Symmetric Objects Research Articles

A Novel Algorithm for Fast Measurement of Material Density in Symmetrical Objects Using X-Ray Radiography

AbstractX-ray radiography has proved to be essential in medical imaging and examination of material structures because it is noninvasive and generates images based on well-understood attenuation characteristics of materials. For radiographs of multiple overlapping materials, unraveling the individual attenuation contributions poses a problem that is commonly handled by either taking many radiographs at different object orientations for computed tomography or multiple images with different photon energies for Multiple Energy X-ray Absorptiometry (MEXA). Alternatively, to perform faster measurements, a novel algorithm has been developed to determine multimaterial systems' density. The algorithm can be effectively applied to perform measurements using only one to four radiographs of the object. A case study has been presented for a layered cylindrical object that involved sensitivity studies on image noise, X-ray generator voltage fluctuations, layer thickness measurement perturbations, and X-ray generator photon energy distribution fluctuations using simulated radiographs and density calculations using actual radiographs. The results from the simulated and experimental results were found to agree with actual density values.

Deflection of Light due to Spheroidal Oblate Static Objects

Deflection of light due to massive objects was predicted by Einstein in his General Theory of Relativity. This deflection of light has been calculated by many researchers in past, for spherically symmetric objects. But, in reality, most of these gravitating objects are not spherical instead they are ellipsoidal ( oblate) in shape. The objective of the present work is to study theoretically the effect of this ellipticity on the trajectory of a light ray. Here, we obtain a converging series expression for the deflection of a light ray due to an ellipsoidal gravitating object, characterised by an ellipticity parameter. As a boundary condition, by setting the ellipticity parameter to be equal to zero, we get back the same expression for deflection as due to Schwarzschild object. It is also found that the additional contribution in deflection angle due to this ellipticity though small, but could be typically higher than the similar contribution caused by the rotation of a celestial object. Therefore for a precise estimate of the deflection due to a celestial object, the calculations presented here would be useful.

Dynamics of uniformally collapsing system and the horizon formation

In astrophysics studies, the stellar system (e.g. stars, etc.) is generally considered a spherically symmetric object, and its evolutions (e.g. collapse) depends on the nature of fluid distribution and kinematical properties that are not precisely known. One generally makes additional restrictions allowing the integration of the field equations for the studies of dynamical models. This work deals with the spherically symmetric stellar system with uniform expansion scalar ([Formula: see text]) describing the uniform collapse of stars. The uniform expansion scalar describes an important physical scenario (a generalization of the OSD model) that will exhibit a new class of collapsing stars. Here, we have parameterized the expansion scalar as exponential, power law and their combination form as a function of time [Formula: see text] so that it describes the collapsing configuration [Formula: see text]. The formation of black-hole, the horizon surface, and the thermal behavior of black-hole has also been discussed. Further, we have discussed the dynamics of uniformly collapsing system for conformally flatness. The uniform motion (collapse) of any stellar system is a fascinating phenomenon therefore the present works will produce new aspects for the studies of collapsing stellar systems and may generate broad interest among astrophysicists.

The Nature of Metal Artifacts in X-ray Computed Tomography and Their Reduction by Optimization of Tomography Systems Parameters

