SKETCH targets the exploration and communication of 3D geometric ideas. Traditionally, people have attacked conceptual design with paper and pencil, not with computers, even though computer models offer numerous advantages. The reasons for this include the low overhead of a single-tool interface (pencil), the lack of special knowledge needed to draw, the ease with which many kinds of changes can be made, and the fact that precision is not required to express an idea. Consider Ann sketching a table with an oval top for Joe. Joe gets an immediate sense of the object, without Ann having to indicate the precise locations of the legs, nor the exact shape of the top. By scribbling over what she has sketched, Ann can make the top round or square or freeform without affecting Joe's perception that the legs are attached to the top. (Imagine doing this in a typical CAD or drawing program.) Nevertheless, pencil and paper are still imperfect. After many changes, the paper can become cluttered. Drastic alterations such as showing the model from different viewpoints require new drawings, and collections of drawn objects cannot be transformed as a unit. While computer models do not have these disadvantages, they are typically considerably more difficult to create.
SKETCH is designed to bridge the gap between hand sketches and computer-based modeling programs, combining some of the features of pencil-and-paper sketching and some of the features of CAD systems to provide a lightweight, gesture-based interface to ``approximate'' 3D polyhedral modeling. Conceptually, our approach is very similar to Landay and Myers' use of sketching to support the early stages of conventional 2D interface design . SKETCH uses a gestural mode of input in which all operations are available directly in the 3D scene through a three-button mouse. The user sketches the salient features of any of a variety of 3D primitives and, following four simple placement rules, SKETCH instantiates the corresponding 3D primitive in the 3D scene. SKETCH allows both geometry and the camera to be gesturally manipulated, and uses an automatic grouping mechanism, similar to that described by Bukowski and Sequin , to make it easier to transform aggregates of geometry. Since the set of geometric primitives is more restricted than those in most CAD systems, the user approximates complex shapes with aggregates of simpler primitives. Since we know these conceptual models are approximations (often to only partially formed mental images) SKETCH renders them with non-photorealistic rendering techniques designed to help viewers see what they want to see.
We also imagine that SKETCH might be used as part of a storyboarding system, for generating a series of scenes and camera views in planning a 3D animation.
The accompanying videotape(1) shows the features of SKETCH and indicates the utility of its simple approach in creating and editing 3D models.
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