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Shape Manipulation of Diffusion Curves Images

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Diffusion curves image (DCI) is a kind of vector graphics with its geometric primitives defined by a set of Bézier curves. As one of the vector graphics, editability is an important property for diffusion curves. However, most of the current DCI shape manipulation methods are typically limited to single curve. It is tedious for users to edit the shape of DCI by deforming each curve individually, especially with the increasing number of curves. In addition, the image contents cannot well preserved because there is no constraints among curves. We present an efficient and flexible DCI manipulation method by combining global and local deformations. The overall shape is easily changed by our global manipulation tool with a few user interactions, while the local manipulation tool is used to modify details. These two tools can be combined to obtain desired effects. In both cases, the constraints during deformation preserve the quality of the editing results. Our experimental results demonstrate that the proposed method provides more convenient control for DCI shape editing than existing techniques based on diffusion curves.
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Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000.
Digital Object Identifier 10.1109/ACCESS.2017.DOI
Shape Manipulation of Diffusion Curves
Images
SHUFANG LU1, XUEFENG DING1, FEI GAO1, JIAZHOU CHEN1
1College of Computer Science and Technology, Zhejiang University of Technology
Corresponding author: Shufang Lu (e-mail: sflu@zjut.edu.cn).
This work was supported by Natural Science Foundation of Zhejiang Province (No. LY19F020027), the Key Research and Development
Program of Zhejiang Province (No. 2018C03055), and Natural Science Foundation of Zhejiang Province (No. LY18F020035).
ABSTRACT Diffusion curves image (DCI) is a kind of vector graphics with its geometric primitives
defined by a set of Bézier curves. As one of the vector graphics, editability is an important property for
diffusion curves. However, most of the current DCI shape manipulation methods are typically limited to
single curve. It is tedious for users to edit the shape of DCI by deforming each curve individually, especially
with the increasing number of curves. In addition, the image contents cannot well preserved because there is
no constraints among curves. We present an efficient and flexible DCI manipulation method by combining
global and local deformations. The overall shape is easily changed by our global manipulation tool with a
few user interactions, while the local manipulation tool is used to modify details. These two tools can be
combined to obtain desired effects. In both cases, the constraints during deformation preserve the quality of
the editing results. Our experimental results demonstrate that the proposed method provides more convenient
control for DCI shape editing than existing techniques based on diffusion curves.
INDEX TERMS Shape Manipulation, Vector Graphics, Diffusion Curves Images
I. INTRODUCTION
Vector graphics provides a representation for describing im-
ages by geometric primitives such as points, lines and curves.
Compared with raster graphics which is defined via a grid
of pixels, vector graphics is resolution-independent, more
compact, and easy for editing. Due to these advantages,
it is extensively used in graphical user interface, internet
applications, three-dimensional modeling and so on.
As one of popular vector graphics primitives, diffusion
curves [1] uses cubic Bézier curves and attached both side
color control points to create high-quality vector images. It
can be created from scratch by drawing tools or through
tracing existing images. The rendering of a set of diffusion
curves results in a diffusion curve image (DCI) with complex
color gradients.
Editability is an important property of vector graphics,
which is the reason why it remains popular among artists
and designers. However, existing work mainly focus on the
DCI creating process [2], [3], rendering acceleration [4],
[5] and image diffusion quality improvement [6], with weak
support for flexible shape manipulation. As a DCI consists of
a sparse set of disconnected paths, most of current methods
are limited to curve-level editing. They just deform a curve
by moving its control points. Despite their capability of
modifying the local details of a DCI, they still have some
inevitable limitations. Obviously, it is a time-consuming pro-
cess because users have to edit each curve individually. Also,
without geometric constraints among curves, it may require
modifications to a large amount of individual curves in order
to obtain desired effects. Moreover, the semantic content
represented by a set of curves in a DCI may be destroyed
due to editing each curve separately and the editing effects
are hard to control or predict.
To address these problems, we present an efficient shape
manipulation tool for DCI by combining both global and
local deformations. The global shape manipulation is treated
as space deformation operation through linear blending affine
transformations. We triangulate the closed domain of the DCI
and adopt bounded biharmonic weights [7] as the weight
function. Then, the global shape is deformed by setting a
cage or a few single points as handles controlled by users.
