2. User-controlled surface fairing of subdivision surfaces in the
presence of creases and geometric constraints.
3. Two-way curve/surface communication where curve con-
straints and surface smoothness can control surfaces and
curves, respectively.
Fig. 3 shows two example designs modeled by our approach.
2. Related work
Current commercial modeling packages provide the means for
designing curves and surfaces, and implement geometric opera-
tions with which a wide variety of shapes can be achieved [2–7].
Although effective for surface design, the manipulators for local
and volumetric editing require the user to achieve and preserve
smooth surfaces manually (Fig. 4a, b). A lack of direct control on
the fairness of the surface makes it challenging for the user to
establish the key feature curves, while producing smooth surfaces
from these curves (Fig. 1). As a result, much effort is spent on
surface manipulation to achieve fair surfaces that result in the
desired feature curves, rather than directly prescribing such
curves, from which suitable surfaces can be generated.
Recent studies [10,11] on sketch-based modeling have pro-
posed methods to create and modify smooth surfaces using
sketching. In free-form surface modeling, these systems seek a
fast transformation from 2D to 3D using a simple base surface
instantiation, followed by an iterative modification of the geo-
metry with pen strokes [8,12–15]. Alternatively, a number of
systems have been proposed that involve primitive sketching,
followed by gestural interactions that implement common CAD
operations [10]. A common feature in both approaches is that the
sketched strokes are used either to define an initial starting shape
such as a contour or silhouette, or to iteratively modify an
existing shape. While these approaches can produce smooth
surfaces, the created surfaces are similarly controlled through
the curves (Fig. 4c, d) rather than a direct shape control of the
surfaces. These approaches still require the user to iteratively
search for suitable curve configurations that lead to desirably
smooth surfaces.
Among the sketch-based blob creation and modification
methods that have been proposed recently [8,12,13,16,17], Fiber-
mesh by Nealen et al. [8] is closely related to our work. It
produces a curvature minimizing closed mesh that interpolates
user-drawn contour and feature curves. While our approach
shares a similar goal of producing energy minimizing surfaces
such as those in Fibermesh [18–20], it differs from these
approaches in a number of ways. First, the curvature variation
minimization approach in Fibermesh produces a single, unique
mesh surface from the set of input curves. The user modifies this
surface by modifying the curves. By contrast, our approach
permits greater latitude in surface modeling by allowing the user
to (1) construct a precise surface topology through a curve
network rather than through an iterative curve addition on a
base surface, (2) explore different surface geometries on the same
curve network through the use of free curves and independent
surface fairing. Moreover, during surface modifications, in con-
trast to Fibermesh’s pick/drag based interactions, we utilize
sketched strokes to directly define and modify the shapes of
curves. We also use a subdivision surface representation which allows
the control of the shape through a coarse control mesh but produces
high complexity sampling at low cost. Fibermesh, on the other hand,
uses a mesh representation which requires the coordinates of all
vertices to be calculated for each modification on the curves.
Works of Nasri et al. [21] and Bein et al. [22] also originate
from a similar motivation. Nasri et al. [21] uses polygonal
complexes to create models that have user-defined outer con-
tours, followed by subdivision for surfacing. However, the pro-
duced geometries are limited to flat inflated models. Bein et al.
[22] present sketch-based tools for creating subdivision surfaces
using operators such as extrusion, revolution, and lofting. The
shape is controlled through vertex/edge additions and vertex/face
dragging. Similar to other sketch-based tools, maintaining the
crease curves
crease curves
Fig. 3. Various results obtained by our approach. Note the feature crease curves and the transitions between smooth and discontinuous regions. The smooth regions far
from crease curves are designed and maintained using our approach.
Fig. 4. Current modeling software presents tools to locally modify surfaces
including (a) modifications using control points and (b) deformation lattices
(Rhinoceros 3D [5]). Recent advances in sketch-based methods utilize modifica-
tions to the curves using (c) pick/drag (Fibermesh [8]) and (d) modifier strokes
(SketchCAD [9]). However, surfaces are still manipulated indirectly through the
curves, preventing a direct control of surface fairness.
G. Orbay, L.B. Kara / Computers & Graphics 36 (2012) 916–929918