5,673 research outputs found
Thick Boundaries in Binary Space and Their Influence on Nearest-Neighbor Search
Binary descriptors allow faster similarity computation than real-valued ones while requiring much less storage. As a result, many algorithms have recently been proposed to binarize floating-point descriptors so that they can be searched for quickly. Unfortunately, even if the similarity between vectors can be computed fast, exhaustive linear search remains impractical for truly large databases and Approximate Nearest Neighbor (ANN) search is still required. It is therefore surprising that relatively little attention has been paid to the efficiency of ANN algorithms on binary vectors and this is the focus of this paper. We first show that binary-space Voronoi diagrams have thick boundaries, meaning that there are many points that lie at the same distance from two random points. This violates the implicit assumption made by most ANN algorithms that points can be neatly assigned to clusters centered around a set of cluster centers. As a result, state-of-the-art algorithms that can operate on binary vectors exhibit much lower performance than those that work with floating point ones. The above analysis is the first contribution of the paper. The second one is two effective ways to overcome this limitation, by appropriately randomizing either a tree-based algorithm or hashing-based one. In both cases, we show that we obtain precision/recall curves that are similar to those than can be obtained using floating point number calculation, but at much reduced computational cost
Evaluation of Hashing Methods Performance on Binary Feature Descriptors
In this paper we evaluate performance of data-dependent hashing methods on
binary data. The goal is to find a hashing method that can effectively produce
lower dimensional binary representation of 512-bit FREAK descriptors. A
representative sample of recent unsupervised, semi-supervised and supervised
hashing methods was experimentally evaluated on large datasets of labelled
binary FREAK feature descriptors
Tuning magnetocrystalline anisotropy of FeSn by alloying
The electronic structure, magnetic properties and phase formation of
hexagonal ferromagnetic FeSn-based alloys have been studied from first
principles and by experiment. The pristine FeSn compound is known to
fulfill all the requirements for a good permanent magnet, except for the
magnetocrystalline anisotropy energy (MAE). The latter is large, but planar,
i.e. the easy magnetization axis is not along the hexagonal c direction,
whereas a good permanent magnet requires the MAE to be uniaxial. Here we
consider FeSnM, where M= Si, P, Ga, Ge, As, Se, In, Sb,
Te and Bi, and show how different dopants on the Sn sublattice affect the MAE
and can alter it from planar to uniaxial. The stability of the doped FeSn
phases is elucidated theoretically via the calculations of their formation
enthalpies. A micromagnetic model is developed in order to estimate the energy
density product (BH)max and coercive field H of a potential
magnet made of FeSnSb, the most promising candidate
from theoretical studies. The phase stability and magnetic properties of the
FeSn compound doped with Sb and Mn has been checked experimentally on the
samples synthesised using the reactive crucible melting technique as well as by
solid state reaction. The FeSn-Sb compound is found to be stable when
alloyed with Mn. It is shown that even small structural changes, such as a
change of the c/a ratio or volume, that can be induced by, e.g., alloying with
Mn, can influence anisotropy and reverse it from planar to uniaxial and back
The APOGEE-2 Survey of the Orion Star Forming Complex: I. Target Selection and Validation with early observations
The Orion Star Forming Complex (OSFC) is a central target for the APOGEE-2
Young Cluster Survey. Existing membership catalogs span limited portions of the
OSFC, reflecting the difficulty of selecting targets homogeneously across this
extended, highly structured region. We have used data from wide field
photometric surveys to produce a less biased parent sample of young stellar
objects (YSOs) with infrared (IR) excesses indicative of warm circumstellar
material or photometric variability at optical wavelengths across the full 420
square degrees extent of the OSFC. When restricted to YSO candidates with H <
12.4, to ensure S/N ~100 for a six visit source, this uniformly selected sample
includes 1307 IR excess sources selected using criteria vetted by Koenig &
Liesawitz and 990 optical variables identified in the Pan-STARRS1 3
survey: 319 sources exhibit both optical variability and evidence of
circumstellar disks through IR excess. Objects from this uniformly selected
sample received the highest priority for targeting, but required fewer than
half of the fibers on each APOGEE-2 plate. We fill the remaining fibers with
previously confirmed and new color-magnitude selected candidate OSFC members.
Radial velocity measurements from APOGEE-1 and new APOGEE-2 observations taken
in the survey's first year indicate that ~90% of the uniformly selected targets
have radial velocities consistent with Orion membership.The APOGEE-2 Orion
survey will include >1100 bona fide YSOs whose uniform selection function will
provide a robust sample for comparative analyses of the stellar populations and
properties across all sub-regions of Orion.Comment: Accepted for publication in ApJ
Efficient Yao Graph Construction
Yao graphs are geometric spanners that connect each point of a given point set to its nearest neighbor in each of k cones drawn around it. Yao graphs were introduced to construct minimum spanning trees in d dimensional spaces. Moreover, they are used for instance in topology control in wireless networks. An optimal ?(n log n)-time algorithm to construct Yao graphs for a given point set has been proposed in the literature but - to the best of our knowledge - never been implemented. Instead, algorithms with a quadratic complexity are used in popular packages to construct these graphs. In this paper we present the first implementation of the optimal Yao graph algorithm. We engineer the data structures required to achieve the ?(n log n) time bound and detail algorithmic adaptations necessary to take the original algorithm from theory to practice. We propose a priority queue data structure that separates static and dynamic events and might be of independent interest for other sweepline algorithms. Additionally, we propose a new Yao graph algorithm based on a uniform grid data structure that performs well for medium-sized inputs. We evaluate our implementations on a wide variety of synthetic and real-world datasets and show that our implementation outperforms current publicly available implementations by at least an order of magnitude
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