With only moving or dragging a few handles, a new shape
of DCI can be easily generated followed by a curve refitting
process. On the other hand, local details are modified by
editing individual curve. To minimize user interaction, we
use two simulation points to replace two curve control points
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that are not located on the curve. In both global and local
deformation modes, we add constraints on the junctions
between connected curves, which makes the editing results
more reliable.
With our DCI shape manipulation method, the user can
first edit the global shape roughly by global deformation and
then modify the details precisely with local shape deforma-
tion. Although it has been proposed to add diffusion curves
to SVG standards, there is still no standard representations
and rendering methods for diffusion curves. We employ the
original diffusion curves representation [1] and a GPU-based
rendering system [4] to make our method concise and ex-
tensible. The color diffusion result is automatically changed
with DCI shape editing because the color inforamtion of
diffusion curves is attached with its geometric structure.
Compared with previous diffusion curves image editing
methods, our method is more effective and flexible in con-
trolling the shape of DCI. It significantly reduces the user
interaction time, especially for the circumstances of creat-
ing different gestures of a same object(i.e. keyframes for
animation). Furthermore, several constraints are used on the
junctions between connected curves in both global and local
schemes to guarantee high-quality editing results. To the best
of our knowledge, ours is the first DCI shape manipulation
technique combining both global and local deformation.
II. RELATED WORK
In this section, we focus on techniques targeting vector
graphics, vector graphics editing and shape deformation, the
topics that are most relevant to our work.
For a long time, vector graphics is defined by the prim-
itives filled with solid color and cannot offer complex col-
or gradients compared with raster graphics. More recent-
ly, new powerful vector graphics representations have been
proposed. One of them is gradient mesh [8]–[10], which
employs triangular or quad mesh as geometric structure and
stores color values on the vertices of mesh. Smooth rendering
results are achieved by interpolated colors across the mesh.
Gradient mesh produces high-quality image, but is not natu-
ral for artists and designers because the relationship between
patches of mesh and specific image contents is unclear. As a
result, it is difficult for us to manually create vector images
represented by gradient meshes and not convenient for subse-
quent editing. Another line of methods are based on diffusion
curves [1]. Diffusion curves employs cubic Bézier curves as
geometric primitives to describe the image boundaries, with
color constraints attached on both sides of each curve. By
solving a Poisson equation, the color control points on either
side of a curve diffuse over the entire image canvas smoothly .
It is intuitive for users because it supports traditional freehand
drawing techniques, and edges/curves are viewed as natural
primitives for encoding and editing images [11]. Subsequent
works on diffusion curves mainly focus on the rendering
acceleration [4], [5], [12], image quality improvement [3],
[13], [14], color diffusion control [15]–[19], new forms of
diffusion curves extension [6], [20], [21], and its application-
s [22], [23].
Easy for editing is one of the most important reasons
that vector graphics remains popular . Vector primitives are
widely used either in raster image editing and vector graphics
image itself. Raster image editing has natural advantages
in manipulating pixels but limits some operations such as
object-based editing. Based on the observation that edges and
contours are directly related to significant visual events in
image content such as object boundaries, vector geometric
primitives are commonly applied for raster image editing op-
erations. Elder et al. [11] first introduced a method for image
editing based on edges rather than pixels. Barrett et al. [24]
proposed a real-time object-based image editing technique
for manipulation and animation of static digital photographs.
In their method, Objects is tessellated into a triangular mesh,
allowing shape modification to be performed by changing the
granularity of editing from the pixel to the object level. Fang
et al. [25] applied shape deformation in digital image editing
and proposed a detail preserving editing method via feature
curves.
The editability of vector graphics is very essential for
designers and artists who create visual content with user
interaction. Shape editing is convenient for gradient mesh
representation because it provides the connectivity for vector
image contents. Space deformation techniques can be easily
applied to mesh deformation. Liao et al. [9] proposed a
subdivision-based representation for vector images by com-
bining the triangular patches and curved boundaries. It per-
forms shape editing for vector images by using as-rigid-as-
possible method [26], but cannot guarantee the continuity be-
tween the adjacent curve segments. Different from gradient
mesh, diffusion curves representation consists of sparse sets
of disconnected curves to describe the geometry information.
