Illustrations are produced to enable a viewer to extract information. For this purpose, they contain not merely straightforward renditions of the data--the presentation is dedicated to atr thematic focus. Thus, portions of the data may be represented with more detail or more comprehensively, others may simplified, shrunken or even left out.
We refer to the data from which an image is generated as a complex information space. We use the term abstraction to denote ``the process by which an extract of a complex information space is refined so as to reflect the importance of the features of the underlying model for the dialog context and the visualization goal at hand'' [32, p. 95,].
Abstraction introduces a distortion to the visualization with respect to the underlying model. The fidelity of a visualization depends on the kind and degree of abstraction applied to single objects or classes of objects.
For computer generated graphics data on the fidelity (the kind and degree of abstraction applied to single objects or classes) can be obtained as a direct by-product of the visualization process. Thus, the abstraction can be evaluated with respect to the degree of inaccuracy and with respect to how the abstraction violates the expectations of the intended user. The assessment of the ``expectations'' of the user is a difficult task and, of course, requires an intensive study of the different information exploration tasks in a given application domain, including heuristic or empirical evaluations. In anatomy, for example, it is important for a student to recognize distortions of topological relations and relative sizes. As a consequence, whether or not the effect of a specific abstraction is crucial and depends heavily on the information exploration task of the user.
Abstraction techniques are the means by which the effect of the abstraction process is achieved. Since there are usually several abstraction techniques which produce the same effect, the designer of an illustration has to select one or a combination of abstraction techniques. To provide visual access to an object, for example, the objects occluding it may be removed or relocated, a cut-away view can be used or a rotation of the model can be employed. This choice is constrained by parameters of the output medium chosen and of human recognition.
Restrictions of the medium result from its general capability to encode various types of information as well from physical restrictions. The presentation space available, for instance, limits the number of annotations which can be placed within a figure.
To meet the restrictions of human recognition it is essential to establish an equilibrium between the level of detail of the objects of interest and the objects depicting their context so that the user can understand the image. On the one hand, it is crucial to reduce the cognitive load for the interpretation of an illustration. On the other hand, enough contextual information must be provided to understand an image.
In anatomy, for example, bones are important for orientation. Even for an image with the focus on sinews, parts of the skeleton should be depicted to enable the viewer to recognize the spatial location of the sinews. Human designers frequently resolve this conflict by drawing objects within the thematic focus and the context in different styles (they often reduce the attraction of unfocused objects by illustrating them gray, with reduced level of detail, or only as silhouette).
Inspired by observations from hand-made illustrations, similar techniques for the generation of rendered images have been designed and implemented. These techniques work on different levels: high level abstraction techniques are concerned with what should be visible and recognizable. Above all, the viewing specification determines the visible portion. Furthermore, the most salient objects should centered. A combination of several images could be used if the objects of the thematic focus cannot be depicted from one view point. Abstraction techniques such as cut-aways, exploded views, simplification, fish-eye zoom and transparency ensure the visibility and discriminating power of the objects within the thematic focus. Lighting specification influence the composition and thus content selection for the graphics generation.
Low level abstraction techniques, on the other hand, deal with how objects should be presented. Colors, textures, brightness values, as well as line-styles and cross-hatching techniques are frequently adapted to support the visualization goal. We refer to these as presentation variables (see also [16]). However, as low level abstraction techniques also determine the contrast of adjacent objects and the contrast of objects to the background, they influence the composition of the illustration, too.
The application of abstraction techniques results in flexible and expressive images, but as they do not correspond to a physically correct image of the depicted object, these illustrations may be misinterpreted. Moreover, according to the definition of abstraction, the information space is carefully adapted to the users information extraction goal.
Since visualization is a sophisticated process, manipulations that restrict image fidelity ought to be described in the caption. This covers the mentioning of single objects whose visibility or position was adapted during the visualization process. If users should be made aware of these modifications, they to be described in a figure caption.