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An electronic version of a magnifying lens using a
one-dimensional Bifocal Display. (a) The original text message; (b) The view of the message using a conventional magnifying lens; and (c) The electronic version of a magnifying lens using a one-dimensional Bifocal Display.
1 Farrand proposed his DECR (Detail-Enhancing & Context-Retaining) lens (1973) and provided an earlier discussion of the lens concept. but he did not present any formal treatment of this approach.
CHAPTER 6: DESIGNING THE INFOLENS 1 1 1
It is pertinent to explain how the lens metaphor can be applied to the visualisation of large information spaces and the design of the InfoLens. Robertson & Mackinlay (1993) highlight the problem of the conventional magnifying lens in viewing data. Figure 6. 1 above, adapted from Figure 2 of their paper, shows that parts of the image near the edges of the lens are obscured by the lens. The area of this obscured region increases, as the lens is moved towards the eye. Figure 6.2 shows the electronic version of a magnifying lens using a one-dimensional Bifocal Display which eliminates this problem. Transformation function I I Transformation function I Magnification function -
-
I I I I I I I I I I I (a) Magnification function I I . --
-
I I Pannng I I-
I I --
L �I - - - Zooming & Adjusting Figure 6.3 (b)The three canonical manipulation operations of a rectangular magnifying lens. (a) The transformation and magnification
function of the lens as it is moved horizontally parallel to the plane of the display surface. (b) The transformation and magnification function of the lens as it is moved in a direction perpendicular to the display surface.
CHAPTER 6: DESIGNING THE INFOLENS Figure 6.4 (a) (c) (e) (b) (d) (0 .
Different views generated by manipulating the electronic magnifying lens. (a) The original undistorted image. (b) A one-dimensional Bifocal Display of the image. (c) As the lens is drawn near the viewer, the window for the detailed view is
widened while the magnification factor remains unchanged in the focus region. (d) The increase in the magnification factor while the window size remains unchanged. (e) The combined effect of widening the viewport and the increase of the magnification factor in the focus region. (f) The effect of panning to the right the focus region of the one-dimensional Bifocal Display in (b).
CHAPTER 6: DESIGNING THE INFOLENS 1 13
The user may move the lens in two ways. First, the lens may be moved freely on a plane parallel to the surface of the displayed information. This has the effect of changing the viewport so that the user can examine a particular area of interest in detail. Second, the user can pull the lens towards (or away from) the eye with the effect of increasing (or decreasing) the display area of the detailed view, at the same time as reducing (or increasing) the size of the out-of-focus view. Another consequence of this action is the zoom-in (or out) effect, where the size of the data displayed within the detailed viewport will be changed. These three canonical operations - zoom, adjust and pan - are the basic forms of manipulations which the user can perform with the lens. Before extensions of these functions are considered, the modification of the conventional magnifying lens is discussed.
Whilst the conventional magnifying lens presents the problem of obscuring part of the information space to be viewed, this can be overcome in implementing an electronic version of the magnifying lens. Transformation functions may be selected for the in-focus and out-of-focus regions, eliminating the obscured region totally. For illustration purposes, in Figure 6.3 the Bifocal Display has been used as the underlying presentation technique for this rectangular lens. The three basic manipulations of this electronic lens may be achieved as it is moved, resulting in a change in the transformation and magnification functions. It is important to note that in the implementation of this electronic lens, there are three possible scenarios when the lens is moved towards the viewer:
(1) the size of the detailed viewport may increase without any change in the magnification factors in the detailed view; or
(2) the magnification factors in the detailed view may increase while maintaining
the size of the viewport; or
(3) a combination of (1) and (2) may occur
The conceptual model of distortion-oriented presentation techniques developed in Chapter 4 suggests that if the size of the display area remains fixed, any changes made in the magnification function in one part of the display will be neutralised by compensatory effects in other parts of the display. This zero-sum gain rule governs the operation of the magnification function of the entire display surface. Using a one-dimensional Bifocal Display, Figure 6.4 shows the three possible effects on the entire display as the lens is drawn towards the viewer. Although the lens metaphor has been illustrated using the Bifocal Display, it is important to emphasise that this metaphor is not confined to this technique and other distortion-oriented presentation techniques could be effectively applied.
CHAPTER 6: DESIGNING THE INFOLENS 1 14
The lens metaphor described so far only deals with the physical movement of a simple magnifying lens. It does not take into account the fact that in the case of a photographic lens, the user can adjust the focus of the lens freely to obtain the clearest possible image of the data under consideration. In the context of data visualisation, the user typically views the data in a form which enables the user to carry out the task at hand. In the discussion of the E3 metrication framework in Chapter 5, it was pointed out that in a visualisation task, the user interacts with the system to elicit information from it. The imormation flows between the display screen and the user, who perceives and interprets the relevant data. The representation of data, therefore, is also pertinent to the lens metaphor. If the data is represented in a form which does not facilitate easy performance of a task, the lens may be considered to be out of focus. Data may be represented in various forms with different degrees of accuracy. It is important to reiterate that data accuracy is not always important in a data visualisation task; a table of numbers, albeit highly precise and accurate, is less effective than a line graph if a user wishes to determine the general trend of the data set.
The concept of a lens filter used in photographic lenses may also be considered to extend the lens metaphor further. Lens fllters are used by photographers to highlight or bring out important features of an object or a scene. In the context of data visualisation, this lens filter allows the user to view the data selectively. For example, the user may be interested to view the data which fall within a certain range of a large database. In their Magic Lens system, Stone, Fisher and Bier (1994) applied the lens fllter metaphor to suppress/display the view of an information space in a way similar to that used by Mitta & Gunning ( 1992). Stone et al's system is flexible in that the filters are movable and their size may be customised easily by the user. In their HomeFinder system, Williamson & Shneiderman (1992) also used a similar concept to visualise data of an information exploration system.
The magnifying/photographic lens metaphor and its extension described in this section give rise to powerful concepts in the visualisation of large information spaces. These ideas may be brought together to form an integrated visualisation system. Such a system would enable the user to perform a wide variety of tasks such as data exploration and decision making with a large information space. In the following sections, the design of such a system, the InfoLens and its interface is proposed.
CHAP1ER 6: DESIGNING TilE INFOLENS