The Experts below are selected from a list of 6654 Experts worldwide ranked by ideXlab platform
Enrico Radi - One of the best experts on this subject based on the ideXlab platform.
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path independent integrals around two circular holes in an infinite plate under biaxial loading conditions
International Journal of Engineering Science, 2011Co-Authors: Enrico RadiAbstract:Abstract An analytic solution is presented for stresses induced in an infinite plate with two unequal circular holes by remote uniform loadings and arbitrary internal pressures in the holes. The solution is obtained by using the general expression for a biharmonic function in bipolar coordinates. The Airy stress function is decomposed in the sum of a fundamental stress function for an infinite plate remotely loaded, which gives non vanishing tractions on the circular boundaries, and an auxiliary stress function required to satisfy the boundary conditions on the pressures at the edges of the holes, which produces vanishing stresses at infinity. Correspondingly, the variations of the stress concentration factor are determined in terms of the holes geometry and loading conditions. The path independent Jk- (k = 1, 2), M- and L-integrals are analytically calculated on a Closed Contour encircling the two holes, under remote loading, in order to evaluate the energy release rates accompanying unit translation, self similar expansion and rotation of the holes, respectively. Results are then presented for varying loading orientation angle, biaxial loading ratio and holes geometry.
Simo Vanni - One of the best experts on this subject based on the ideXlab platform.
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radial frequency analysis of Contour shapes in the visual cortex
PLOS Computational Biology, 2016Co-Authors: Viljami Salmela, Linda Henriksson, Simo VanniAbstract:Cumulative psychophysical evidence suggests that the shape of Closed Contours is analysed by means of their radial frequency components (RFC). However, neurophysiological evidence for RFC-based representations is still missing. We investigated the representation of radial frequency in the human visual cortex with functional magnetic resonance imaging. We parametrically varied the radial frequency, amplitude and local curvature of Contour shapes. The stimuli evoked clear responses across visual areas in the univariate analysis, but the response magnitude did not depend on radial frequency or local curvature. Searchlight-based, multivariate representational similarity analysis revealed RFC specific response patterns in areas V2d, V3d, V3AB, and IPS0. Interestingly, RFC-specific representations were not found in hV4 or LO, traditionally associated with visual shape analysis. The modulation amplitude of the shapes did not affect the responses in any visual area. Local curvature, SF-spectrum and contrast energy related representations were found across visual areas but without similar specificity for visual area that was found for RFC. The results suggest that the radial frequency of a Closed Contour is one of the cortical shape analysis dimensions, represented in the early and mid-level visual areas.
Per Sundell - One of the best experts on this subject based on the ideXlab platform.
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twistor space observables and quasi amplitudes in 4d higher spin gravity
Journal of High Energy Physics, 2011Co-Authors: Nicolo Colombo, Per SundellAbstract:Vasiliev equations facilitate globally defined formulations of higher-spin gravity in various correspondence spaces associated with different phases of the theory. In the four-dimensional case this induces a correspondence between a generally covariant formulation in spacetime with higher-derivative interactions to a formulation in terms of a deformed symplectic structure on a noncommutative doubled twistor space, whereby space-time boundary conditions correspond to sectors of an associative star-product algebra. In this paper, we look at observables given by integrals over twistor space defining composite zero-forms in spacetime that do not break any local symmetries and that are Closed on shell. They are nonlocal observables that can be evaluated in single coordinate charts in spacetime and interpreted as building blocks for dual amplitudes. To regularize potential divergencies arising in their curvature expansion from integration over twistor space, we propose a Closed-Contour prescription that respects associativity and hence higher-spin gauge symmetry. Applying this regularization scheme to twistor-space plane waves, we show that there exists a class of dual amplitudes given by supertraces. In particular, we examine next-to-leading corrections, and find cancellations that we interpret using transgression properties in twistor space.
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twistor space observables and quasi amplitudes in 4d higher spin gravity
arXiv: High Energy Physics - Theory, 2010Co-Authors: Nicolo Colombo, Per SundellAbstract:Vasiliev equations facilitate globally defined formulations of higher-spin gravity in various correspondence spaces associated with different phases of the theory. In the four-dimensional case this induces a map from a generally covariant formulation in spacetime with higher-derivative interactions to a formulation in terms of a deformed symplectic structure on a noncommutative doubled twistor space, sending spacetime boundary conditions to various sectors of an associative star-product algebra. We look at observables given by integrals over twistor space defining composite zero-forms in spacetime that do not break any local symmetries and that are Closed on shell. They can be evaluated locally in spacetime and interpreted as building blocks for dual amplitudes. To regularize potential twistor-space divergencies arising in their curvature expansion, we propose a Closed-Contour prescription that respects associativity and hence higher-spin gauge symmetry. As a sample calculation, we examine next-to-leading corrections to quasi-amplitudes for twistor-space plane waves, and find cancellations that we interpret using transgression properties in twistor space.
