Scrolling is one of the most common tasks performed when writing, editing or browsing a document. As it is done so often, even a small improvement results in a significant overall saving in time. Hence, if it is not optimised, a large amount of time and money is being wasted on the mundane task of scrolling. This provides a strong motivation to ensure that the methods we are using are indeed the best. In a study of five hours of web use, \citeasnoun{bjwc:1} observed that users spent 40 minutes scrolling. They commented that ``An obvious case where widget design could make a difference is scrolling.''

The traditional techniques used in document navigation are panning, a scrollbar, and keyboard based scrolling. Modern Windows operating systems have standardised the concept of a new type of scrolling system: rate-based scrolling. In Windows XP, pressing the middle mouse button inside a window that has a scrollbar invokes rate-based scrolling. It has therefore become important to understand how rate-based scrolling complements the existing scroll techniques, and how to optimise rate-based systems. 

Rate-based scrolling is an alternative approach where the displacement of the mouse from the point where scrolling began is mapped to the speed of scrolling. Thus, the further the mouse is moved when in scrolling mode, the faster the document scrolls. This method successfully provides smooth scrolling within large documents. However, rate-based scrolling suffers from a limitation on its speed. Attempting to scroll very rapidly means that the information becomes blurred. This imposes a relatively low maximum limit on the speed of scrolling.

A solution to this problem which attempts to retain all the advantages of the rate-based system is Speed Dependent Automatic Zooming (SDAZ). The concept of SDAZ was first investigated by \citeasnoun{ih:1} who suggested that the system invoke an automatic zoom-out once the scrolling speed exceeded a predefined velocity. The zoom-out reduces the problem of blurring as the document may be scrolled more rapidly without the information moving across the screen as quickly. SDAZ thus retains the advantages of rate-based scrolling while removing the problems associated with rapid scrolling, hence allowing the rapid acquisition of targets that are a great distance away.

The major difficultly surrounding SDAZ is its calibration. Implementing an SDAZ system requires the developer to make a number of non-trivial decisions regarding the functioning of the system. We had initially thought to compare SDAZ to other forms of scrolling, but were immediately plagued with the problem of calibration --- the difficultly of which had not been clearly stated in the previous literature.

This report elaborates on the issues in calibration of SDAZ systems. We look at the mathematical equations and constants driving the system, and also the human-system interface questions. We provide a comprehensive review of the difficulties and attempt to obtain an optimal implementation of SDAZ. Using our implementation of SDAZ, we compare SDAZ against the more standard scrolling techniques: scrollbars, and rate-based scrolling. To do this comparison we use the experimental paradigm developed by \citeasnoun{hcbm:1}, which models scrolling using Fitts' Law.

Fitts' Law \cite{fitts:1} has been a well recognised formula for modelling the acquisition of visible targets in computer interfaces since the work of \citeasnoun{macfitts:1}. However, \citename{hcbm:1} proposed that this model could be extended to model scrolling --- where the targets being acquired are not initially visible. They believe that the Fitts' Law parameters of distance and tolerance are suitable for being used to provide experimenters with a practical method to quantify scrolling techniques. We have adopted their paradigm for our formal evaluation of SDAZ.

Previous work on SDAZ and related topics is described in Chapter 2. In Chapter 3 we propose a classification of document navigation goals and discuss how rate-based scrolling and SDAZ complement the traditional document navigation techniques in aiding users to achieve these goals.
%In Chapter 4 we present a pilot study which validates the idea of an automatic zoom-out, and also serves to demonstrate some of the difficulties involved in the calibration of SDAZ.
We examine both the mathematical issues surrounding the calibration of SDAZ systems and the human-system interface aspects of the system in Chapter 4. In this investigation we attempt to obtain an optimal implementation through user feedback. In Chapter 5 we describe our formal evaluation of SDAZ against both rate-based and normal scrolling. A discussion of our results follows in Chapter 6, and our Conclusions with suggestions for future work in Chapter 7.