A significant gap in the known methods for assessing the levels of metal artifacts in X-ray computed tomography and approaches to their reduction is an almost complete disregard for the physical nature of this artifact—the proximity to zero of radioscopic transparency. The proposed work fills this gap. A mathematical model has been developed for evaluating metal artifacts in X-ray computed tomography as applied to the geometry of a parallel beam. The simulation model was transformed into an algorithm, and a Mathcad program was designed to simulate images of the internal structure of the test objects. The algorithm for estimating the studied artifact includes the stages of generating sinograms and estimating the distributions of the linear coefficient over the sections of the object based on the back projection method with filtering. The efficiency of the metal artifacts simulation algorithm is demonstrated in the example of symmetric and asymmetric objects with low- and high-density inclusions and inclusions from materials with high atomic number values. The possibility of reducing metal artifacts with the help of a rational choice of the maximum energy of X-ray radiation and the ADC bit depth is illustrated. For example, for an aluminum cylinder 200 mm in diameter with a central cylindrical cavity 80 mm in diameter, cylindrical inserts 12 mm in diameter with material densities from 1.5 g/cm3 to 10 g/cm3, and effective atomic numbers of materials from 13 to 47, the numerical simulation method proved the following: it is practically unattainable to significantly reduce the level of metal artifacts by increasing the ADC capacity to the maximum X-ray energy of 160 keV; the desired effect is achieved by simultaneously increasing the maximum X-ray energy to 225 keV and the ADC capacity to 24 or 32; increasing the maximum X-ray energy from 160 keV to 225 keV leads to an increase in the energy absorbed in the material of the test object by 26%. The results of this research can be used at the design stage of X-ray computed tomography systems designed to control objects with fragments of low radiation transparency.

Solution of the inverse problem definition surfaces at electron beam additive manufacturing

In this work, the inverse problem of surface restoration by the X-ray bremsstrahlung signal is posed in relation to the process of bead surfacing in additive manufacturing, and its solution is given using the iterative restoration method. Numerical implementation is carried out on the example of recovery of three types of symmetrical objects: Gaussian surface, hemisphere, cylinder. The comparison showed a good agreement between the reconstructed surfaces and the given ones.

Detect Orientation of Symmetric Objects from Monocular Camera to Enhance Landmark Estimations in Object SLAM

Object simultaneous localization and mapping (SLAM) introduces object-level landmarks to the map and helps robots to further perceive their surroundings. As one of the most preferred landmark representations, ellipsoid has a dense mathematical expression and can represent the occupied space of objects with high accuracy. However, the orientations of ellipsoid approximations often fail to coincide with the orientation of objects. To further improve the performance of object SLAM systems with ellipsoid landmarks, we innovatively propose a strategy that first extracts the orientations of those symmetric human-made objects in a single frame and then implements the results of the orientation as a back-end constraint factor of the ellipsoid landmarks. Experimental results obtained show that, compared with the baseline, the proposed orientation detection method can reduce the orientation error by more than 46.5% in most tested datasets and improves the accuracy of mapping. The average translation, rotation and shape error improved by 63.4%, 61.7% and 42.4%, respectively, compared with quadric-SLAM. With only 9 ms additional time cost of each frame, the object SLAM system integrated with our proposed method can still run in real time.

Physical process first law and the entropy change of Rindler horizons

The physical process version of the first law can be obtained for bifurcate Killing horizons with certain assumptions. Especially, one has to restrict to the situations where the horizon evolution is quasi-stationary, under perturbations. We revisit the analysis of this assumption considering the horizon perturbations of Rindler horizon by a spherically symmetric object. We demonstrate that even if the quasi-stationary assumption holds, the change in entropy, in four space–time dimensions, diverges when considered between asymptotic cross-sections. However, these divergences do not appear in higher dimensions. We also analyze these features in the presence of a positive cosmological constant. In the process, we prescribe a recipe to establish the physical process first law in such ill-behaved scenarios.

Asymmetry as an indicator of stress: From population statistics to clinical life-saving applications

Most symmetrical objects can be efficiently described in terms of their deviation from a specific symmetry group, whether it be a mirror, radial, or translatory symmetry, among other groups. Fundamentally, asymmetry is an individual trait, but the asymmetry distribution of a given population may provide valuable information about the well-being of that population. Quantification of these deviations from perfect symmetry evolved from counts and linear measures of distances to landmarks conducive to structures with consistent topology, and then to Continuous Symmetry Measures (CSM) conducive to structures with no consistent topology. We demonstrate the usefulness of this approach on quantification of leaf veins that mirror bifurcating structures. Deviations from a given symmetry group can be described in terms of (i) Fluctuating Asymmetries (FA) or (ii) broken asymmetries. Fluctuating Asymmetry (FA) is a controversial indicator of stress, and therefore tackling the problem needs a large number of species and populations in habitats with well-known stressors. We found such a site at “Evolution Canyon”, Israel, and we examine and discuss a study of twenty-four species that live in the canyon’s opposing slopes. We conclude with examples from asymmetry as a neurophysiological bioindicator by presenting several studies on Amyotrophic Lateral Sclerosis (ALS), Parkinson’s disease, and stroke. We show how machine-learning methods, applied on asymmetry indicators (in addition to the traditional signal processing features), can improve the sensitivity of the system and provide reliable diagnostic results.

