1,881 research outputs found
Using step width to compare locomotor biomechanics between extinct, non-avian theropod dinosaurs and modern obligate bipeds
How extinct, non-avian theropod dinosaurs locomoted is a subject of considerable interest, as is the manner in which it evolved on the line leading to birds. Fossil footprints provide the most direct evidence for answering these questions. In this study, step width—the mediolateral (transverse) distance between successive footfalls—was investigated with respect to speed (stride length) in non-avian theropod trackways of Late Triassic age. Comparable kinematic data were also collected for humans and 11 species of ground-dwelling birds. Permutation tests of the slope on a plot of step width against stride length showed that step width decreased continuously with increasing speed in the extinct theropods (p < 0.001), as well as the five tallest bird species studied (p < 0.01). Humans, by contrast, showed an abrupt decrease in step width at the walk–run transition. In the modern bipeds, these patterns reflect the use of either a discontinuous locomotor repertoire, characterized by distinct gaits (humans), or a continuous locomotor repertoire, where walking smoothly transitions into running (birds). The non-avian theropods are consequently inferred to have had a continuous locomotor repertoire, possibly including grounded running. Thus, features that characterize avian terrestrial locomotion had begun to evolve early in theropod history
Support Neighbor Loss for Person Re-Identification
Person re-identification (re-ID) has recently been tremendously boosted due
to the advancement of deep convolutional neural networks (CNN). The majority of
deep re-ID methods focus on designing new CNN architectures, while less
attention is paid on investigating the loss functions. Verification loss and
identification loss are two types of losses widely used to train various deep
re-ID models, both of which however have limitations. Verification loss guides
the networks to generate feature embeddings of which the intra-class variance
is decreased while the inter-class ones is enlarged. However, training networks
with verification loss tends to be of slow convergence and unstable performance
when the number of training samples is large. On the other hand, identification
loss has good separating and scalable property. But its neglect to explicitly
reduce the intra-class variance limits its performance on re-ID, because the
same person may have significant appearance disparity across different camera
views. To avoid the limitations of the two types of losses, we propose a new
loss, called support neighbor (SN) loss. Rather than being derived from data
sample pairs or triplets, SN loss is calculated based on the positive and
negative support neighbor sets of each anchor sample, which contain more
valuable contextual information and neighborhood structure that are beneficial
for more stable performance. To ensure scalability and separability, a
softmax-like function is formulated to push apart the positive and negative
support sets. To reduce intra-class variance, the distance between the anchor's
nearest positive neighbor and furthest positive sample is penalized.
Integrating SN loss on top of Resnet50, superior re-ID results to the
state-of-the-art ones are obtained on several widely used datasets.Comment: Accepted by ACM Multimedia (ACM MM) 201
Intensity limits of the PSI Injector II cyclotron
We investigate limits on the current of the PSI Injector II high intensity
separate-sector isochronous cyclotron, in its present configuration and after a
proposed upgrade. Accelerator Driven Subcritical Reactors, neutron and neutrino
experiments, and medical isotope production all benefit from increases in
current, even at the ~ 10% level: the PSI cyclotrons provide relevant
experience. As space charge dominates at low beam energy, the injector is
critical. Understanding space charge effects and halo formation through
detailed numerical modelling gives clues on how to maximise the extracted
current. Simulation of a space-charge dominated low energy high intensity (9.5
mA DC) machine, with a complex collimator set up in the central region shaping
the bunch, is not trivial. We use the OPAL code, a tool for charged-particle
optics calculations in large accelerator structures and beam lines, including
3D space charge. We have a precise model of the present production) Injector
II, operating at 2.2 mA current. A simple model of the proposed future
(upgraded) configuration of the cyclotron is also investigated.
We estimate intensity limits based on the developed models, supported by
fitted scaling laws and measurements. We have been able to perform more
detailed analysis of the bunch parameters and halo development than any
previous study. Optimisation techniques enable better matching of the
simulation set-up with Injector II parameters and measurements. We show that in
the production configuration the beam current scales to the power of three with
the beam size. However, at higher intensities, 4th power scaling is a better
fit, setting the limit of approximately 3 mA. Currents of over 5 mA, higher
than have been achieved to date, can be produced if the collimation scheme is
adjusted
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