3,702 research outputs found
Subset Warping: Rubber Sheeting with Cuts
Image warping, often referred to as "rubber sheeting" represents the deformation of a domain image space into a range image space. In this paper, a technique is described which extends the definition of a rubber-sheet transformation to allow a polygonal region to be warped into one or more subsets of itself, where the subsets may be multiply connected. To do this, it constructs a set of "slits" in the domain image, which correspond to discontinuities in the range image, using a technique based on generalized Voronoi diagrams. The concept of medial axis is extended to describe inner and outer medial contours of a polygon. Polygonal regions are decomposed into annular subregions, and path homotopies are introduced to describe the annular subregions. These constructions motivate the definition of a ladder, which guides the construction of grid point pairs necessary to effect the warp itself
Active skeleton for bacteria modeling
The investigation of spatio-temporal dynamics of bacterial cells and their
molecular components requires automated image analysis tools to track cell
shape properties and molecular component locations inside the cells. In the
study of bacteria aging, the molecular components of interest are protein
aggregates accumulated near bacteria boundaries. This particular location makes
very ambiguous the correspondence between aggregates and cells, since computing
accurately bacteria boundaries in phase-contrast time-lapse imaging is a
challenging task. This paper proposes an active skeleton formulation for
bacteria modeling which provides several advantages: an easy computation of
shape properties (perimeter, length, thickness, orientation), an improved
boundary accuracy in noisy images, and a natural bacteria-centered coordinate
system that permits the intrinsic location of molecular components inside the
cell. Starting from an initial skeleton estimate, the medial axis of the
bacterium is obtained by minimizing an energy function which incorporates
bacteria shape constraints. Experimental results on biological images and
comparative evaluation of the performances validate the proposed approach for
modeling cigar-shaped bacteria like Escherichia coli. The Image-J plugin of the
proposed method can be found online at http://fluobactracker.inrialpes.fr.Comment: Published in Computer Methods in Biomechanics and Biomedical
Engineering: Imaging and Visualizationto appear i
Image inpainting based on coherence transport with adapted distance functions
We discuss an extension of our method Image Inpainting Based on Coherence Transport. For the latter method the pixels of the inpainting domain have to be serialized into an ordered list. Up till now, to induce the serialization we have used the distance to boundary map. But there are inpainting problems where the distance to boundary serialization causes unsatisfactory inpainting results. In the present work we demonstrate cases where we can resolve the difficulties by employing other distance functions which better suit the problem at hand
Unwind: Interactive Fish Straightening
The ScanAllFish project is a large-scale effort to scan all the world's
33,100 known species of fishes. It has already generated thousands of
volumetric CT scans of fish species which are available on open access
platforms such as the Open Science Framework. To achieve a scanning rate
required for a project of this magnitude, many specimens are grouped together
into a single tube and scanned all at once. The resulting data contain many
fish which are often bent and twisted to fit into the scanner. Our system,
Unwind, is a novel interactive visualization and processing tool which
extracts, unbends, and untwists volumetric images of fish with minimal user
interaction. Our approach enables scientists to interactively unwarp these
volumes to remove the undesired torque and bending using a piecewise-linear
skeleton extracted by averaging isosurfaces of a harmonic function connecting
the head and tail of each fish. The result is a volumetric dataset of a
individual, straight fish in a canonical pose defined by the marine biologist
expert user. We have developed Unwind in collaboration with a team of marine
biologists: Our system has been deployed in their labs, and is presently being
used for dataset construction, biomechanical analysis, and the generation of
figures for scientific publication
The three dimensional skeleton: tracing the filamentary structure of the universe
The skeleton formalism aims at extracting and quantifying the filamentary
structure of the universe is generalized to 3D density fields; a numerical
method for computating a local approximation of the skeleton is presented and
validated here on Gaussian random fields. This method manages to trace well the
filamentary structure in 3D fields such as given by numerical simulations of
the dark matter distribution on large scales and is insensitive to monotonic
biasing. Two of its characteristics, namely its length and differential length,
are analyzed for Gaussian random fields. Its differential length per unit
normalized density contrast scales like the PDF of the underlying density
contrast times the total length times a quadratic Edgeworth correction
involving the square of the spectral parameter. The total length scales like
the inverse square smoothing length, with a scaling factor given by 0.21 (5.28+
n) where n is the power index of the underlying field. This dependency implies
that the total length can be used to constrain the shape of the underlying
power spectrum, hence the cosmology. Possible applications of the skeleton to
galaxy formation and cosmology are discussed. As an illustration, the
orientation of the spin of dark halos and the orientation of the flow near the
skeleton is computed for dark matter simulations. The flow is laminar along the
filaments, while spins of dark halos within 500 kpc of the skeleton are
preferentially orthogonal to the direction of the flow at a level of 25%.Comment: 17 pages, 11 figures, submitted to MNRA
Skeleton as a probe of the cosmic web: the 2D case
We discuss the skeleton as a probe of the filamentary structures of a 2D
random field. It can be defined for a smooth field as the ensemble of pairs of
field lines departing from saddle points, initially aligned with the major axis
of local curvature and connecting them to local maxima. This definition is thus
non local and makes analytical predictions difficult, so we propose a local
approximation: the local skeleton is given by the set of points where the
gradient is aligned with the local curvature major axis and where the second
component of the local curvature is negative.
We perform a statistical analysis of the length of the total local skeleton,
chosen for simplicity as the set of all points of space where the gradient is
either parallel or orthogonal to the main curvature axis. In all our numerical
experiments, which include Gaussian and various non Gaussian realizations such
as \chi^2 fields and Zel'dovich maps, the differential length is found within a
normalization factor to be very close to the probability distribution function
of the smoothed field. This is in fact explicitly demonstrated in the Gaussian
case.
This result might be discouraging for using the skeleton as a probe of non
Gausiannity, but our analyses assume that the total length of the skeleton is a
free, adjustable parameter. This total length could in fact be used to
constrain cosmological models, in CMB maps but also in 3D galaxy catalogs,
where it estimates the total length of filaments in the Universe. Making the
link with other works, we also show how the skeleton can be used to study the
dynamics of large scale structure.Comment: 15 pages, 11 figures, submitted to MNRA
Role of critical points of the skin friction field in formation of plumes in thermal convection
The dynamics in the thin boundary layers of temperature and velocity is the
key to a deeper understanding of turbulent transport of heat and momentum in
thermal convection. The velocity gradient at the hot and cold plates of a
Rayleigh-B\'{e}nard convection cell forms the two-dimensional skin friction
field and is related to the formation of thermal plumes in the respective
boundary layers. Our analysis is based on a direct numerical simulation of
Rayleigh-B\'{e}nard convection in a closed cylindrical cell of aspect ratio
and focused on the critical points of the skin friction field. We
identify triplets of critical points, which are composed of two unstable nodes
and a saddle between them, as the characteristic building block of the skin
friction field. Isolated triplets as well as networks of triplets are detected.
The majority of the ridges of line-like thermal plumes coincide with the
unstable manifolds of the saddles. From a dynamical Lagrangian perspective,
thermal plumes are formed together with an attractive hyperbolic Lagrangian
Coherent Structure of the skin friction field. We also discuss the differences
from the skin friction field in turbulent channel flows from the perspective of
the Poincar\'{e}-Hopf index theorem for two-dimensional vector fields
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