Geometric-aware dense matching network for 6D pose estimation of objects from RGB-D images

6D pose estimation for certain targets from RGB-D images is a fundamental problem in computer vision. Current methods emphasize learning the overall expression of the targets, which leads to poor performance under occlusion and truncation conditions. In this paper, we propose using a geometric-aware dense matching network to obtain visible dense correspondences between a RGB-D image and 3D model to address difficult predictions from unseen keypoints. Two geometrical structures are considered for dense matching. (1) The neighbor area of the correspondences is treated as suboptimal matches in addition to the correspondence to reduce the influence of the error caused by ground truth calibration. (2) The distance consistency of the correspondences is leveraged to eliminate the ambiguity from the symmetrical objects. Experiments on LM-O dataset (77.1% ADD(S)-0.1d) and YCB-V dataset (97.6% ADD(S)) show the effectiveness and advantages of our proposed method.11The source code will soon be available at https://github.com/Ray0089/geometric-aware-dense-matching.

From Outside In: Stretching Students’ Conceptual Understanding of Molecular Symmetry with 2D and 3D Manipulatives

The main difficulty in studying molecular symmetry is the mental manipulation of molecules when searching for symmetry. Numerous methods of teaching molecular symmetry require students to identify symmetry elements of objects or molecules. As an alternative, we propose activity that identifies symmetry using elements outside the objects, together with accompanying worksheets containing guiding questions to help them distinguish between transforming symmetrical and asymmetrical objects across/around these elements. This novel activity can be used as a supplement to molecular symmetry instruction at the tertiary level.

Gaussian Mixture Midway-Merge for Object SLAM With Pose Ambiguity

In this letter, we propose a novel method to merge a Gaussian mixture on matrix Lie groups and present its application for a simultaneous localization and mapping problem with symmetric objects. The key idea is to predetermine the weighted mean called a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">midway point</i> and merge Gaussian mixture components at the associated tangent space. Through this rule, the covariance matrix captures the original density more accurately, and the need for the back-projection is spared when compared to the conventional merge. We highlight the midway-merge by numerically evaluating dissimilarity metrics of density functions before and after the merge on the rotational group. Furthermore, we experimentally discover that the rotational error of symmetric objects follows heavy-tailed behavior and formulate the Gaussian sum filter to model it by a Gaussian mixture noise. The effectiveness of our approach is validated through virtual and real-world datasets.

Experimental Verification of the Relationship Formation Model in the Process of Visual Perception

The transcendental psychology approach to the study of perception (A.I. Mirakyan) assumes that in the perception there are so-called structurally-generative processes. Their study is based on an axiomatic methodology, which includes general natural principles for the formation of relationships that provide conditions and opportunities for creating forms in visual perception. The conducted experiment was devoted to the experimental verification of a theoretical model presenting a hierarchy of center-symmetrical and temporary relationships. Experiments included perception of short-term displayed contour symmetric objects that vary in size at a fairly high speed. The method was based on the detection of the place of the Fröhlich effect (onset or offset) for stimuli increasing and decreasing in size. The studied model indicated the possibility of the appearance of subjects of the reversed Fröhlich effect (it is offset as representational momentum but reversed in time) for stimuli decreasing in size at high speeds. The experiments showed that for a speed of 15 deg/s, the percentage of such subjects was 8%, and for a speed of 30 deg/s—22%, while for a speed of 60 deg/s this percentage exceeds 65%. For the last speed, there were a significant number of tests with the reversed Fröhlich effect, which was predicted by the model. Thus, we see possibility to verify the proposed theoretical model, although the conclusions made require further theoretical and experimental verification. Transcendental psychology approach provides alternative explanations for phenomena, which are determined by the principles of Gestalt psychology.

