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  • Matching Non-rigidly Deformable Shapes Across Images: A Globally Optimal Solution

    IEEE Computer Society Conference on Computer Vision and Pattern Recognition 2008, Anchorage, Alaska. c©IEEE.

    Thomas Schoenemann and Daniel Cremers Department of Computer Science

    University of Bonn, Germany

    Abstract

    While global methods for matching shapes to images have recently been proposed, so far research has focused on small deformations of a fixed template.

    In this paper we present the first global method able to pixel-accurately match non-rigidly deformable shapes across images at amenable run-times. By finding cycles of optimal ratio in a four-dimensional graph – spanned by the image, the prior shape and a set of rotation angles – we si- multaneously compute a segmentation of the image plane, a matching of points on the template to points on the seg- menting boundary, and a decomposition of the template into a set of deformable parts.

    In particular, the interpretation of the shape template as a collection of an a priori unknown number of deformable parts – an important aspect of higher-level shape repre- sentations – emerges as a byproduct of our matching al- gorithm. On real-world data of running people and walk- ing animals, we demonstrate that the proposed method can match strongly deformed shapes, even in cases where sim- ple shape measures and optic flow methods fail.

    1. Introduction

    Related work Following a series of seminal works in the late 80’s [8, 1, 10, 1, 14], image segmentation by mini- mizing appropriate energies has become a central focus in Computer Vision research. More recently, it was shown that under certain conditions globally optimal solutions can be obtained [2].

    However, for most real-world applications of image seg- mentation purely low-level information does not provide the desired segmentations. For segmenting and tracking fa- miliar objects, researchers have therefore introduced prior knowledge about the shape of objects [13, 17, 5, 4]. The re- sulting segmentation processes were shown to reliably seg- ment the objects of interest despite missing or misleading information, e.g. due to noise, background clutter or partial

    prior shape matched to generated partitioning and image another image alignment into parts

    Figure 1. Given a silhouette in one image, we want to iden- tify a corresponding silhouette in another image. The proposed method simultaneously determines this matching, a point align- ment between the matched silhouettes and a decomposition into deformable parts (see color-coded image).

    occlusions. Nevertheless, most existing approaches suffer from the following limitations:

    • Respective energies are often minimized locally. Not only does this require appropriate initialization, there is also no guarantee regarding the quality of computed so- lutions.

    • Respective shape priors are typically based on rather sim- ple geometric distances. As a consequence, they often re- quire large amounts of training data to sufficiently cover the space of permissible shapes. Collecting the necessary training silhouettes as well as inferring alignments and statistical models is a tedious process. In addition, the resulting segmentation process only allows for very little generalization to novel views.

    • The notion of point correspondences in the matching of shapes is often treated superficially. It is either handled by a reparameterization in the learning process [5] or completely ignored, reverting to an implicit representa- tion optimized by the level set method [13].

    Two combinatorial solutions to optimally find shapes in images were proposed in [3, 6]. Yet, both have practical limitations: The first one only applies to open curves. The second suffers from a quadratic memory complexity, pro- hibiting pixel-accurate globally optimal segmentations in

    1

  • reasonable-sized images for many years to come. In [16] we proposed a more efficient combinatorial solu-

    tion which allows to generate pixel-accurate segmentations in a matter of seconds. Yet, it merely allows for rather sim- ple elastic deformations of a single template. In particular, rotation is only handled as a global transformation. If parts of the shape rotate locally – like the fingers of a hand – performance deteriorates significantly. Such methods are ill suited for tracking deformable objects like the silhouettes of walking (or running) persons and animals.

    A large body of literature is dedicated to the study of shape. Sophisticated shape distances typically take into ac- count the correspondence of point pairs. Moreover, the par- titioning of shapes into a collection of deformable parts was identified to play an important role in human notions of shape similarity. The identification of point or part corre- spondences is a challenging combinatorial problem [7, 11].

    Contribution In this work we introduce the first pixel- accurate globally optimal algorithm to impose a shape simi- larity measure in image segmentation which considers large scale deformations. By minimizing a single functional, the proposed algorithm computes an optimal matching of tem- plate points to image pixels, while simultaneously partition- ing the silhouette into a set of deformable parts.

