2,282 research outputs found
The THUMOS Challenge on Action Recognition for Videos "in the Wild"
Automatically recognizing and localizing wide ranges of human actions has
crucial importance for video understanding. Towards this goal, the THUMOS
challenge was introduced in 2013 to serve as a benchmark for action
recognition. Until then, video action recognition, including THUMOS challenge,
had focused primarily on the classification of pre-segmented (i.e., trimmed)
videos, which is an artificial task. In THUMOS 2014, we elevated action
recognition to a more practical level by introducing temporally untrimmed
videos. These also include `background videos' which share similar scenes and
backgrounds as action videos, but are devoid of the specific actions. The three
editions of the challenge organized in 2013--2015 have made THUMOS a common
benchmark for action classification and detection and the annual challenge is
widely attended by teams from around the world.
In this paper we describe the THUMOS benchmark in detail and give an overview
of data collection and annotation procedures. We present the evaluation
protocols used to quantify results in the two THUMOS tasks of action
classification and temporal detection. We also present results of submissions
to the THUMOS 2015 challenge and review the participating approaches.
Additionally, we include a comprehensive empirical study evaluating the
differences in action recognition between trimmed and untrimmed videos, and how
well methods trained on trimmed videos generalize to untrimmed videos. We
conclude by proposing several directions and improvements for future THUMOS
challenges.Comment: Preprint submitted to Computer Vision and Image Understandin
Scaling behavior in steady-state contractile actomyosin network flow
Contractile actomyosin network flows are crucial for many cellular processes
including cell division and motility, morphogenesis and transport. How local
remodeling of actin architecture tunes stress production and dissipation and
regulates large-scale network flow remains poorly understood. Here, we generate
contractile actomyosin networks with rapid turnover in vitro, by encapsulating
cytoplasmic Xenopus egg extracts into cell-sized 'water-in-oil' droplets.
Within minutes, the networks reach a dynamic steady-state with continuous
inward flow. The networks exhibit homogenous, density-independent contraction
for a wide range of physiological conditions, indicating that the
myosin-generated stress driving contraction is proportional to the effective
network viscosity. We further find that the contraction rate approximately
scales with the network turnover rate, but this relation breaks down in the
presence of excessive crosslinking or branching. Our findings suggest that
cells use diverse biochemical mechanisms to generate robust, yet tunable, actin
flows by regulating two parameters: turnover rate and network geometry
Robust Market Equilibria with Uncertain Preferences
The problem of allocating scarce items to individuals is an important
practical question in market design. An increasingly popular set of mechanisms
for this task uses the concept of market equilibrium: individuals report their
preferences, have a budget of real or fake currency, and a set of prices for
items and allocations is computed that sets demand equal to supply. An
important real world issue with such mechanisms is that individual valuations
are often only imperfectly known. In this paper, we show how concepts from
classical market equilibrium can be extended to reflect such uncertainty. We
show that in linear, divisible Fisher markets a robust market equilibrium (RME)
always exists; this also holds in settings where buyers may retain unspent
money. We provide theoretical analysis of the allocative properties of RME in
terms of envy and regret. Though RME are hard to compute for general
uncertainty sets, we consider some natural and tractable uncertainty sets which
lead to well behaved formulations of the problem that can be solved via modern
convex programming methods. Finally, we show that very mild uncertainty about
valuations can cause RME allocations to outperform those which take estimates
as having no underlying uncertainty.Comment: Extended preprint of an article accepted to AAAI-20. Contains
supplementary material as appendices. Due to figures, this manuscript is best
printed in colo
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