160,631 research outputs found
Learning viewpoint invariant perceptual representations from cluttered images
In order to perform object recognition, it is necessary to form perceptual representations that are sufficiently specific to distinguish between objects, but that are also sufficiently flexible to generalize across changes in location, rotation, and scale. A standard method for learning perceptual representations that are invariant to viewpoint is to form temporal associations across image sequences showing object transformations. However, this method requires that individual stimuli be presented in isolation and is therefore unlikely to succeed in real-world applications where multiple objects can co-occur in the visual input. This paper proposes a simple modification to the learning method that can overcome this limitation and results in more robust learning of invariant representations
Orientation-dependent handedness and chiral design
Chirality occupies a central role in fields ranging from biological
self-assembly to the design of optical metamaterials. The definition of
chirality, as given by Lord Kelvin, associates chirality with the lack of
mirror symmetry: the inability to superpose an object on its mirror image.
While this definition has guided the classification of chiral objects for over
a century, the quantification of handed phenomena based on this definition has
proven elusive, if not impossible, as manifest in the paradox of chiral
connectedness. In this work, we put forward a quantification scheme in which
the handedness of an object depends on the direction in which it is viewed.
While consistent with familiar chiral notions, such as the right-hand rule,
this framework allows objects to be simultaneously right and left handed. We
demonstrate this orientation dependence in three different systems - a
biomimetic elastic bilayer, a chiral propeller, and optical metamaterial - and
find quantitative agreement with chirality pseudotensors whose form we
explicitly compute. The use of this approach resolves the existing paradoxes
and naturally enables the design of handed metamaterials from symmetry
principles
Large-scale magnetic fields in Bok globules
Context: The role of magnetic fields in the star formation process is a
contentious matter of debate. In particular, no clear observational proof
exists of a general influence by magnetic fields during the initial collapse of
molecular clouds. Aims: Our aim is to examine magnetic fields and their
influence on a wide range of spatial scales in low-mass star-forming regions.
Method: We trace the large-scale magnetic field structure on scales of
10^3-10^5 AU in the local environment of Bok globules through optical and
near-infrared polarimetry and combine these measurements with existing
submillimeter measurements, thereby characterizing the small-scale magnetic
field structure on scales of 10^2-10^3 AU. Results: For the first time, we
present polarimetric observations in the optical and near-infrared of the three
Bok globules B335, CB68, and CB54, combined with archival observations in the
submillimeter and the optical. We find a significant polarization signal
(P>=2%, P/sigma(P)>3) in the optical and near-infrared for all three globules.
Additionally, we detect a connection between the structure on scales of
10^2-10^3 AU to 10^3-10^4 AU for both B335 and CB68. Furthermore, for CB54, we
trace ordered polarization vectors on scales of ~10^5 AU. We determine a
magnetic field orientation that is aligned with the CO outflow in the case of
CB54, but nearly perpendicular to the CO outflow for CB68. For B335 we find a
change in the magnetic field oriented toward the outflow direction, from the
inner core to the outer regions. Conclusion: We find strongly aligned
polarization vectors that indicate dominant magnetic fields on a wide range of
spatial scales.Comment: 9 pages, 7 figures, accepted by A&
Cortical Dynamics of Boundary Segmentation and Reset: Persistence, Afterimages, and Residual Traces
Using a neural network model of boundary segmentation and reset, Francis, Grossberg, and Mingolla (1994) linked the percept of persistence to the duration of a boundary segmentation after stimulus offset. In particular, the model simulated the decrease of persistence duration with an increase in stimulus duration and luminance. Thc present article reveals further evidence for the neural mechanisms used by the theory. Simulations show that the model reset signals generate orientational afterimages, such as the MacKay effect, when the reset signals can be grouped by a subsequent boundary segmentation that generates illusory contours through them. Simulations also show that the same mechanisms explain properties of residual traces, which increase in duration with stimulus duration and luminance. The model hereby discloses previously unsuspected mechanistic links between data about persistence and afterimages, and helps to clarify the sometimes controversial issues surrounding distinctions between persistence, residual traces, and afterimages.Air Force Office of Scientific Research (F49620-92-J-0499); Office of Naval Research (N00014-91-J-4100, N00014-92-J-4015
Shear-invariant Sliding Contact Perception with a Soft Tactile Sensor
Manipulation tasks often require robots to be continuously in contact with an
object. Therefore tactile perception systems need to handle continuous contact
data. Shear deformation causes the tactile sensor to output path-dependent
readings in contrast to discrete contact readings. As such, in some
continuous-contact tasks, sliding can be regarded as a disturbance over the
sensor signal. Here we present a shear-invariant perception method based on
principal component analysis (PCA) which outputs the required information about
the environment despite sliding motion. A compliant tactile sensor (the TacTip)
is used to investigate continuous tactile contact. First, we evaluate the
method offline using test data collected whilst the sensor slides over an edge.
