authors 
Bier, Eric A. 
year 
1988 
title 
SnapDragging. Interactive Geometric design in Two and Three Dimensions 
source 
University of California, Berkeley 
summary 
Graphic artists, mechanical designers, architects, animators, authors of technical papers and others create geometric designs (illustrations and solid models) as a major part of their daily efforts. Some part of this shape construction must be done with precision. For instance, certain line segments should be horizontal, parallel or congruent. In recent years, interactive computer programs have been used to speed up the production of precise geometric designs. These programs take advantage of highspeed graphics, equation solving, and computer input peripherals to reduce the time needed to describe point positions to the machine. Previous techniques include rounding the cursor to points on a rectangular grid, solving networks of constraints, and supporting stepbystep draftingstyle constructions. Snapdragging is a modification of the drafting approach that takes advantage of powerful workstations to reduce the time needed to make precise illustrations. Using a single gravity mapping, a cursor can be snapped to either points, lines or surface. The gravity algorithm achieves good performance by computing intersection points on the fly. 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 alignments 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 through the action of an automatic hotness rule. When snapdragging is used, shapes can often be constructed using a few more keystrokes than would be needed to sketch them freehand. Objects can be edited at arbitrary orientations and sizes. The number of primitive operations is small, making it possible to provide keyboard combinations for quickly activating most of these operations. The user interface works nearly identically in two or three dimensions. In three dimensions, snapdragging works with a twodimensional pointing device in a single perspective view. 
series 
thesis:PhD 
email 
bier@parc.com 
references 
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last changed 
2003/02/12 21:37 
