Results 1  10
of
452
Snakes, Shapes, and Gradient Vector Flow
 IEEE TRANSACTIONS ON IMAGE PROCESSING
, 1998
"... Snakes, or active contours, are used extensively in computer vision and image processing applications, particularly to locate object boundaries. Problems associated with initialization and poor convergence to boundary concavities, however, have limited their utility. This paper presents a new extern ..."
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Cited by 755 (16 self)
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Snakes, or active contours, are used extensively in computer vision and image processing applications, particularly to locate object boundaries. Problems associated with initialization and poor convergence to boundary concavities, however, have limited their utility. This paper presents a new external force for active contours, largely solving both problems. This external force, which we call gradient vector flow (GVF), is computed as a diffusion of the gradient vectors of a graylevel or binary edge map derived from the image. It differs fundamentally from traditional snake external forces in that it cannot be written as the negative gradient of a potential function, and the corresponding snake is formulated directly from a force balance condition rather than a variational formulation. Using several twodimensional (2D) examples and one threedimensional (3D) example, we show that GVF has a large capture range and is able to move snakes into boundary concavities.
On Nonreflecting Boundary Conditions
 J. COMPUT. PHYS
, 1995
"... Improvements are made in nonreflecting boundary conditions at artificial boundaries for use with the Helmholtz equation. First, it is shown how to remove the difficulties that arise when the exact DtN (DirichlettoNeumann) condition is truncated for use in computation, by modifying the truncated ..."
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Cited by 219 (4 self)
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Improvements are made in nonreflecting boundary conditions at artificial boundaries for use with the Helmholtz equation. First, it is shown how to remove the difficulties that arise when the exact DtN (DirichlettoNeumann) condition is truncated for use in computation, by modifying the truncated condition. Second, the exact DtN boundary condition is derived for elliptic and spheroidal coordinates. Third, approximate local boundary conditions are derived for these coordinates. Fourth, the truncated DtN condition in elliptic and spheroidal coordinates is modified to remove difficulties. Fifth, a sequence of new and more accurate local boundary conditions is derived for polar coordinates in two dimensions. Numerical results are presented to demonstrate the usefulness of these improvements.
Integrable Structure of Conformal Field Theory II. Qoperator and DDV equation
, 1996
"... This paper is a direct continuation of [1] where we begun the study of the integrable structures in Conformal Field Theory. We show here how to construct the operators Q \Sigma () which act in highest weight Virasoro module and commute for different values of the parameter . These operators appear ..."
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Cited by 168 (18 self)
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This paper is a direct continuation of [1] where we begun the study of the integrable structures in Conformal Field Theory. We show here how to construct the operators Q \Sigma () which act in highest weight Virasoro module and commute for different values of the parameter . These operators appear to be the CFT analogs of the Q  matrix of Baxter [2], in particular they satisfy famous Baxter's T \Gamma Q equation. We also show that under natural assumptions about analytic properties of the operators Q() as the functions of the Baxter's relation allows one to derive the nonlinear integral equations of Destride Vega (DDV) [3] for the eigenvalues of the Qoperators. We then use the DDV equation to obtain the asymptotic expansions of the Q  operators at large ; it is remarkable that unlike the expansions of the T operators of [1], the asymptotic series for Q() contains the "dual" nonlocal Integrals of Motion along with the local ones. We also discuss an intriguing relation between the ...
Seismic waveform inversion in the frequency domain, Part 1: Theory and verification in a physical scale model
 Geophysics
, 1999
"... and verification in a physical scale model ..."
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Computational anatomy and functional architecture of striate cortex: a spatial mapping approach to perceptual coding
 Vision Research
, 1980
"... AbstractThe spatial inhomogeneity of the retinostriate syslem is summarized by the vector cortical magnification factor. The logarithm of retinal eccentricity provides a good fit to the integrated cortical magnification factor. Under the assumption that the cortical map is analytic (conformal), th ..."