Therefore, it is easy to edit each curve individually in DCI,
but hard to edit the semantic content which includes a large
number of curves. Despite several existing diffusion curves
methods spent some efforts in DCI editing, most of them
are still limited to curve-level editing. To facilitate content
creation for the artist, the pioneer diffusion curves method [1]
provides the user with several standard editing tools includ-
ing editing of single curve geometry and curve splitting. The
individual diffusion curve editing operations in [6], [13] are
intuitively be done by a click-and-drag metaphor. In their
methods, the Bézier curve control point that is the closest to
the clicked position is determined. Moreover, Pang et al. [5],
Boyé et al. [16] and Prévost et al. [27] used the triangle
mesh as a intermediate representation to assist the rendering
process. But as Boyé et al. mentioned in their work [16],
the triangle mesh updates lead to flickering artifacts during
editing operations and is not suitable for real-time DCI edit-
ing. Sun et al. [12] presented a fast multipole representation
for diffusion curves and points as well as an interactive
editing tool of merging different DCIs together. Xie et al. [2]
developed a hierarchical diffusion curves representation to
make their results more editable, but it is limited to gen-
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erating visual abstraction rather than editing the shape of
DCI. Lu et al. [14] proposed a diffusion-curve-based image
vectorization framework for RGBD images. It supports the
editing of objects that are segmented by using both color and
depth information. In this paper, we provide a more flexible
and efficient DCI shape editing system in terms of global and
local aspects.
Shape deformation is extensively used on visual design,
animation and so on. Generalized barycentric coordinates,
which defines coordinate of points with respect to polytopes,
provides an efficient way to compute smooth space deforma-
tion. There are many different constructions of barycentric
coordinates, and the well-known methods and their exten-
sions include Mean Value Coordinates [28]–[30], Harmon-
ic Coordinates [31]–[34], Green Coordinates [35], complex
barycentric coordinates [36], [37], and Biharmonic Coor-
dinates [7], [38]. The deformation equation is linear com-
bination of real or complex-valued coordinates with some
coefficients. Another way to perform space deformation is
variational methods, which solve a variational problem by
finite element discretization over a mesh [26], [39], [40]
or by smooth basis functions [33], [36], [38]. In addition,
some methods which are based on quasi-conformal maps and
their applications have been developed recently [41]–[49].
With trade-off between computational consumption, visual
effect and interactivity, Biharmonic Coordinates is used in
our method.
III. PROPOSED METHOD
In this section, we will introduce the details of our DCI shape
manipulation method. Our method provides two modes to
manipulate the shape of DCI: global shape editing and local
shape editing. Fig. 1 shows an overview of our DCI shape
manipulation framework. In global shape editing mode, by
setting and moving a few handle points, the global shape of
DCI is controlled by the underlying linear space deformation
method. We adopt bounded biharmonic weights as the weight
functions and refit diffusion curves after deformation. In local
shape editing mode, individual curve is deformed to modify
details of DCI. By adding two simulation points, the Bézier
curve control point that is the closest to the clicked position is
selected. Several constraints are performed during the global
and local deformation to produce high-quality results. Users
can combine these two editing modes to adjust the editing
result until achieving a desired shape.
A. GLOBAL SHAPE DEFORMATION
1) Linear Blending Deformation
The global shape editing is treated as the space deformation
operation and we employ the linear method by blending
affine transformations at arbitrary handles. Compared with
some quadratic methods, such as as-rigid-as-possible shape
manipulation [26], linear methods are usually more efficient
due to the simplicity. We denote each point of the given shape
by pand it is transformed to the new position p0by the
following weighted combinations:
p0=
m
X
i=1
ωi(p)Tip(1)
Given the handle points Hi(i= 1,2,3, , m),Tiis the affine
transformation for each handle points Hiand ωi(p)is the
weight function associated with Hievaluated at point p. The
weights ωiis obtained as minimizers of the Laplacian energy
with constraints on the closed domain :
argmin
ωi,i=1,2,3,...,m
m
X
i=1
1
2Z
||ωi||2dV
Subject to:
ωi(Hk) = δi,k
ωi(F)is linear FFG
m
X
i=1
ωi(p)=1 p
0ωi(p)1, i = 1, ..., m p
(2)
where the Hkis a handle point, and δi,k is Kronecker delta.
