945 research outputs found
A Framework for Symmetric Part Detection in Cluttered Scenes
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.Comment: 10 pages, 8 figure
Optical Gyrotropy and the Nonlocal Hall Effect in Chiral Charge Ordered TiSe
It has been suggested that materials which break spatial inversion symmetry,
but not time reversal symmetry, will be optically gyrotropic and display a
nonlocal Hall effect. The associated optical rotary power and the suggested
possibility of inducing a Kerr effect in such materials, in turn are central to
recent discussions about the nature of the pseudogap phases of various cuprate
high-temperature superconductors. In this letter, we show that optical
gyrotropy and the nonlocal Hall effect provide a sensitive probe of broken
inversion symmetry in -TiSe. This material was recently found to
possess a chiral charge ordered phase at low temperatures, in which inversion
symmetry is spontaneously broken, while time reversal symmetry remains unbroken
throughout its phase diagram. We estimate the magnitude of the resulting
gyrotropic constant and optical rotary power and suggest that -TiSe may
be employed as a model material in the interpretation of recent Kerr effect
measurements in cuprate superconductors.Comment: 5 pages, 3 figure
Bilateral symmetry of object silhouettes under perspective projection
Symmetry is an important property of objects and is exhibited in different forms e.g., bilateral, rotational, etc. This paper presents an algorithm for computing the bilateral symmetry of silhouettes of shallow objects under perspective distortion, exploiting the invariance of the cross ratio to projective transformations. The basic idea is to use the cross ratio to compute a number of midpoints of cross sections and then fit a straight line through them. The goodness-of-fit determines the likelihood of the line to be the axis of symmetry. We analytically estimate the midpoint’s location as a function of the vanishing point for a given object silhouette. Hence finding the symmetry axis amounts to a 2D search in the space of vanishing points. We present experiments on two datasets as well as internet images of symmetric objects that validate our approach. under perspectivities, we analytically compute a set of midpoints of the object as a function of the vanishing point. Then, we fit a straight line passing through the midpoints. The goodness-of-fit defines the likelihood of this line to be a symmetry axis. Using the proposed method, searching for the symmetry axis is reduced to searching for a vanishing point. Our approach is global in the sense that we consider the whole silhouette of the object rather than small parts of it. The results show that the method presented here is capable of finding axes of symmetry of considerably distorted perspective images. 2 Related Work
Symmetry based semantic analysis of engineering drawings
pre-printEngineering drawings have posed significant challenges to image analysis for many decades. The goal is to take images of scanned engineering drawings and interpret them so as to understand their contents (e.g., characters, digits, line segments, box segments etc.). This is known as semantic analysis. We propose a new approach here which takes advantage of the man-made nature of drawings: there is a tremendous amount of symmetry. We exploit this insight to enhance our previously reported system, the Non-Deterministic Agent System (NDAS), with symmetry-based analysis tools. Agents work independently but use each others results to produce the final result (e.g., form segmentation, character analysis, structural analysis, boundary segmentation, etc.). We use the wreath product representation both to characterize symmetry as well as to structure a Bayesian network model of the uncertainty. This approach permits wide application to perform semantic analysis of engineering drawings
Sensitivity of the stress response function to packing preparation
A granular assembly composed of a collection of identical grains may pack
under different microscopic configurations with microscopic features that are
sensitive to the preparation history. A given configuration may also change in
response to external actions such as compression, shearing etc. We show using a
mechanical response function method developed experimentally and numerically,
that the macroscopic stress profiles are strongly dependent on these
preparation procedures. These results were obtained for both two and three
dimensions. The method reveals that, under a given preparation history, the
macroscopic symmetries of the granular material is affected and in most cases
significant departures from isotropy should be observed. This suggests a new
path toward a non-intrusive test of granular material constitutive properties.Comment: 15 pages, 11 figures, some numerical data corrected, to appear in J.
Phys. Cond. Mat. special issue on Granular Materials (M. Nicodemi Editor
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