think-cell technical report TC2003/01 A GUI-based Interaction ...

3.2 Computer Supported Layout STATE OF THE ART
idea of snapping the drawing caret to prominent locations. To determine geomet-
rically prominent locations, snap-dragging uses the ruler-and-compass metaphor
traditionally used by draftsmen. Based on explicit hints given by the user and some
heuristics about typical editing behavior, snap-dragging constructs transient align-
ment objects – points, lines and circles – on the fly. A gravity function drags the
caret to align with one or more of these objects, enabling fast and accurate drawing
with little time spent in moving the tools and setting up the construction.
Bier and Stone’s snap-dragging forgets about the relationships between shapes
immediately after a shape is constructed, whereas Gleicher [Gle92] takes snap-
dragging one step further and presents augmented snapping. Roughly spoken, in
Gleicher’s prototype system Briar the snap-dragging technique is used to specify
constraints that are maintained from then on. This way, geometric relationships be-
tween drawing objects are invariant against editing operations like drag-and-drop.
The specification of constraints through direct manipulation implies important sim-
plifications compared to traditional constraint-based systems: Because constraints
start out satisfied, there are no constraint-satisfaction problems to solve or state
jumps to explain. There is no concern about conflicting or unsatisfiable constraints,
since there exists at least one configuration which meets the constraints.
An essential feature of snap-dragging as well as augmented snapping is the im-
mediate on-screen feedback to user actions and intentions. In snap-dragging, align-
ment objects are visualized such that they can easily be distinguished from concrete
objects. In augmented snapping, the effect of applying a drag-and-drop operation
while existing constraints are being maintained, is continuously displayed during
the dragging action.
Other systems have been realized that support quick and geometrically precise
drawing without using any grid or gravity function and without the explicit no-
tion of constraints. Notably, Igarashi et al. have demonstrated various work in this
field. Interactive beautification [IMKT97] infers relations between new strokes and
existing line segments and thus generates geometrically accurate 2D shapes from
plain and generally imprecise drawing input. In contrast to snap-dragging, inter-
active beautification “beautifies” an irregular input stroke after it has been drawn.
Relations between the new stroke and existing objects are not explicitly shown. In
Teddy [IMT99], on the other hand, two-dimensional drawing gestures are “inflated”
into 3D models. Because the user interface is entirely based on direct manipulation
gestures, no traditional UI widgets – buttons, sliders, menus, dialogs – are required
for 3D construction using this technique.
Support for imprecise mouse movement and maintenance of spatial relations
between on-screen objects is not restricted to the domain of freehand drawing. In
[BNB00], Badros et al. present SCWM, an “intelligent constraint-enabled window
manager”. SCWM is a window manager for the X window system that handles
relations between windows and enforces them when windows are moved or resized.
SCWM implements a UI for constraint specification and visualization of active con-
straints. Constraints are implicitly inferred by augmented snapping and explicitly
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