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We often talk about how pictures "depict" things. But, even the process of depiction seems to differ from one picture to another. On a computer, we may use a paint system to guide a virtual brush over the screen, a video camera to capture a live image, a spreadsheet to automatically generate a corresponding bar chart, or a rendering system that models the interactions of synthetic lights, objects, and cameras. Is there some un-derlying property that these processes all share?

Computer scientists are used to thinking of pictures in terms of their representation: an array of pixels, a list or hierarchy of graphics primitives, or even a program written in a language such as PostScript. But these descriptions capture only the form in which a picture is encoded for computer display. Might there be something deeper that we can ask about what it means to be a picture? For example, how do pictures do what they do? Or, for that matter, what exactly is it that they do? And how can we make pictures and picture-making programs better at doing it?

Christine and Thomas Strothotte address these issues in this book, in which they ex-amine the place of pictures in computing systems. They provide a wealth of infor-mation on the psychological interpretation of pictures, on semiotics, and on design. To put pic-tures in perspective, they have constructed a classification system that is based in part on how a picture is used. Their taxonomy ranges from presentational pictures, which show things that we can see, to abstract-graphical pictures, which show abstrac-tions of things that we cannot see, to pictograms, whose abstractions stand for more than they show.

Throughout, they stress the notion that a picture's human viewer brings something extremely important to the table in pictorial communication: In addition to the information explicitly communicated by a picture, there is also information that is added by the viewer through the inevitable, but often unpredictable, process of reasoning. Furthermore, they emphasize that use of pictures in computer systems is a two-party game, played by both the computer and the viewer. And if we let the viewer play, then the computer must also be able to interpret and respond to the viewer's input, which requires that the computer have some understanding of the pictures that it presents. The idea that the computer needs to understand something about its pictures is crucial. The authors discuss systems in which this understanding is provided by the viewer, who conveys her analyses back to the computer in a formal language, as well as ones in which the computer generates pic-tures from its own underlying knowledge about what is to be conveyed.

As static 2D images give way to dynamic 3D ones, and interactive virtual worlds be-come part of our everyday experience, we are building a future in which pictures play an increasingly fundamental role. As this happens, the questions that this book explores are becoming ever more important.