Anisotropic two-fluid stellar objects in general relativity

We apply the $1+1+2$ covariant semitetrad approach to describe a general static and spherically symmetric relativistic stellar object which contains two fluids with anisotropic pressure. The corresponding Tolman-Oppenheimer-Volkoff equations are then obtained in covariant form for the anisotropic case. These equations are used to obtain new exact solutions using direct resolution and reconstruction techniques. Finally, we show that three of the generating theorems known for the single-fluid case can also be used to obtain two-fluid solutions from single-fluid ones.

A VLA Light Curve Spanning 43.8 yr for the Compact Symmetric Object OQ 208

Compact symmetric objects are young, jetted active galactic nuclei defined in part by their slow radio continuum variability. Radio light curves spanning decades can help characterize their evolutionary trends. Here, we present a preliminary VLA light curve at 1.465 GHz for the compact symmetric object OQ 208, a frequently observed calibrator. The light curve spans 43.8 yr, or about 17% of OQ 208's estimated age. The light curve shows two distinct steady levels separated by a decade-long rise at a rate of about 1% yr−1, corresponding to a cumulative increase of about 12% in flux density.

DeepJoin

We introduce DeepJoin, an automated approach to generate high-resolution repairs for fractured shapes using deep neural networks. Existing approaches to perform automated shape repair operate exclusively on symmetric objects, require a complete proxy shape, or predict restoration shapes using low-resolution voxels which are too coarse for physical repair. We generate a high-resolution restoration shape by inferring a corresponding complete shape and a break surface from an input fractured shape. We present a novel implicit shape representation for fractured shape repair that combines the occupancy function, signed distance function, and normal field. We demonstrate repairs using our approach for synthetically fractured objects from ShapeNet, 3D scans from the Google Scanned Objects dataset, objects in the style of ancient Greek pottery from the QP Cultural Heritage dataset, and real fractured objects. We outperform six baseline approaches in terms of chamfer distance and normal consistency. Unlike existing approaches and restorations generated using subtraction, DeepJoin restorations do not exhibit surface artifacts and join closely to the fractured region of the fractured shape. Our code is available at: https://github.com/Terascale-All-sensing-Research-Studio/DeepJoin.

The different effects of breaking an object at different time points.

Whether the Object-based Correspondence Effect caused by task-irrelevant handles of graspable objects is attributed to the spatial-based or motor-based accounts, or both, has been controversial thus far. In this study, we investigated this question from the time dynamic perspective with the modified stimulus-response (S-R) compatibility paradigm. Specifically, we used three different types of objects in three behavioral experiments: objects with a handle, symmetrical objects, and objects with unilateral protrusion. The objects were broken or remained intact at three different time points (i.e., 50 ms, 150 ms, and 250 ms). The results showed that the object with an intact handle had the correspondence effect at 150 ms, while the correspondence effect disappeared when the handle was broken (Experiment 1). However, no similar pattern was found for symmetric objects (Experiment 2) and objects with unilateral protrusion (Experiment 3). Meanwhile, similar compatibility effects were found in all three experiments when the breakage occurred at 50 ms, which suggests that spatial-based, attention-related factors play a key role in early visual information representation. Our findings suggest that both early spatial-based and later motor-based mechanisms are necessary for the object-based correspondence effect, corroborating the development of visual information representation over time.

