174 research outputs found
Optimization of starshades: focal plane versus pupil plane
We search for the best possible transmission for an external occulter
coronagraph that is dedicated to the direct observation of terrestrial
exoplanets. We show that better observation conditions are obtained when the
flux in the focal plane is minimized in the zone in which the exoplanet is
observed, instead of the total flux received by the telescope. We describe the
transmission of the occulter as a sum of basis functions. For each element of
the basis, we numerically computed the Fresnel diffraction at the aperture of
the telescope and the complex amplitude at its focus. The basis functions are
circular disks that are linearly apodized over a few centimeters (truncated
cones). We complemented the numerical calculation of the Fresnel diffraction
for these functions by a comparison with pure circular discs (cylinder) for
which an analytical expression, based on a decomposition in Lommel series, is
available. The technique of deriving the optimal transmission for a given
spectral bandwidth is a classical regularized quadratic minimization of
intensities, but linear optimizations can be used as well. Minimizing the
integrated intensity on the aperture of the telescope or for selected regions
of the focal plane leads to slightly different transmissions for the occulter.
For the focal plane optimization, the resulting residual intensity is
concentrated behind the geometrical image of the occulter, in a blind region
for the observation of an exoplanet, and the level of background residual
starlight becomes very low outside this image. Finally, we provide a tolerance
analysis for the alignment of the occulter to the telescope which also favors
the focal plane optimization.This means that telescope offsets of a few
decimeters do not strongly reduce the efficiency of the occulter
Large margin filtering for signal sequence labeling
Signal Sequence Labeling consists in predicting a sequence of labels given an
observed sequence of samples. A naive way is to filter the signal in order to
reduce the noise and to apply a classification algorithm on the filtered
samples. We propose in this paper to jointly learn the filter with the
classifier leading to a large margin filtering for classification. This method
allows to learn the optimal cutoff frequency and phase of the filter that may
be different from zero. Two methods are proposed and tested on a toy dataset
and on a real life BCI dataset from BCI Competition III.Comment: IEEE International Conference on Acoustics Speech and Signal
Processing (ICASSP), 2010, Dallas : United States (2010
Generalized conditional gradient: analysis of convergence and applications
The objectives of this technical report is to provide additional results on
the generalized conditional gradient methods introduced by Bredies et al.
[BLM05]. Indeed , when the objective function is smooth, we provide a novel
certificate of optimality and we show that the algorithm has a linear
convergence rate. Applications of this algorithm are also discussed
Importance sampling strategy for non-convex randomized block-coordinate descent
As the number of samples and dimensionality of optimization problems related
to statistics an machine learning explode, block coordinate descent algorithms
have gained popularity since they reduce the original problem to several
smaller ones. Coordinates to be optimized are usually selected randomly
according to a given probability distribution. We introduce an importance
sampling strategy that helps randomized coordinate descent algorithms to focus
on blocks that are still far from convergence. The framework applies to
problems composed of the sum of two possibly non-convex terms, one being
separable and non-smooth. We have compared our algorithm to a full gradient
proximal approach as well as to a randomized block coordinate algorithm that
considers uniform sampling and cyclic block coordinate descent. Experimental
evidences show the clear benefit of using an importance sampling strategy
Joint Distribution Optimal Transportation for Domain Adaptation
This paper deals with the unsupervised domain adaptation problem, where one
wants to estimate a prediction function in a given target domain without
any labeled sample by exploiting the knowledge available from a source domain
where labels are known. Our work makes the following assumption: there exists a
non-linear transformation between the joint feature/label space distributions
of the two domain and . We propose a solution of
this problem with optimal transport, that allows to recover an estimated target
by optimizing simultaneously the optimal coupling
and . We show that our method corresponds to the minimization of a bound on
the target error, and provide an efficient algorithmic solution, for which
convergence is proved. The versatility of our approach, both in terms of class
of hypothesis or loss functions is demonstrated with real world classification
and regression problems, for which we reach or surpass state-of-the-art
results.Comment: Accepted for publication at NIPS 201
Decoding Finger Movements from ECoG Signals Using Switching Linear Models
One of the most interesting challenges in ECoG-based Brain-Machine Interface is movement prediction. Being able to perform such a prediction paves the way to high-degree precision command for a machine such as a robotic arm or robotic hands. As a witness of the BCI community increasing interest toward such a problem, the fourth BCI Competition provides a dataset which aim is to predict individual finger movements from ECoG signals. The difficulty of the problem relies on the fact that there is no simple relation between ECoG signals and finger movements. We propose in this paper, to estimate and decode these finger flexions using switching models controlled by an hidden state. Switching models can integrate prior knowledge about the decoding problem and helps in predicting fine and precise movements. Our model is thus based on a first block which estimates which finger is moving and another block which, knowing which finger is moving, predicts the movements of all other fingers. Numerical results that have been submitted to the Competition show that the model yields high decoding performances when the hidden state is well estimated. This approach achieved the second place in the BCI competition with a correlation measure between real and predicted movements of 0.42
Distributed image reconstruction for very large arrays in radio astronomy
Current and future radio interferometric arrays such as LOFAR and SKA are
characterized by a paradox. Their large number of receptors (up to millions)
allow theoretically unprecedented high imaging resolution. In the same time,
the ultra massive amounts of samples makes the data transfer and computational
loads (correlation and calibration) order of magnitudes too high to allow any
currently existing image reconstruction algorithm to achieve, or even approach,
the theoretical resolution. We investigate here decentralized and distributed
image reconstruction strategies which select, transfer and process only a
fraction of the total data. The loss in MSE incurred by the proposed approach
is evaluated theoretically and numerically on simple test cases.Comment: Sensor Array and Multichannel Signal Processing Workshop (SAM), 2014
IEEE 8th, Jun 2014, Coruna, Spain. 201
Optimal Transport for Domain Adaptation
Domain adaptation from one data space (or domain) to another is one of the
most challenging tasks of modern data analytics. If the adaptation is done
correctly, models built on a specific data space become more robust when
confronted to data depicting the same semantic concepts (the classes), but
observed by another observation system with its own specificities. Among the
many strategies proposed to adapt a domain to another, finding a common
representation has shown excellent properties: by finding a common
representation for both domains, a single classifier can be effective in both
and use labelled samples from the source domain to predict the unlabelled
samples of the target domain. In this paper, we propose a regularized
unsupervised optimal transportation model to perform the alignment of the
representations in the source and target domains. We learn a transportation
plan matching both PDFs, which constrains labelled samples in the source domain
to remain close during transport. This way, we exploit at the same time the few
labeled information in the source and the unlabelled distributions observed in
both domains. Experiments in toy and challenging real visual adaptation
examples show the interest of the method, that consistently outperforms state
of the art approaches
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