An iterative approach for fitting multiple connected ellipse structure to silhouette
Richard Yi Da Xu
*
, Michael Kemp
School of Computing and Mathematics, Charles Sturt University, Australia
article info
Article history:
Available online 18 January 2010
Keywords:
Multiple connected ellipse fitting
Iterative Closest Point
Contour fitting
abstract
In many image processing applications, the structures conveyed in the image contour can often be
described by a set of connected ellipses. Previous fitting methods to align the connected ellipse structure
with a contour, in general, lack a continuous solution space. In addition, the solution obtained often sat-
isfies only a partial number of ellipses, leaving others with poor fits. In this paper, we address these two
problems by presenting an iterative framework for fitting a 2D silhouette contour to a pre-specified con-
nected ellipses structure with a very coarse initial guess. Under the proposed framework, we first
improve the initial guess by modeling the silhouette region as a set of disconnected ellipses using mixture
of Gaussian densities or the heuristic approaches. Then, an iterative method is applied in a similar fashion
to the Iterative Closest Point (ICP) (Alshawa, 2007; Li and Griffiths, 2000; Besl and Mckay, 1992) algo-
rithm. Each iteration contains two parts: first part is to assign all the contour points to the individual
unconnected ellipses, which we refer to as the segmentation step and the second part is the non-linear
least square approach that minimizes both the sum of square distances between the contour points
and ellipse’s edge as well as minimizing the ellipse’s vertex pair (s) distances, which we refer to as the
minimization step. We illustrate the effectiveness of our methods through experimental results on several
images as well as applying the algorithm to a mini database of human upper-body images.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Object recognition and shape registration is an important image
processing topic and its finding has been incorporated into many
real-life applications. Over the last decade, various techniques have
been developed for registering 2D and 3D models to images, from
simple geometrical shape models such as a vehicle (Yiu et al.,
2005) to more complex models such as human limbs tracking (Ber-
nier et al., 2009; Fossati et al., 2008) and hand model fitting (Du
and Charbon, 2007).
Many works have used the joint ellipse models to represent
complex shapes (Fossati et al., 2008; Xu and Kemp, 2009; Jeune
et al., 2004). The advantage of using a connected ellipse represen-
tation is that the ellipse is easily parameterized and the parameters
convey information including the location, orientation and varia-
tion of data. The connectivity requirements of the adjacent ellipses
further reduce the number of degree of freedoms, and hence con-
strain their movements to reflect real-world problems. For exam-
ple, the upper and lower human limbs should always be joined.
However, some of the recent works suffer from two drawbacks:
firstly, they lack a continuous solution space to ensure only dis-
crete ‘‘valid” ellipse parameters are achieved and secondly most
of these methods require an accurate initialization.
A recent representative example which lacks in continuous
solution space is found in (Fossati et al., 2008), where the authors
use Generalized Expectation Maximization (GEM) to assign edge
pixels to body parts and to find the body pose that maximizes
the likelihood of the resultant assignments. However, in the Max-
imization (M)-step of the algorithm, a ‘‘better” pose parameter is
selected from the predefined data sets to increase the likelihood
probability. The use of only discrete number of classes can result
in misalignment between the image and the model, especially
when the number of predefined models is small. In this paper,
we propose an effective and generalized approach to the fitting
algorithm. Our algorithm does not use the predefined model
parameter classes like (Fossati et al., 2008), but considers an infi-
nite number of possible multiple ellipse representations under
the connectivity requirement.
It is natural to present the problem of fitting as finding a solu-
tion for ellipse structure that minimizes the sum of squares of dis-
tances between the data points and the ellipse structure, in which
a numerical method can be used to find the solution in a non-linear
least square fashion. For example, in (Jeune et al., 2004), the
authors fit multiple ellipsoids to the noisy 3D hand points using
the geometrically constrained Levenberg–Marquardt (L–M) (Mad-
sen et al., 2004) algorithm to reflect the fact that fingers can only
swing around the joints within an angular threshold. However,
when minimizing a single objective function for a complex struc-
ture containing many connected ellipses, the solution often aligns
0167-8655/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.patrec.2010.01.017
* Corresponding author. Fax: +61 2 633 84649.
E-mail addresses: rxu@csu.edu.au (R.Y.D. Xu), mkemp@csu.edu.au (M. Kemp).
Pattern Recognition Letters 31 (2010) 1860–1867
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Pattern Recognition Letters
journal homepage: www.elsevier.com/locate/patrec