    The key idea is to map every possible solution to a cy- cle in a four-dimensional graph spanned by the image, the prior shape and a set of rotation angles. The optimal cycle in the graph is then found in effectively linear time. Due to a penalty on changes in the rotation angle, the silhouette is simultaneously partitioned into the (automatically deter- mined) optimal number of parts. On real-world sequences we demonstrate that our algorithm provides accurate match- ing of substantially deformed persons and animals.

    2. Ratio Energies for Shape Knowledge in Im- age Segmentation

    The central contribution of this work is a generaliza- tion of the method in [16] for matching shapes to images: Where previously an essentially rigid model was used, we are able to handle shape templates consisting of an (a priori unknown) number of deformable parts.

    In [16] the countour of a rigid template, given as a curve S : S1 → IR2, is elastically matched to an image I : Ω ⊂ IR2 → IR, i.e. an object silhouette C : S1 → Ω is determined. We only consider uniform parameteriza- tions of C, i.e. with constant derivative ||Cs(s)|| every- where. Simultaneously a monotone matching m : S1 → S1 is determined which brings points on C in correspondence with points on S: A point C(s) is matched to the point S(m(s)). The two functions are combined into a single function Γ : S1 → Ω × S1, with Γ(s) = (C(s),m(s)).

    The optimal pair of image contour and matching Γ is found by minimizing an energy in the form of a line integral. This integral consists of a data term encouraging the contour to coincide with image edges, a shape similarity term and a regularization term for the matching:

    E(Γ) = Edata(Γ) + ν Eshape(Γ) + λ Ereg,m(Γ), (1)

    The data term is realized as a positive edge detector function assigning low values to high image gradients:

    g(x) = 1

    1 + |∇I(x)| , (2)

    The shape term corresponds to a comparison of the tan- gent angles αC(s) on C with the corresponding tangent an- gle αS(m(s)) on S. The squared cyclic distance on S1 is taken. The regularity term for m penalizes length distor- tions: While a part of C may correspond to a part of S of different length, the amount of distortion is penalized. This amount is given by the derivative of a scaled version of the matching function m̃(s) = l(S)l(C)m(s). The used penalty function is

    Ψ(m̃′)=

     m̃′ − 1 if K ≥ m̃′ ≥ 1( m̃′

    )−1−1 if 1K ≤ m̃′ < 1 ∞ otherwise

    (3)

    where K is a pre-defined constant limiting the maximal length distortion. Together, these terms result in the min- imization problem

    min Γ=(C,m)

    ∫ S1

    g(C(s)) ds + λ ∫

    S1 Ψ(m̃′(s)) ds

    + ν ∫

    S1 |αC(s)− αS(m(s))|2 ds (4)

    This functional can be written as a ratio energy: Instead of the uniform parameterization one can parameterize by arc length and divide by the length of C. The (globally) optimal Γ can be found by the Minimum Ratio Cycle algorithm [12] on a suitable regular graph.

    For matching deformable shapes across images, the model described above has two severe limitations:

    • The shape dissimilarity encoded in (4) merely allows for small elastic deformations of a rigid template. While in- variance to global rotations can be introduced by rerun- ning the algorithm for various rotations of the template, local rotations of parts of the shape (the fingers of a hand or the legs of a running person) are not supported.

    • When allowing more severe deformations (as done in this work), the edge indicator function g(x) is too weak as a data term: If the image contains regions with high gradi- ents like the twigs or leaves of trees, the optimal object silhouette is placed within these regions with no shape deformation at all.

  • Both drawbacks will be resolved in the following section.

    3. Matching Non-rigidly Deformable Shapes Across Images

    In this section we generalize the above framework from elastically deforming templates to flexible templates con- sisting of an a priori unknown number of deformable parts. To this end we increase the dimension of the optimization space by adding a rotation function a(·) modelling the local rotation of parts. The extended function

    Γ : S1 → Ω× S1 × S1, (5)

    with Γ(s) = ( C(s),m(s), a(s)

    ) , now associates eac