11,813 research outputs found
Computing motion in the primate's visual system
Computing motion on the basis of the time-varying image intensity is a difficult problem for both artificial and biological vision systems. We will show how one well-known gradient-based computer algorithm for estimating visual motion can be implemented within the primate's visual system. This relaxation algorithm computes the optical flow field by minimizing a variational functional of a form commonly encountered in early vision, and is performed in two steps. In the first stage, local motion is computed, while in the second stage spatial integration occurs. Neurons in the second stage represent the optical flow field via a population-coding scheme, such that the vector sum of all neurons at each location codes for the direction and magnitude of the velocity at that location. The resulting network maps onto the magnocellular pathway of the primate visual system, in particular onto cells in the primary visual cortex (V1) as well as onto cells in the middle temporal area (MT). Our algorithm mimics a number of psychophysical phenomena and illusions (perception of coherent plaids, motion capture, motion coherence) as well as electrophysiological recordings. Thus, a single unifying principle ‘the final optical flow should be as smooth as possible’ (except at isolated motion discontinuities) explains a large number of phenomena and links single-cell behavior with perception and computational theory
Variational optic flow computation with a spatio-temporal smoothness constraint
Nonquadratic variational regularization is a well-known and powerful approach for the discontinuity-preserving computation of optic flow. In the present paper, we consider an extension of flow-driven spatial smoothness terms to spatio-temporal regularizers. Our method leads to a rotationally invariant and time symmetrie convex optimization problem. It has a unique minimum that can be found in a stable way by standard algorithms such as gradient descent. Since the convexity guarantees global convergence, the result does not depend on the flow initialization. An iterative algorithm is presented that is not difficult to implement. Qualitative and quantitative results for synthetic and real-world scenes show that our spatio-temporal approach (i) improves optic flow fields significantly, (ii) smoothes out background noise efficiently, and (iii) preserves true motion boundaries. The computational costs are only 50 % higher than for a pure spatial approach applied to all subsequent image pairs of the sequence
Methods for characterising microphysical processes in plasmas
Advanced spectral and statistical data analysis techniques have greatly
contributed to shaping our understanding of microphysical processes in plasmas.
We review some of the main techniques that allow for characterising fluctuation
phenomena in geospace and in laboratory plasma observations. Special emphasis
is given to the commonalities between different disciplines, which have
witnessed the development of similar tools, often with differing terminologies.
The review is phrased in terms of few important concepts: self-similarity,
deviation from self-similarity (i.e. intermittency and coherent structures),
wave-turbulence, and anomalous transport.Comment: Space Science Reviews (2013), in pres
Automatic segmentation of the left ventricle cavity and myocardium in MRI data
A novel approach for the automatic segmentation has been developed to extract the epi-cardium and endo-cardium boundaries of the left ventricle (lv) of the heart. The developed segmentation scheme takes multi-slice and multi-phase magnetic resonance (MR) images of the heart, transversing the short-axis length from the base to the apex. Each image is taken at one instance in the heart's phase. The images are segmented using a diffusion-based filter followed by an unsupervised clustering technique and the resulting labels are checked to locate the (lv) cavity. From cardiac anatomy, the closest pool of blood to the lv cavity is the right ventricle cavity. The wall between these two blood-pools (interventricular septum) is measured to give an approximate thickness for the myocardium. This value is used when a radial search is performed on a gradient image to find appropriate robust segments of the epi-cardium boundary. The robust edge segments are then joined using a normal spline curve. Experimental results are presented with very encouraging qualitative and quantitative results and a comparison is made against the state-of-the art level-sets method
Particle Acceleration at Relativistic Shocks in Extragalactic Systems
Diffusive shock acceleration (DSA) at relativistic shocks is expected to be
an important acceleration mechanism in a variety of astrophysical objects
including extragalactic jets in active galactic nuclei and gamma ray bursts.
These sources remain strong and interesting candidate sites for the generation
of ultra-high energy cosmic rays. In this paper, key predictions of DSA at
relativistic shocks that are salient to the issue of cosmic ray ion and
electron production are outlined. Results from a Monte Carlo simulation of such
diffusive acceleration in test-particle, relativistic, oblique, MHD shocks are
presented. Simulation output is described for both large angle and small angle
scattering scenarios, and a variety of shock obliquities including superluminal
regimes when the de Hoffman-Teller frame does not exist. The distribution
function power-law indices compare favorably with results from other
techniques. They are found to depend sensitively on the mean magnetic field
orientation in the shock, and the nature of MHD turbulence that propagates
along fields in shock environs. An interesting regime of flat spectrum
generation is addressed, providing evidence for its origin being due to shock
drift acceleration. The impact of these theoretical results on gamma-ray burst
and blazar science is outlined. Specifically, Fermi gamma-ray observations of
these cosmic sources are already providing significant constraints on important
environmental quantities for relativistic shocks, namely the frequency of
scattering and the level of field turbulence.Comment: 11 pages, 6 figures, to appear in Proc. of the 8th International
Astrophysics Conference "Shock Waves in Space and Astrophysical Environments"
(2010), eds. X. Ao, R. Burrows and G. P. Zank (AIP Conf. Proc., New York
Chandra Observation of the Merging Cluster A168: A Late Stage in the Evolution of a Cold Front
We present Chandra observations of the cool cluster A168, for which previous
X-ray imaging and optical studies indicated a merger of two subclusters nearly
in the plane of the sky. We derive a temperature map for A168, which shows that
the merger has proceeded beyond the core passage and is near subcluster
turnaround. It also reveals an unusual feature -- the gas core of one of the
subclusters forms a tongue-like structure extending ahead (in the direction of
motion) of the subcluster center. The coolest cluster gas is found in a
crescent-shaped region at the tip of this tongue, and forms a cold front in
pressure equilibrium with the external gas. In contrast with this feature's
forward location, previously observed merger cold fronts (e.g., A3667,
1E0657--56) lagged behind their host subclusters, as expected in the presense
of ram pressure. We propose that A168 illustrates a much later stage in the
evolution of a cold front, when its host subcluster approaches the apocenter of
the merger orbit where the ram pressure on its gas drops sharply. As a result,
a large chunk of the subcluster gas ``slingshots'' past the dark matter center,
becomes unbound from the subcluster and expands adiabatically, as seen in some
recent hydrodynamic simulations.Comment: 4 pages, 3 figures, to be published in ApJ Letter
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