X-Ray and GeV-γ-Ray Emission Property of TeV Compact Symmetric Object PKS 1413+135 and Implication for Episodic Jet Activity

PKS 1413+135, a compact symmetric object (CSO) with a two-sided parsec-scale structure in its miniature radio morphology, is spatially associated with 4FGL J1416.1+1320 and recently detected with MAGIC telescopes. We comprehensively analyze its X-ray and gigaelectronvolt γ-ray observation data to reveal its high-energy radiation physics. It is found that the source is in a low-flux stage before MJD 58500 and experiences violent outbursts after MJD 58500 in the gigaelectronvolt band. The flux at 10 GeV varies by ∼3 orders of magnitude, and the gigaelectronvolt-flux variation is accompanied by clear spectral variation, which is characterized as a soft log-parabola spectrum in the low-flux state and a hard power-law spectrum in the bright flares. The amplitude of the variability of X-rays is lower than that of γ-rays, and no correlation of variability between γ-rays and X-rays is observed. Fitting the broadband spectral energy distribution during a gigaelectronvolt outburst with a multi-zone leptonic model, we show that the gigaelectronvolt γ-rays are attributed to the external Compton process while the X-rays are a hybrid of several components. The predicted teraelectronvolt γ-ray flux during the gigaelectronvolt outburst is consistent with the detection of MAGIC telescopes. These results, together with its CSO radio morphology, imply that PKS 1413+135 has episodic nuclear jet activities. The weak γ-ray emission before MJD 58500 may be from its sub-parsec-/parsec-scale jet component powered by previous activities, and the violent outbursts with short timescale variability after MJD 58500 could be attributed to the recently restarted jet activity.

Correspondence between the number of no-slip critical points and nature of rear stagnation point of a symmetric object

For flow around an isolated object, the points of zero vorticity/shear stress located at fluid–solid interface, i.e., the separation, reattachment points inclusive of forward and rear stagnation points are refered to as no-slip critical points. The total number, n (≥2), of such points is an even number. For flow past a diamond-section object, it is shown here that a change of the value of n by 2 alters the nature of its rear stagnation point. The rear stagnation point acts as a separation point for n = 2, 6, 10, etc. and as an attachment point for n = 4, 8, 12, etc. A pair of hypothetical mean wakes is proposed and their viability discussed with reference to results available in literature. Concerning flow past two in-line diamond cylinders, the formation of an “anti-wake” at the leading edge of the downstream cylinder renders its forward stagnation point to act as a separation point, which, otherwise for an isolated object, invariably serves as an attachment point. The forebody and afterbody of a symmetric object act as independent entities in influencing the nature of no-slip stagnation points.

ER-Pose: Learning edge representation for 6D pose estimation of texture-less objects

6D pose estimation from a single RGB image is a fundamental task in computer vision. We introduce a two-stage 6D pose estimation method for texture-less objects. Instead of utilizing the object mask in almost current monocular methods, we propose an edge representation for texture-less objects. An object is represented by the combination of visible edges corresponding to the object’s 6D pose, allowing the neural network to focus more on the object’s invariant global shape and structure, rather than indistinguishable local patches with image noise and similar texture. Given an RGB image, the proposed method predicts the direction and distance to a certain object keypoint from all object pixels within the range of object edge representation, establishes voting-based sparse 2D-3D correspondences, and solves the object pose with PnP algorithm. In the experiments, we found that directly replacing the object mask with the edge representation can bring a performance improvement in two current two-stage pipelines without any modification. Further evaluations on three different benchmark datasets containing symmetric and occluded objects show our method outperforms the state-of-the-art methods using RGB images only.

The structure at the centre of the giant radio galaxy GRS J0844+4627: a compact symmetric object?

ABSTRACT We observed the core region of the giant radio galaxy GRS J0844+4627 with e-MERLIN at 1.52 and 5.07 GHz. These observations revealed that the apparent single feature at the centre of GRS J0844+4627, as seen by GMRT, consists of two components separated by 2.7 kpc in projection. Follow-up observations at 1.66 GHz using the EVN unveiled the complex morphologies of the two components. In particular, the south-western component identified with the SDSS J084408.85+462744.2 galaxy morphologically resembles a compact symmetric object (CSO) with a projected linear size of 115 pc. If the CSO hypothesis turns out to be correct, then the overall radio structure of GRS J0844+4627 is triple–double. Given that CSOs are considered young objects, GRS J0844+4627 would appear as a recently restarted active galaxy.