N.e. O'connor - One of the best experts on this subject based on the ideXlab platform.
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A multiscale representation method for nonrigid shapes with a single Closed Contour
IEEE Transactions on Circuits and Systems for Video Technology, 2004Co-Authors: T. Adamek, N.e. O'connorAbstract:In this paper, we discuss the criteria that should be satisfied by a descriptor for nonrigid shapes with a single Closed Contour. We then propose a shape representation method that fulfills these criteria. In the proposed approach, Contour convexities and concavities at different scale levels are represented using a two-dimensional (2-D) matrix. The representation can be visualized as a 2-D surface, where "hills" and "valleys" represent Contour convexities and concavities, respectively. The optimal matching of two shape representations is achieved using dynamic programming and a dissimilarity measure is defined based on this matching. The proposed algorithm is very efficient and invariant to several kinds of transformations including some articulations and modest occlusions. The retrieval performance of the approach is illustrated using the MPEG-7 shape database, which is one of the most complete shape databases currently available. Our experiments indicate that the proposed representation is well suited for object indexing and retrieval in large databases. Furthermore, the representation can be used as a starting point to obtain more compact descriptors.
Sven Dickinson - One of the best experts on this subject based on the ideXlab platform.
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a framework for symmetric part detection in cluttered scenes
Symmetry, 2015Co-Authors: Sanja Fidler, Alex Levinshtein, Cristian Sminchisescu, Sven DickinsonAbstract:The role of symmetry in computer vision has waxed and waned in importance during the evolution of the field from its earliest days. At first figuring prominently in support of bottom-up indexing, it fell out of favour as shape gave way to appearance and recognition gave way to detection. With a strong prior in the form of a target object, the role of the weaker priors offered by perceptual grouping was greatly diminished. However, as the field returns to the problem of recognition from a large database, the bottom-up recovery of the parts that make up the objects in a cluttered scene is critical for their recognition. The medial axis community has long exploited the ubiquitous regularity of symmetry as a basis for the decomposition of a Closed Contour into medial parts. However, today’s recognition systems are faced with cluttered scenes and the assumption that a Closed Contour exists, i.e., that figure-ground segmentation has been solved, rendering much of the medial axis community’s work inapplicable. In this article, we review a computational framework, previously reported in [1–3], that bridges the representation power of the medial axis and the need to recover and group an object’s parts in a cluttered scene. Our framework is rooted in the idea that a maximally-inscribed disc, the building block of a medial axis, can be modelled as a compact superpixel in the image. We evaluate the method on images of cluttered scenes.
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a framework for symmetric part detection in cluttered scenes
arXiv: Computer Vision and Pattern Recognition, 2015Co-Authors: Sanja Fidler, Alex Levinshtein, Cristian Sminchisescu, Sven DickinsonAbstract:The role of symmetry in computer vision has waxed and waned in importance during the evolution of the field from its earliest days. At first figuring prominently in support of bottom-up indexing, it fell out of favor as shape gave way to appearance and recognition gave way to detection. With a strong prior in the form of a target object, the role of the weaker priors offered by perceptual grouping was greatly diminished. However, as the field returns to the problem of recognition from a large database, the bottom-up recovery of the parts that make up the objects in a cluttered scene is critical for their recognition. The medial axis community has long exploited the ubiquitous regularity of symmetry as a basis for the decomposition of a Closed Contour into medial parts. However, today's recognition systems are faced with cluttered scenes, and the assumption that a Closed Contour exists, i.e. that figure-ground segmentation has been solved, renders much of the medial axis community's work inapplicable. In this article, we review a computational framework, previously reported in Lee et al. (2013), Levinshtein et al. (2009, 2013), that bridges the representation power of the medial axis and the need to recover and group an object's parts in a cluttered scene. Our framework is rooted in the idea that a maximally inscribed disc, the building block of a medial axis, can be modeled as a compact superpixel in the image. We evaluate the method on images of cluttered scenes.
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Multiscale Symmetric Part Detection and Grouping
International Journal of Computer Vision, 2013Co-Authors: Alex Levinshtein, Cristian Sminchisescu, Sven DickinsonAbstract:Skeletonization algorithms typically decompose an object’s silhouette into a set of symmetric parts, offering a powerful representation for shape categorization. However, having access to an object’s silhouette assumes correct figure-ground segmentation, leading to a disconnect with the mainstream categorization community, which attempts to recognize objects from cluttered images. In this paper, we present a novel approach to recovering and grouping the symmetric parts of an object from a cluttered scene. We begin by using a multiresolution superpixel segmentation to generate medial point hypotheses, and use a learned affinity function to perceptually group nearby medial points likely to belong to the same medial branch. In the next stage, we learn higher granularity affinity functions to group the resulting medial branches likely to belong to the same object. The resulting framework yields a skeletal approximation that is free of many of the instabilities that occur with traditional skeletons. More importantly, it does not require a Closed Contour, enabling the application of skeleton-based categorization systems to more realistic imagery.