The FGdenotes the set of all boundary of cage.
We use the bounded biharmonic weights [7] as the weight
function because it is flexible and supports handles of arbi-
trary topology. In our method, we use cage or single points
to control the global shape of DCI. Fig. 1 and Fig. 2 show
the process of global shape editing with these two kinds of
topology schemes respectively.
Minimizing the biharmonic energy amounts to solving the
Euler-largrange equations, which are the biharmonic PDEs
in this case: 2ωi= 0. In order to solve the constrained
varitional problem numerically with quadratic programming,
we discretize it using linear finite elements.
2) Space discretization
Bounded Biharmonic Weight functions require space dis-
cretization for weight computation. As diffusion curves is
an open structure, a domain enclosing all curves is required
to compute the weight function values. For the cage topol-
ogy, the cage itself is a closed structure(see Fig.1(b)). For
single points, the image boundary is treated as the closed
region(see Fig.2(c)). We use the triangulation technique [50]
to discretize the closed space and the weight value of each
vertex in the triangle mesh is obtained by solving Eq.( 2).
We can change the path of cubic Bézier curves direct-
ly or via moving its control points. For the two different
controlling ways, there are two different constraints during
triangulation process.
The first one is to use the control points of Bézier curves as
additional constraint points, and these control points will be
the vertices of mesh after triangulation. Therefore, the path
of Bézier curves is changed by mapping their control points
to new positions. For each single curve, it is a directly way
since the path of Bézier curve is controlled by its control
points. However, it has some drawbacks for the global shape
deformation. Let C= [c1c2c3c4]represent the four
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FIGURE 1: An overview of our DCI shape manipulation method.
(a) (b) (c) (d) (e)
FIGURE 2: Global deformation result with single points. (a) is the input DCI, (b) shows blue handles points, (c) is the space
discretization result, (d) is the deformation result with blue arrows indicate the moving direction of handle points, and (e) is the
corresponding rendering result.
control points of a Bézier curve (see Fig. 4). c1and c4is
the start point and the end point of the curve and they are
located on the curve. But the other two control points c2
and c3are usually not located on the curve, even far away
from the curve. The bounded biharmonic weights is distance-
dependent, so for each handle point its influence decreases
with the increase of distance from it. As shown in Fig. 3, the
yellow curve is defined by A= [A1A2A3A4], and the
green curve is defined by B= [B1B2B3B4]. If we
use the control points as the triangulation constraints, the red
handle point Phas more effects on the yellow curve although
green curve is closer to P, because the control point A3of
the yellow path is closer to Pthan the control points of the
green curve. As a result, it may generate unpredictable and
undesirable results after deformation. As shown in Fig. 8(e),
the continuity is destroyed and local textures are not well
preserved after editing, which resulting in visible artifacts
indicated by the green arrow.
The second option is to use the sampling points on paths of
Bézier curves as additional constraints for triangulation. As
we observe that the following refitting result is not smooth
with a small number of sampling points, in our experimen-
t, we sample 100 points of each curve as the additional
constraints during the triangulation process. Fig. 1(c) and
Fig. 2(c) show two area discretization results. In this way,
the handle points directly influence the path of Bézier curve
rather than the control points, and the problem shown in
Fig. 3 can be avoided. With the same editing by dragging
the same control point, the deformation result based on this
triangulation scheme is improved as shown in Fig. 8(h).
3) Curves Refitting
For each individual curve, let S(p)represent the undeformed
shape with each sampling point p. With the arrows in
Fig. 1(d) and Fig. 2(d) indicate the moving directions of the
handles, all sampling points, that is the discrete representa-
tion of the curves are mapped to new positions. The deformed
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FIGURE 3: Illustration of using control points as triangula-
tion constraints.
shape is:
S(p0) =
m
X
i=1
ωi(S(p)TiS(p)(3)
Then we need to find a cubic Bézier curve that is the closest
to the deformed polygonal path through refitting process.
