Real-time Tracking Using Level Sets
Yonggang Shi, W. Clem Karl
∗
Electrical and Computer Engineering Department
Boston University
Boston, MA 02215
email:{yshi,wckarl}@bu.edu
Abstract
In this paper we propose a novel implementation of
the level set method that achieves real-time level-set-based
video tracking. In our fast algorithm, the evolution of the
curve is realized by simple operations such as switching
elements between two linked lists and there is no need to
solve any partial differential equations. Furthermore, a
novel procedure based on Gaussian filtering is introduced
to incorporate boundary smoothness regularization. By re-
placing the standard curve length penalty with this new
smoothing procedure, further speedups are obtained. An-
other advantage of our fast algorithm is that the topology
of the curves can be controlled easily. For the tracking of
multiple objects, we extend our fast algorithm to maintain
the desired topology for multiple object boundaries based
on ideas from discrete topology. With our fast algorithm, a
real-time system has been implemented on a standard PC
and only a small fraction of the CPU power is used for
tracking. Results from standard test sequences and our real-
time system are presented.
1. Introduction
Real-time tracking of object boundaries is an impor-
tant task in many vision applications such as video surveil-
lance, video conferencing, and human-computer interac-
tion, etc. Parametric contours have been applied success-
fully to achieve real-time performance[12, 7, 6, 19], but
they have difficulties in handling topological changes such
as the merging and splitting of object regions. For this pur-
pose, the level set method[15] is a more powerful technique
and various models have been proposed[2, 16, 4, 14, 9, 21].
But the high computational cost of the level set method has
∗
This work was partially supported by The Air Force Office of Scien-
tific Research under Grant F49620-03-1-0257, The National Institutes of
Health under Grant NINDS 1 R01 NS34189, and The Engineering research
centers program of the NSF under award EEC-9986821
limited its popularity in real-time scenarios. In this pa-
per, we propose a novel approach to implement the level
set method. Our approach does not need to solve any par-
tial differential equations (PDEs), thus reducing the compu-
tation dramatically compared with optimized narrow band
techniques proposed before. With our approach, real-time
level-set-based video tracking can be achieved.
Tracking models using level sets can be classified as
edge-based or region-based. In [16], a geodesic model that
combines motion and edge information was proposed. Such
edge-based models can be traced back to the snake model
in [13]. Based on morphing images, an early work on
region-based tracking was proposed in [2]. Using the dif-
ference between the current frame and a reference back-
ground, a region-based model was proposed in [4]. By
generalizing the region competition[22] idea, statistical ap-
proaches become quite popular recently. In [14, 21], the
feature distributions of both the object and background re-
gions were used for tracking. In [9], a predefined distri-
bution for the object region was tracked by minimizing a
Kullback-Leibler or Bhattacharyya distance.
Considerable work has been done to improve the speed
of level-set-based curve evolution. The basic idea has been
to limit the solution of the level set PDE to a narrow band
around the zero level set[1, 17, 20, 16], but issues such as
narrow band construction, reinitialization and step size con-
trol still make existing level set methods computationally
too expensive for real-time tracking.
The novel implementation we propose in this paper
avoids the above problems. Our approach is based on the
key observation that implicitly represented curves can be
moved by simply switching elements between two linked
lists. We also propose a Gaussian filtering process to replace
curvature-based smoothing, and this further speeds up our
algorithm. Another advantage of our method is that ideas
for controlling topological changes in [11] can be incorpo-
rated into our algorithm easily, which can be important for
the tracking of multiple objects and medical imaging appli-
cations. With our fast level set implementation, we have