2 7

A major application area of computer graphics is visualization, where computergenerated images are used to help people understand both spatial and non-spatial data. Visualization is used when the goal is to augment human capabilities in situations where the problem is not sufficiently well defined for a computer to handle algorithmically. If a totally automatic solution can completely replace human judgement, then visualization is not typically required. Visualization can be used to generate new hypotheses when exploring a completely unfamiliar dataset, to confirm existing hypotheses in a partially understood dataset, or to present information about a known dataset to another audience. Visualization allows people to offload cognition to the perceptual system, using carefully designed images as a form of external memory. The human visual system is a very high-bandwidth channel to the brain, with a significant amount of processing occurring in parallel and at the pre-conscious level. We can thus use external images as a substitute for keeping track of things inside our own heads. For an example, let us consider the task of understanding the relationships between a subset of the topics in the splendid book Gödel, Escher, Bach: The Eternal Golden Braid (Hofstadter, 1979); see Figure 27.1. When we see the dataset as a text list, at the low level we must read words and compare them to memories of previously read words. It is hard to keep track of just these dozen topics using cognition and memory alone, let alone the hundreds of topics in the full book. The higher-level problem of identifying neighborhoods, for instance finding all the topics two hops away from the target topic Paradoxes, is very difficult.