave positive feedback regarding this type of data access, they also showed difficulties in orienting themselves in the information space. We further investigated this issue by implementing a zoomable overview+detail starfield display. Thus, while navigating the detail view, the users could keep track of their current position and scale via an additional overview window. In a controlled experiment with 24 participants, we compared the usability of this approach with a detail-only starfield and found that the separate overview window was not able to improve user satisfaction. Moreover, due to the smaller size of the detail view and the time needed for visual switching, it worsened task-completion times. This result led us to implement a rectangular fisheye view for starfield displays. The interface has the advantage that it displays both detail and context in a single view without requiring visual switching between separate windows. Another usability evaluation with 24 participants was conducted to compare the focus+context solution with an improved detail-only ZUI. While task-completion times were similar between the interfaces, the fisheye was strongly preferred by the users. This result may encourage interface designers to employ distortion strategies when displaying abstract information spaces on small screens. Our research also indicates that zoomable starfield displays provide an elegant and effective solution for data retrieval on devices such as smartphones and PDAs.

The second part of the research deals with map-based ZUIs, for which we investigated different approaches for improving the interaction design. Maps are currently the most common application domain for ZUIs. Standard interfaces for controlling such interfaces on pen-operated devices usually rely on sequential interaction, i.e. the users can either zoom or pan. A more advanced technique is speed-dependent automatic zooming (SDAZ), which combines rate-based panning and zooming into a single operation and thus enables concurrent interaction. Yet another navigation strategy is to allow for concurrent, but separate, zooming and panning. However, due to the limitations of stylus input, this feature requires the pen-operated device to be enhanced with additional input dimensions. We propose one unimanual approach based on pen pressure, and one bimanual approach in which users pan the view with the pen while manipulating the scale by tilting the device. In total, we developed four interfaces (standard, SDAZ, pressure, and tilting) and compared them in a usability study with 32 participants. The results show that SDAZ performed well for both simple speed tasks and more complex navigation scenarios, but that the coupled interaction led to much user frustration. In a preference vote, the participants strongly rejected the interface and stated that they found it difficult and irksome to control. In contrast, the novel pressure and tilt interfaces were much appreciated. However, in solving the test tasks the participants took hardly any advantage of parallel interaction. For a map view of 600x600 pixels, this resulted in task-completion times comparable to those for the standard interface. For a smaller 300x300 pixels view, the standard interface was actually significantly faster than the two novel techniques. This ratio is also reflected in the preference votes. While for the larger 600x600 pixels view the tilt interface was the most popular, the standard interface was rated highest for the 300x300 pixels view. Hence, on a smaller display, precise interaction may have an increased impact on the interface usability.
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