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Cited by 112 (6 self)
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AbstractThe spatial inhomogeneity of the retinostriate syslem is summarized by the vector cortical magnification factor. The logarithm of retinal eccentricity provides a good fit to the integrated cortical magnification factor. Under the assumption that the cortical map is analytic (conformal), this implies that a complex logarithmic function of retinal coordinates describes the twodimensional structure of the cortical representation of a visual stimulus. This hypothesis is in good agreement with the measured global structure of rhesus, squirrel, and owl monkey retinostriate mappings, as well as that of the upper visual field of the cat. The geometric structure of the local hypercolumnar unit of striate cortex may also be characterized in terms of the complex Logarithmic mapping: thus. the retinocortical system may be thought of as a concatenated cornplex logarithmic mapping. A simple developmental mechanism is capable of constructing a map of this form, and the general mathematical properties of conformal mappings allow some Insight into the nature of the minimal coding requirements which must be specified to encode a neural map. Complex logarithmic mapping yields a cortical &quot;Gestalt &quot; which is pseudoinvariant to size, rotation. and projection scaling: these symmetries. for a given fixation point. result in a linear shift of an invariant
Accuracy of finitedifference modeling of the acoustic wave equation
 Geophysics
, 1974
"... Recent interest in finitedifference modeling of the wave equation has raised questions regarding the degree of match between finitedifference solutions and solutions obtained by the more classical analytical approaches. This problem is studied by means of a comparison of seismograms computed for ..."
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Cited by 72 (0 self)
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Recent interest in finitedifference modeling of the wave equation has raised questions regarding the degree of match between finitedifference solutions and solutions obtained by the more classical analytical approaches. This problem is studied by means of a comparison of seismograms computed for receivers located in the vicinity of a WIdegree wedge embedded in an infinite twodimensional acoustic medium. The calculations were carried out both by the finitedifference method and by a more conventional eigenfunction expansion technique. The results indicate the solutions arc in good agreement provided that the grid interval for the finitedifference method is sufficiently small. If the grid is too coarse, the signals computed by the finitedifference method become strongly dispersed, and agreement between the
Convergent Multiplicative Processes Repelled from Zero
 Power Laws and Truncated Power Laws, Journal de Physique I France
, 1997
"... Levy and Solomon have found that random multiplicative processes wt = λ1λ2...λt (with λj> 0) lead, in the presence of a boundary constraint, to a distribution P(wt) in the form of a power law w −(1+µ) t. We provide a simple exact physically intuitive derivation of this result based on a random wa ..."
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Cited by 63 (13 self)
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Levy and Solomon have found that random multiplicative processes wt = λ1λ2...λt (with λj> 0) lead, in the presence of a boundary constraint, to a distribution P(wt) in the form of a power law w −(1+µ) t. We provide a simple exact physically intuitive derivation of this result based on a random walk analogy and show the following: 1) the result applies to the asymptotic (t → ∞) distribution of wt and should be distinguished from the central limit theorem which is a statement on the asymptotic distribution of the reduced variable 1 √ (log wt − 〈log wt〉); 2) the two necessary and sufficient conditions for P(wt) t to be a power law are that 〈log λj 〉 < 0 (corresponding to a drift wt → 0) and that wt not be allowed to become too small. We discuss several models, previously thought unrelated, showing the common underlying mechanism for the generation of power laws by multiplicative processes: the variable log wt
The scale representation
 IEEE Transactions on Signal Processing
, 1993
"... scaleable automated quality assurance technique for semantic ..."
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Cited by 60 (3 self)
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scaleable automated quality assurance technique for semantic
Timedomain reconstruction for thermoacoustic tomography in a spherical geometry
 IEEE Med. Imag
, 2002
"... coustic tomography in a spherical configuration is presented. Thermoacoustic waves from biological tissue samples excited by microwave pulses are measured by a wideband unfocused ultrasonic transducer, which is set on a spherical surface enclosing the sample. Sufficient data are acquired from diffe ..."
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Cited by 59 (8 self)
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coustic tomography in a spherical configuration is presented. Thermoacoustic waves from biological tissue samples excited by microwave pulses are measured by a wideband unfocused ultrasonic transducer, which is set on a spherical surface enclosing the sample. Sufficient data are acquired from different directions to reconstruct the microwave absorption distribution. An exact reconstruction solution is derived and approximated to a modified backprojection algorithm. Experiments demonstrate that the reconstructed images agree well with the original samples. The spatial resolution of the system reaches 0.5 mm. Index Terms—Microwave, reconstruction, thermoacoustic, tomography.
Controlled electromagnetic sources for measuring electrical conductivity beneath the oceans, 1, Forward problem and model
, 1982
"... Exact closedform expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a onedimensional earth are derived from the Maxwell equations. Numerical methods for the evaluation of the solutions are given, including corr ..."
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Cited by 53 (4 self)
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Exact closedform expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a onedimensional earth are derived from the Maxwell equations. Numerical methods for the evaluation of the solutions are given, including correction for the finite size of real sources. Simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic, petrologic, and laboratory data, and their electromagnetic response is modeled. Horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range, and the horizontal electric field offers superior received signal performance. Reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges. Estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0.0110 Hz are presented. The ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle, especially low conductivity zones, is demonstrated. If the mantle conductivity is low enough, horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved.