127 research outputs found
Hexagonal structure for intelligent vision
Using hexagonal grids to represent digital images have been studied for more than 40 years. Increased processing capabilities of graphic devices and recent improvements in CCD technology have made hexagonal sampling attractive for practical applications and brought new interests on this topic. The hexagonal structure is considered to be preferable to the rectangular structure due to its higher sampling efficiency, consistent connectivity and higher angular resolution and is even proved to be superior to square structure in many applications. Since there is no mature hardware for hexagonal-based image capture and display, square to hexagonal image conversion has to be done before hexagonal-based image processing. Although hexagonal image representation and storage has not yet come to a standard, experiments based on existing hexagonal coordinate systems have never ceased. In this paper, we firstly introduced general reasons that hexagonally sampled images are chosen for research. Then, typical hexagonal coordinates and addressing schemes, as well as hexagonal based image processing and applications, are fully reviewed. © 2005 IEEE
The Many Faces of Heterogeneous Ice Nucleation: Interplay Between Surface Morphology and Hydrophobicity
What makes a material a good ice nucleating agent? Despite the importance of
heterogeneous ice nucleation to a variety of fields, from cloud science to
microbiology, major gaps in our understanding of this ubiquitous process still
prevent us from answering this question. In this work, we have examined the
ability of generic crystalline substrates to promote ice nucleation as a
function of the hydrophobicity and the morphology of the surface. Nucleation
rates have been obtained by brute-force molecular dynamics simulations of
coarse-grained water on top of different surfaces of a model fcc crystal,
varying the water-surface interaction and the surface lattice parameter. It
turns out that the lattice mismatch of the surface with respect to ice,
customarily regarded as the most important requirement for a good ice
nucleating agent, is at most desirable but not a requirement. On the other
hand, the balance between the morphology of the surface and its hydrophobicity
can significantly alter the ice nucleation rate and can also lead to the
formation of up to three different faces of ice on the same substrate. We have
pinpointed three circumstances where heterogeneous ice nucleation can be
promoted by the crystalline surface: (i) the formation of a water overlayer
that acts as an in-plane template; (ii) the emergence of a contact layer
buckled in an ice-like manner; and (iii) nucleation on compact surfaces with
very high interaction strength. We hope that this extensive systematic study
will foster future experimental work aimed at testing the physiochemical
understanding presented herein.Comment: Main + S
Reversible, fast, and high-quality grid conversions
A new grid conversion method is proposed to resample between two 2-D periodic lattices with the same sampling density. The main feature of our approach is the symmetric reversibility, which means that when using the same algorithm for the converse operation, then the initial data is recovered exactly. To that purpose, we decompose the lattice conversion process into (at most) three successive shear operations. The translations along the shear directions are implemented by 1-D fractional delay operators, which revert to simple 1-D convolutions, with appropriate filters that yield the property of symmetric reversibility. We show that the method is fast and provides high-quality resampled images. Applications of our approach can be found in various settings, such as grid conversion between the hexagonal and the Cartesian lattice, or fast implementation of affine transformations such as rotations
Optical transmission matrix measurement sampled on a dense hexagonal lattice
The optical transmission matrix (TM) characterizes the transmission
properties of a sample. We show a novel experimental procedure for measuring
the TM of light waves in a slab geometry based on sampling the light field on a
hexagonal lattice at the Rayleigh criterion. Our method enables the efficient
measurement of a large fraction of the complete TM without oversampling while
minimizing sampling crosstalk and the associated distortion of the statistics
of the matrix elements. The procedure and analysis described here is
demonstrated on a clear sample which serves as an important reference for other
systems and geometries such as dense scattering media.Comment: 12 pages, 6 figure
Lattice-Based High-Dimensional Gaussian Filtering and the Permutohedral Lattice
High-dimensional Gaussian filtering is a popular technique in image processing, geometry processing and computer graphics for smoothing data while preserving important features. For instance, the bilateral filter, cross bilateral filter and non-local means filter fall under the broad umbrella of high-dimensional Gaussian filters. Recent algorithmic advances therein have demonstrated that by relying on a sampled representation of the underlying space, one can obtain speed-ups of orders of magnitude over the naïve approach. The simplest such sampled representation is a lattice, and it has been used successfully in the bilateral grid and the permutohedral lattice algorithms. In this paper, we analyze these lattice-based algorithms, developing a general theory of lattice-based high-dimensional Gaussian filtering. We consider the set of criteria for an optimal lattice for filtering, as it offers a good tradeoff of quality for computational efficiency, and evaluate the existing lattices under the criteria. In particular, we give a rigorous exposition of the properties of the permutohedral lattice and argue that it is the optimal lattice for Gaussian filtering. Lastly, we explore further uses of the permutohedral-lattice-based Gaussian filtering framework, showing that it can be easily adapted to perform mean shift filtering and yield improvement over the traditional approach based on a Cartesian grid.Stanford University (Reed-Hodgson Fellowship)Nokia Research Cente
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