The deformed shape will usually not be represented by
Bézier curve exactly. For each individual curve, let S(c)
represent the undeformed shape with each control point cof
Bézier curve. Therefore, our goal is to find the closest shape
S(c0)whose path best fit the deformed shape. This problem
is treated as the energy optimization to obtain the unknown
control points c0as follows:
E(S) = ||S(c0)S(p0)||2.(4)
E(S)is the integrated distance between the fitting shape
S(c0)and deformed shape S(p0).
In order to preserve the local details of DCI, it is very im-
portant to keep the smoothness at the junctions between con-
nected curves. We employ C0and C1continuity constraints
as the linear constraints in the energy minimization process.
Let D= [d1d2d3d4]represent the curve which shares
an endpoint with curve C, and c4=d1. For deformed curves
C0= [c0
1c0
2c0
3c0
4]and D0= [d0
1d0
2d0
3d0
4], we
set c0
4=d0
1and c0
4c0
3=d0
2c0
1as the linear constraints
to enforce C0continuity and C1continuity, respectively. On
the overall domain, in order to solve for all cubic Bézier
curves simultaneously, we weight the energy for each curve.
Therefore, the complete refitting energy minimization prob-
lem with all constraints is as follows:
argmin
S
n
X
j=1
ηjE(Sj)
Subject to: c0
4=d0
1
c0
4c0
3=d0
2c0
1
(5)
where nis the number of curves in the DCI, Sjis the shape
of j-th curve, and ηjis the length of Sj.
We adopt the numerical solver proposed by Liu et al. [51]
to solve Eq.(5), in which the linear equality constraints are
enforced via the Lagrange multiplier method. The different
FIGURE 4: Illustration of our local shape editing method.
(a) (b)
FIGURE 5: Comparison of local editing without and with
angle preservation.
part is that we do not use the tangent magnitude ratios and
angle preservation constraints for live manipulation, as we
find that these constraints have a small and limited nega-
tive effect on refitting result but are time-consuming. Also,
the negative effect can be further remedied by our local
shape editing method (see Section III-B). Fig. 1(d) and 2(d)
show the curve refitting results after global deformation, and
Fig. 1(e) and 2(e) are the corresponding rendering results
with deformed diffusion curves. After the refitting step, the
deformed paths are represented by Bézier curves rather than
discrete points.
B. LOCAL SHAPE EDITING
The overall shape of a DCI can be easily changed by the
global shape editing mode, as shown in Fig. 1 and 2. But
it is hard to perform more precise editing operations, such as
modifying local details. To make the shape editing tool more
powerful, our method also supports a local shape editing tool.
Since the individual curve is independent from each other,
it is intuitive to change its path by moving the control points.
However, it is difficult for users to accurately select the con-
trol points especially where the area is densely covered by a
large number of curves. Instead of moving the control points
directly, we use two simulation points s1and s2to implicitly
represent the two control points c2and c3that usually are not
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located on the curve as shown in Fig. 4. s1and s2are located
on the 1/3and 2/3of the curve, respectively. When the user
clicks on the DCI (i.e. the red point in Fig. 4), the simulation
point s1that is closest to the clicking position is selected
automatically. Then the displacement vector from clicking
position to the new position is added to the corresponding
control point c2. Therefore, the curve that is closest to the
clicking position is automatically chosen for deformation.
Similar to global shape editing, we also set constraints
at the junctions between connected individual curves. For
connected Bézier curves C= [c1c2c3c4]and D=
[d1d2d3d4]with c4=d1, the local shape editing tool
enforces C0continuity as global deformation described in
Section III-A3. In addition, we preserve the angle at junctions
as well. Specifically, as dragging is a translation operation
on the selected point, we apply same displacement vector
to the control points c3and d2when dragging the junction
point c4(d1) to the new position c0
4(d0
1). The angle constraint
is expressed as follows:
c0
3=c3+ (c0
4c4)
d0
2=d2+ (c0
4c4)(6)
With junction point A0, Fig. 5(a) is the result without
angle preserving . When dragging point A0to B0while other
control points are fixed, the shape at the junction B0has great
change which is not desired. Fig. 5(b) shows the improved
deformation result by preserving the angle at junction A0
during dragging. The smoothness at junction B0is kept and
local details are preserved better. Fig. 1(f) shows the local
shape editing result by modifying the red highlighted curve.
