174,042 research outputs found
Finite element surface registration incorporating curvature, volume preservation, and statistical model information
We present a novel method for nonrigid registration of 3D surfaces and images. The method can be used to register surfaces by means of their distance images, or to register medical images directly. It is formulated as a minimization problem of a sum of several terms representing the desired properties of a registration result: smoothness, volume preservation, matching of the surface, its curvature, and possible other feature images, as well as consistency with previous registration results of similar objects, represented by a statistical deformation model. While most of these concepts are already known, we present a coherent continuous formulation of these constraints, including the statistical deformation model. This continuous formulation renders the registration method independent of its discretization. The finite element discretization we present is, while independent of the registration functional, the second main contribution of this paper. The local discontinuous Galerkin method has not previously been used in image registration, and it provides an efficient and general framework to discretize each of the terms of our functional. Computational efficiency and modest memory consumption are achieved thanks to parallelization and locally adaptive mesh refinement. This allows for the first time the use of otherwise prohibitively large 3D statistical deformation models
Conformal Invariance and Renormalization Group in Quantum Gravity Near Two Dimensions
We study quantum gravity in dimensions in such a way to preserve
the volume preserving diffeomorphism invariance. In such a formulation, we
prove the following trinity: the general covariance, the conformal invariance
and the renormalization group flow to Einstein theory at long distance. We
emphasize that the consistent and macroscopic universes like our own can only
exist for matter central charge . We show that the spacetime
singularity at the big bang is resolved by the renormalization effect and
universes are found to bounce back from the big crunch. Our formulation may be
viewed as a Ginzburg-Landau theory which can describe both the broken and the
unbroken phase of quantum gravity and the phase transition between them.Comment: 32 pages, TIT-HEP-256, KEK-TH-395, YITP/U-94-1
Supermembranes and Super Matrix Models
We review recent developments in the theory of supermembranes and their
relation to matrix models.Comment: Invited lecture presented at the Corfu Workshop, September 20 - 26,
1998, of the TMR Project "Quantum Aspects of Gauge Theories, Supersymmetry
and Unification" (ERBFMRXCT96-0045), to appear in the proceedings. Latex 41
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Decodability Attack against the Fuzzy Commitment Scheme with Public Feature Transforms
The fuzzy commitment scheme is a cryptographic primitive that can be used to
store biometric templates being encoded as fixed-length feature vectors
protected. If multiple related records generated from the same biometric
instance can be intercepted, their correspondence can be determined using the
decodability attack. In 2011, Kelkboom et al. proposed to pass the feature
vectors through a record-specific but public permutation process in order to
prevent this attack. In this paper, it is shown that this countermeasure
enables another attack also analyzed by Simoens et al. in 2009 which can even
ease an adversary to fully break two related records. The attack may only be
feasible if the protected feature vectors have a reasonably small Hamming
distance; yet, implementations and security analyses must account for this
risk. This paper furthermore discusses that by means of a public
transformation, the attack cannot be prevented in a binary fuzzy commitment
scheme based on linear codes. Fortunately, such transformations can be
generated for the non-binary case. In order to still be able to protect binary
feature vectors, one may consider to use the improved fuzzy vault scheme by
Dodis et al. which may be secured against linkability attacks using
observations made by Merkle and Tams
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