4,187 research outputs found
Learning to Extract Motion from Videos in Convolutional Neural Networks
This paper shows how to extract dense optical flow from videos with a
convolutional neural network (CNN). The proposed model constitutes a potential
building block for deeper architectures to allow using motion without resorting
to an external algorithm, \eg for recognition in videos. We derive our network
architecture from signal processing principles to provide desired invariances
to image contrast, phase and texture. We constrain weights within the network
to enforce strict rotation invariance and substantially reduce the number of
parameters to learn. We demonstrate end-to-end training on only 8 sequences of
the Middlebury dataset, orders of magnitude less than competing CNN-based
motion estimation methods, and obtain comparable performance to classical
methods on the Middlebury benchmark. Importantly, our method outputs a
distributed representation of motion that allows representing multiple,
transparent motions, and dynamic textures. Our contributions on network design
and rotation invariance offer insights nonspecific to motion estimation
Theory of light-activated catalytic Janus particles
We study the dynamics of active Janus particles that self-propel in solution
by light-activated catalytic decomposition of chemical "fuel." We develop an
analytical model of a photo-active self-phoretic particle that accounts for
"self-shadowing" of the light by the opaque catalytic face of the particle. We
find that self-shadowing can drive "phototaxis" (rotation of the catalytic cap
towards the light source) or "anti-phototaxis," depending on the properties of
the particle. Incorporating the effect of thermal noise, we show that the
distribution of particle orientations is captured by a Boltzmann distribution
with a nonequilibrium effective potential. Furthermore, the mean vertical
velocity of phototactic (anti-phototactic) particles exhibits a superlinear
(sublinear) dependence on intensity. Overall, our findings show that
photo-active particles exhibit a rich "tactic" response to light, which could
be harnessed to program complex three-dimensional trajectories
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