Then, the method is used within a contour-following task applied to 6 objects
with varying curvatures; all contours are successfully traced. The method
demonstrates generalisation capabilities and could underlie a more
sophisticated controller for challenging manipulation or exploration tasks in
unstructured environments. A video showing the work described in the paper can
be found at https://youtu.be/wrTM61-pieUComment: Accepted in ICRA 201
Magnetic fields in Bok globules: Multi-wavelength polarimetry as tracer across large spatial scales
[abridged] The role of magnetic fields in the process of star formation is a
matter of continuous debate. Clear observational proof of the general influence
of magnetic fields on the early phase of cloud collapse is still pending. First
results on Bok globules with simple structures indicate dominant magnetic
fields across large spatial scales (Bertrang+2014).
The aim of this study is to test the magnetic field influence across Bok
globules with more complex density structures. We apply near-infrared
polarimetry to trace the magnetic field structure on scales of 10^4-10^5au in
selected Bok globules. The combination of these measurements with archival data
in the optical and sub-mm wavelength range allows us to characterize the
magnetic field on scales of 10^3-10^6au.
We present polarimetric data in the near-infrared wavelength range for the
three Bok globules CB34, CB56, and [OMK2002]18, combined with archival
polarimetric data in the optical wavelength range for CB34 and CB56, and in the
sub-millimeter wavelength range for CB34 and [OMK2002]18. We find a strong
polarization signal (P>2%) in the near-infrared and strongly aligned
polarization segments on large scales (10^4-10^6au) for all three globules.
This indicates dominant magnetic fields across Bok globules with complex
density structures.
To reconcile our findings in globules, the lowest mass clouds known, and the
results on intermediate (e.g., Taurus) and more massive (e.g., Orion) clouds,
we postulate a mass dependent role of magnetic fields, whereby magnetic fields
appear to be dominant on low and high mass but rather sub-dominant on
intermediate mass clouds.Comment: 7 pages, 6 figures; Accepted by A&
A Methodology to Support the Maintenance of Object -Oriented Systems Using Impact Analysis.
Object-Oriented (OO) systems are difficult to understand due to the complex nature of the relationships that object-orientation supports. Inheritance, polymorphism, encapsulation, information hiding, aggregation, and association combine to make maintenance of OO systems difficult. Due to the presence of these characteristics in OO systems, maintenance activities on OO systems often have unexpected or unseen effects on the system. These effects can ripple through system components, complicating maintenance and testing of the system. The ability to trace the effects of maintenance provides the maintainer with knowledge that assists in debugging and testing modified and affected components. In this research, we show that the architecture of an OO system provides an effective framework for determining the impact of system changes. We developed the Comparative Software Maintenance (CSM) methodology to support the maintenance of OO systems. Through this methodology, we model relationships and structures, analyze the models to determine components that change as a result of maintenance, and perform impact analysis to determine components that are candidates for re-testing as a result of maintenance activity. The methodology includes a new data model, called Extended Low-Level Software Architecture (ELLSA), that facilitates impact analysis. CSM locates potential side effects, ripple effects, and other effects of maintenance on class structures, methods, and objects. The comprehensive architecture model enables CSM to perform either predictive, pre-modification impact analysis or post-modification impact analysis. The improved impact analysis process found in the methodology determines impact of changes to the component level. We apply the results of impact analysis to determine component level testing requirements. CSM enhances program understanding through the use of ELLSA. It also provides assistance for capturing complex dependencies found in object-oriented code. The methodology is implemented in JFlex. The automation provided by JFlex makes the application of CSM feasible
Geometry and material effects in Casimir physics - Scattering theory
We give a comprehensive presentation of methods for calculating the Casimir
force to arbitrary accuracy, for any number of objects, arbitrary shapes,
susceptibility functions, and separations. The technique is applicable to
objects immersed in media other than vacuum, to nonzero temperatures, and to
spatial arrangements in which one object is enclosed in another. Our method
combines each object's classical electromagnetic scattering amplitude with
universal translation matrices, which convert between the bases used to
calculate scattering for each object, but are otherwise independent of the
details of the individual objects. This approach, which combines methods of
statistical physics and scattering theory, is well suited to analyze many
diverse phenomena. We illustrate its power and versatility by a number of
examples, which show how the interplay of geometry and material properties
helps to understand and control Casimir forces. We also examine whether
electrodynamic Casimir forces can lead to stable levitation. Neglecting
permeabilities, we prove that any equilibrium position of objects subject to
such forces is unstable if the permittivities of all objects are higher or
lower than that of the enveloping medium; the former being the generic case for
ordinary materials in vacuum.Comment: 44 pages, 11 figures, to appear in upcoming Lecture Notes in Physics
volume in Casimir physic
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