Ying Leung, Swinburne Computer Human Interaction Laboratory, School of Computer Science and Software Engineering, Swinburne University of Technology, P.O. Box 218, Hawthorn 3122,Victoria, Australia. Phone +61 3 9214 8765 Fax: +61 3 9819 0823 Email: yleung@swin.edu.au Home Page: Ying Leung [HREF 1]
One of the key problems relates to the vast amount of information that the Web contains and the methods by which a user can access this information. It has been established that one of the major usability problems with hypermedia information spaces such as the World Wide Web is the user's risk of disorientation whilst navigating (Nielsen, 1990). Help should be provided to maintain a user's sense of orientation and facilitate navigation within the context of the total information space. Hence there is a need for support for both the local navigation task of moving between pairs of specific nodes as well as the global navigation task which involves movements that spans many nodes (Kahn, 1995). Most browsers display a limited amount of orientation information consisting of document name and URL as well as the URLs of possible links from the current page. In addition, many browsers provide a history list of pages previously visited which enables the user to establish some notion of location and also provides the ability to backtrack. Whilst these orientation clues do provide adequate support for navigation at the local level, they lack the contextual overview required for global orientation. Users, when presented with multiple paths to the same or different end points will quickly experience cognitive overloading as they struggle to maintain orientation (Hedberg & Harper, 1991).
The use of overview diagrams as an aid to navigation have provided a solution to then problem of global orientation (Nielsen, 1990). Overview diagrams are a graphical map of the structure of the hypermedia systems showing all nodes and links. Such diagrams act as a two dimensional table of contents and are particularly suitable for providing a level of orientation in non-linear hypermedia information spaces. Overview diagrams support the user's mental model of how the information space is structured by providing orientation at a visual level. There is evidence to suggest that mental models are a powerful aid to reduce learning time and improve users' overall performance (Rumelhart and Norman 1981, Halasz and Moran 1982, Foss et al, 1982).
The World Wide Web provides the technical support for overview diagrams through the use of image maps. A bit-mapped image of a Web space may be created by the Web page developer and made accessible to the user either as a simple static map of the Web space to provide orientation or as a dynamic map that can augment the traditional method of navigation by the encoding of graphical hot-spot links. A cursory examination of the Web shows that the use of image maps in this way is becoming popular.
Although image map overviews are an excellent tool for orienting and navigating a hypermedia system such as the World Wide Web, there are some drawbacks when they are applied to large information spaces which involve many nodes and links (Mukherjea et al, 1994). Large image maps give rise to the need to use of scroll bars to enable the user to change views of the map and as the entire map cannot be display in its entirety. Researchers (Beard and Walker II, 1990) have provided empirical evidence to suggest that users perform sub-optimally with scroll bars in tasks which require navigating a large two-dimensional space.
This paper proposes the use of the Bifocal Display as a visualisation aid to assist with navigating the WWW. A number of WWW pages have been implemented using this presentation technique to illustrate the concept.
Various techniques have been proposed to overcome this visualisation difficulty associated with large visual information systems. Leung (1996) illustrates that the three key techniques which can be used to facilitate visualising large information spaces are distortion, encoding and thresholding. Distorted presentations involve a geometric scaling of the data to generate a distorted display; they consist of a detail view of a small portion of the data space within a demagnified global scene. Encoded presentations allow attributes of the data to be displayed through graphical representation, thus saving valuable screen space. Thresholding presentations provide a systematic way of suppressing or revealing the information to be presented, reducing screen clutter.
The Bifocal Display was first proposed by Spence and Apperley (1982) to overcome the classical "window" problem associated with viewing large databases through the concurrent presentation of local detail with global context. The essence of this technique is to allow attributes of the data to be displayed through representation, thus saving valuable screen space. The Bifocal Display concept has been extended by Leung (1989) in a two dimensional graphical form using the implementation of an interactive London Underground map (Figure 1).
The two dimensional bifocal display comprises of nine distinct regions as shown in Figure 2. The central focus region provides a detailed view of part of the entire information space. The surrounding 8 regions are demagnified by a constant value in either x, y or x and y dimensions depending upon the position relative to the central focus region. With an interactive map display, the focus region together with the 8 surrounding regions shift as the user moves the mouse to view an desired area of the information space.
In the context of the WWW, the bifocal displays can therefore be applied in two ways concurrently - representing data in graphical form or demagnifying areas of the information space which are 'out-of-focus'. Both techniques have the desired effects of saving valuable screen space and allow concurrent display of the global scene together with local detail to be presented in context.
The original graphical overview map (Figure 3) displayed all of the links and nodes in the system using different colours to represent the levels of the hierarchy. Due to the size and complexity of the system, the use of scroll bars is essential to view the overall structure.
A dynamic bifocal display [HREF 5] of this map has been implemented using the Java language which removes the need for scroll bars. This program implements the full distortion-oriented bifocal display technology by allowing the user to establish overall orientation within the system and to then navigate through to the detail of the section of interest by dynamically controlling the focal area of the map by mouse movements.
The central focal region is outlined by a white rectangle as shown in Figure 4. This region is not distorted in any way and provides the user with a window by which to examine the detail of the whole map. The area of the map outside of the central focus region is distorted in such a manner that the user can place the central focus region in the context of the whole map. The links between the nodes do not lose continuity over the boundry of the focal area thereby allowing the user to visually follow a path.
The Java program has two modes:
(a) Focus Control
This mode allows the user to move the central focus region by the use of the mouse. The point the user selects with the mouse becomes the centre of the new central focus region and the rest of the map distorts accordingly as shown in Figure 5.
(b) Page Navigation
This mode allows the user to select a specific node on the map with the mouse in order to jump to a corresponding URL.
The user selects the appropriate mode by the use of a small toolbar positioned at the top right of the map.
The co-ordinates of the image map required for the URL jumps were hard coded into the program. It is proposed to create a program which would take the output of a standard image map application and translate it into the format required for the Java program. Further work to automate the manual creation of the GIF images is required.
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