One more local editing result is shown in Fig. 6, in which the
shapes of the front two legs of the chair are edited.
IV. RESULTS AND DISCUSSIONS
More DCI global shape editing results are shown in Fig. 7.
Cage is used as handles to deform the vector images of
Lotus, Butterfly, Leaf and Cup, while single points are used
as handles to control the images of Dolphin, Armchair, Frog
and Panda. By moving the handle points, our editing tool
easily modifies the global shape of DCI with a few user
interactions. On the other hand, images details can be further
modified by the provided local editing method. Users can
alternately perform the two editing operations to achieve
desired editing results. Compared to other DCI editing tools,
which only offer single curve editing function, our DCI shape
editing method is more efficient and convenient by signifi-
cantly reducing the user interactions. Furthermore, with the
constraints in both global and local editing modes, image
features and details are better preserved after deformation.
Our editing system is implemented on a 3.3 GHz Intel
Dual Core i5 CPU and an NVIDIA GTX960 GPU. The
user interface is implemented on browser with JavaScript
and diffusion curves rendering is performed on GPU by em-
ploying WebGL. The geometry processing library libigl [52]
is used to compute bounded biharmonic weights. The cubic
(a) (b)
(c) (d)
FIGURE 6: The local editing result. (a) is the input, (b) is
the local shape editing result with modifying highlighted red
curves, (c) and (d )are the rendering results before and after
performing local shape editing.
Bézier curves refitting is implemented in Python with Numpy
package.
The statistics and performance of the global shape edit-
ing results in this paper are shown in Table 1. The space
discretization and bounded biharmonic weights calculation
are performed in the precomputation step, and it takes more
precomputation time with the increasing number of handles
and curves. Fortunately, it is just performed once and do not
affect the efficiency of live deformation. In the experiments,
we notice that the live shape manipulation is smooth when
the number of curves is not large, probably less than 300.
As shown in Table 1, after moving handles, the update of
calculating new positions of all sample points on curves and
refitting them back to diffusion curves representation is very
fast. The FPS is the average frame number per second when
continually performing the shape editing manipulation. Due
to the modification of geometric information, the rendering
system need to resample the points on curves and send these
data to GPU solver to compute every pixel of DCI again.
When frequently performing shape editing interaction on a
large number of diffusion curves, the frame rate will drop
down due to the rendering computation cost. As a result, the
DCI rendering is the most time-consuming part, which is the
bottleneck of our method.
Fig. 8 compares global deformation results with differ-
ent triangulation constraints during the process of area dis-
cretization as mentioned in Section III-A2. Given an input of
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FIGURE 7: A variety of global shape editing results. For each example, left is the DCI before editing and right is the editing
result after global deformation. The blue arrows indicate the moving directions of handle points.
TABLE 1: Statistics and performance of the global shape editing results shown in this paper. #Paths is the number of path
that consists of a series of end-to-end diffusion curves, #Curves is the number of diffusion curves, #Handles is the number of
handle points, Precomputation/h(s) is the bind time per handle in seconds.Updating(s) is the time including calculating the new
positions of all the sample points and refitting them back to diffusion curves representation.
Dataset #Paths #Curves #Handles Precomputation/h(s) Updating(s) Rendering(FPS)
Poivron 38 109 8 1.66 3e-3 60
Lotus 22 113 40 1.64 1.6e-2 52
Butterfly 89 176 23 1.65 1.7e-2 55
Leaf 39 197 13 3.01 5e-3 49
Cup 99 132 18 1.72 1.9e-2 58
Dolphin 39 171 5 3.61 2.5e-3 53
Armchair 48 117 2 4.51 1.9e-3 60
Frog 32 79 2 3.63 1.2e-4 60
Panda 34 310 8 3.07 3.3e-3 33
VOLUME 4, 2016 7
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(a) (b) (c)
(d) (e) (f)
(g) (h) (i)
FIGURE 8: The global deformation results comparison between two area discretization schemes. The first row is the initial
state, the second row is the deformation result by using control points as triangulation constraints, and the third row is the result
by using sampling points of curves as triangulation constraints. Each row from left to right: a DCI, a close-up of the highlighted
box region, and corresponding geometrical curves.
