Snap-dragging: interactive geometric design in two and three dimensions

Abstract

A new technique has been developed for designing precise two and three-dimensional shapes rapidly and interactively. A synthesis of the best properties of grid-based systems, constraint networks, and drafting has resulted in a new technique called "snap-dragging". <p>To aid precise construction, a set of lines, circles, planes, and spheres, called 'alignment objects' are constructed by the system at a set of slopes, angles, and distances specified by the user. These alignment objects are constructed at each vertex or edge that the user has declared to be 'hot' (of interest). Vertices and edges can also be made hot by the system automatically through some heuristics. <p>The user can snap the cursor onto these alignment objects, onto their points or lines of intersections, or onto scene objects using an adjustable 'gravity mapping'. Finally, the user can translate, rotate, scale, and skew shapes, by specifying which operation is desired, then grabbing a point on the object and dragging it to a suitable, precise location. <p>Snap-dragging was incorporated into a prototype two-dimensional illustration system at Xerox PARC which has been used successfully by a community of users since 1986