48,820 research outputs found
Tracking in a space variant active vision system
Without the ability to foveate on and maintain foveation, active vision for applications such as surveillance, object recognition and object tracking are difficult to build. Although foveation in cartesian coordinates is being actively pursued by many, multi-resolution high accuracy foveation in log polar space has not been given much attention. This paper addresses the use of foveation to track a single object as well as multiple objects for a simulated space variant active vision system. Complex logarithmic mapping is chosen firstly because it provides high resolution and wide angle viewing. Secondly, the spatially variant structure of log polar space leads to an object increasing in size as it moves towards the fovea. This is important as we know which object is closer to the fovea at any instant in time.<br /
Competing ferromagnetic and nematic alignment in self-propelled polar particles
We study a Vicsek-style model of self-propelled particles where ferromagnetic
and nematic alignment compete in both the usual "metric" version and in the
"metric-free" case where a particle interacts with its Voronoi neighbors. We
show that the phase diagram of this out-of-equilibrium XY model is similar to
that of its equilibrium counterpart: the properties of the fully-nematic model,
studied before in [F. Ginelli, F. Peruani, M. Baer, and H. Chat\'e, Phys. Rev.
Lett. 104, 184502 (2010)], are thus robust to the introduction of a modest bias
of interactions towards ferromagnetic alignment. The direct transitions between
polar and nematic ordered phases are shown to be discontinuous in the metric
case, and continuous, belonging to the Ising universality class, in the
metric-free version
Long-Range Ordering of Vibrated Polar Disks
Vibrated polar disks have been used experimentally to investigate collective
motion of driven particles, where fully-ordered asymptotic regimes could not be
reached. Here we present a model reproducing quantitatively the single, binary
and collective properties of this granular system. Using system sizes not
accessible in the laboratory, we show in silico that true long-range order is
possible in the experimental system. Exploring the model's parameter space, we
find a phase diagram qualitatively different from that of dilute or point-like
particle systems.Comment: 5 pages, 4 figure
A Study of H2 Emission in Three Bipolar Proto-Planetary Nebulae: IRAS 16594-4656, Hen 3-401, and Rob 22
We have carried out a spatial-kinematical study of three proto-planetary
nebulae, IRAS 16594-4656, Hen 3-401, and Rob 22. High-resolution H2 images were
obtained with NICMOS on the HST and high-resolution spectra were obtained with
the Phoenix spectrograph on Gemini-South. IRAS 16594-4656 shows a
"peanut-shaped" bipolar structure with H2 emission from the walls and from two
pairs of more distant, point-symmetric faint blobs. The velocity structure
shows the polar axis to be in the plane of the sky, contrary to the impression
given by the more complex visual image and the visibility of the central star,
with an ellipsoidal velocity structure. Hen 3-401 shows the H2 emission coming
from the walls of the very elongated, open-ended lobes seen in visible light,
along with a possible small disk around the star. The bipolar lobes appear to
be tilted 10-15 deg with respect to the plane of the sky and their kinematics
display a Hubble-like flow. In Rob 22, the H2 appears in the form of an "S"
shape, approximately tracing out the similar pattern seen in the visible. H2 is
especially seen at the ends of the lobes and at two opposite regions close to
the unseen central star. The axis of the lobes is nearly in the plane of the
sky. Expansion ages of the lobes are calculated to be approximately 1600 yr
(IRAS 16594-4656), 1100 yr (Hen 3-401), and 640 yr (Rob 22), based upon
approximate distances
Invisible Collisions
This activity relates an elastic collision to the change in a satellite’s or spacecraft’s speed and direction resulting from a planetary fly-by, often called a “gravity assist” maneuver. Both hands-on and online interactive methods are used to explore these topics. Educational levels: High school
Self-propelled rods exhibit a novel phase-separated state characterized by the presence of active stresses and the ejection of polar clusters
We study collections of self-propelled rods (SPR) moving in two dimensions
for packing fractions less than or equal to 0.3. We find that in the
thermodynamical limit the SPR undergo a phase transition between a disordered
gas and a novel phase-separated system state. Interestingly, (global)
orientational order patterns -- contrary to what has been suggested -- vanish
in this limit. In the found novel state, the SPR self-organize into a highly
dynamical, high-density, compact region - which we call aggregate - which is
surrounded by a disordered gas. Active stresses build inside aggregates as
result of the combined effect of local orientational order and active forces.
This leads to the most distinctive feature of these aggregates: constant
ejection of polar clusters of SPR. This novel phase-separated state represents
a novel state of matter characterized by large fluctuations in volume and
shape, related to mass ejection, and exhibits positional as well as
orientational local order. SPR systems display new physics unseen in other
active matter systems due to the coupling between density, active stresses, and
orientational order (such coupling cannot be reduced simply to a coupling
between speed and density).Comment: to appear in PR
Human saccadic eye movements and tracking by active foveation in log polar space
One of the possible models of the human visual system (HVS) in the computer vision literature has a high resolution fovea and exponentially decreasing resolution periphery. The high resolution fovea is used to extract necessary information in order to solve a vision task and the periphery may be used to detect motion. To obtain the desired information, the fovea is guided by the contents of the scene and other knowledge to position the fovea over areas of interest. These eye movements are called saccades and corrective saccades. A two stage process has been implemented as a mechanism for changing foveation in log polar space. Initially, the open loop stage roughly foveates on the best interest feature and then the closed loop stage is invoked to accurately iteratively converge onto the foveation point. The open loop stage developed for the foveation algorithm is applied to saccadic eye movements and a tracking system. Log polar space is preferred over Cartesian space as: (1) it simultaneously provides high resolution and a wide viewing angle; and (2) feature invariance occurs in the fovea which simplifies the foveation process
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