a DCI with its cage shown in Fig. 8(a), Fig. 8(b-c) are the
zooming-in result of highlighted red box and its diffusion
curves. Same deformation indicated by the blue arrow is
applied on both triangulation results, and the output are
shown in Fig. 8(d) and Fig. 8(g), followed by a close-up of the
highlighted boxes and corresponding diffusion curves. When
using the control points as the triangulation constraints, the
spatial distance between handle points and control points
is not consistent with the distance between handle points
and curve paths, which resulting in visible artifacts indicated
by green arrows in Fig. 8(e) Instead, we directly sample
points on curves and use them as additional constraints for
triangulation with the improved results shown in Fig. 8(h).
V. CONCLUSIONS
In this paper, we have presented a shape editing system for
diffusion curves images. The proposed shape editing method
is efficient and convenient by providing both global and local
editing modes. A new shape can be easily created by only a
few user interactions, and the local details are well preserved
8VOLUME 4, 2016
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by setting constraints on the junctions between connected
curves. Moreover, the deformed result is always represented
by the data structure of diffusion curves, so it is easy to
integrate our editing tool into other DCI editing and rendering
systems.
In the future, there are still some aspects that could be
improved. First, real-time user feedback is essential for live
shape manipulation. With the increasing number of handles
and curves, the frame rate drops down as the DCI rendering
and bounded biharmonic weights calculation take most of
the time. We will develop a fast DCI shape editing system
by accelerating rendering process while reducing weights
computation time. Secondly, our tool is applied on the single-
layer DCI, and all the curves in the region will be deformed
and refitted during global deformation. If there are unclear
boundaries between the object we want to edit and the
background, to select the region of interest is a difficult task.
Thus, multi-layer representation of DCI can be developed to
avoid this problem.
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SHUFANG LU is an associate professor at Col-
lege of Computer Science and Technology, Zhe-
jiang University of Technology. She received her
BSc degree in software engineering from Wuhan
University, and MSc and PhD degrees in computer
science and technology from Zhejiang University.
Her research interests include computer graphics
and computer vision.
XUEFENG DING is a postgraduate student of the
College of Computer Science & Technology, Zhe-
jiang University of Technology. He received his
BSc degree in software engineering from Zhejiang
University of Technology. His research interests
include computer graphics and computer vision.
FEI GAO is a professor at the College of Comput-
er Science and Technology at Zhejiang University
of Technology. He received his PhD degree in
mechanical engineering from Zhejiang Universi-
ty in 2004. His research interests include image
processing, computer vision, and computer-aided
design.
JIAZHOU CHEN is an associate professor in
Zhejiang University of Technology. He received
his double Ph.D. degrees in INRIA Bordeaux
Sud-Ouest, France, and the State Key Laboratory
of CAD and CG at Zhejiang University, China,
in 2012. His research interests include computer
graphics and visual media computing.
10 VOLUME 4, 2016
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Smooth space deformation has become a vital tool for the animation and design of 2D and 3D shapes. Linear methods, under the umbrella term of "linear blend skinning", are the de facto standard for 3D animations. Unfortunately such approaches do not trivially extend to deforming vector graphics, such as the cubic Bézier splines prevalent in 2D or subdivision surfaces in 3D. We propose a variational approach to reposition the control points of cubic Bézier splines and Catmull-Clark subdivision surfaces - or any linear subdivision curves or surfaces - to produce curves or surfaces which match a linear blend skinning deformation as closely as possible. Exploiting the linearity of linear blend skinning, we show how this optimization collapses neatly into the repeated multiplication of a matrix per handle. We support C0, C1, G1, and fixed-angle continuity constraints between adjacent Bézier curves in a spline. Complexity scales linearly with respect to the number of input curves and run-time performance is fast enough for real-time editing and animation of high-resolution shapes. © 2014, Association for Computing Machinery. All rights reserved.