Langis Gagnon

Registration and fusion of retinal images-an evaluation study

We present the results of a study on the application of registration and pixel-level fusion techniques to retinal images. The images are of different modalities (color, fluorescein angiogram), different resolutions, and taken at different times (from a few minutes during an angiography examination to several years between two examinations). We propose a new registration method based on global point mapping with blood vessel bifurcations as control points and a search for control point matches that uses local structural information of the retinal network. Three transformation types (similarity, affine, and second-order polynomial) are evaluated on each image pair. Fourteen pixel-level fusion techniques have been tested and classified according to their qualitative and quantitative performance. Four quantitative fusion performance criteria are used to evaluate the gain obtained with the grayscale fusion.

Speckle filtering of SAR images: a comparative study between complex-wavelet-based and standard filters

We present a comparative study between a complex Wavelet Coefficient Shrinkage (WCS) filter and several standard speckle filters that are widely used in the radar imaging community. The WCS filter is based on the use of Symmetric Daubechies wavelets which share the same properties as the real Daubechies wavelets but with an additional symmetry property. The filtering operation is an elliptical soft- thresholding procedure with respect to the principal axes of the 2D complex wavelet coefficient distributions. Both qualitative and quantitative results (signal to mean square error ratio, equivalent number of looks, edgemap figure of merit) are reported. Tests have been performed using simulated speckle noise as well as real radar images. It is found that the WCS filter performs equally well as the standard filters for low-level noise and slightly outperforms them for higher-level noise.

Solitary pulses in an amplified nonlinear dispersive medium

An analytical solution is obtained for solitary pulse propagation in an amplified nonlinear dispersive system. For a homogeneously broadened gain medium, this solitary pulse has a hyperbolic secant amplitude and a hyperbolic tangent instantaneous frequency variation. The pulse is a gain-guided pulse in either the positive or the negative dispersion regime as well as in the self-focusing or self-defocusing regime. A dark solitary pulse that has a hyperbolic tangent amplitude and a similar instantaneous frequency variation is also obtained.

Real-time eye blink detection with GPU-based SIFT tracking

This paper reports on the implementation of a GPU-based, real-time eye blink detector on very low contrast images acquired under near-infrared illumination. This detector is part of a multi-sensor data acquisition and analysis system for driver performance assessment and training. Eye blinks are detected inside regions of interest that are aligned with the subjects eyes at initialization. Alignment is maintained through time by tracking SIFT feature points that are used to estimate the affine transformation between the initial face pose and the pose in subsequent frames. The GPU implementation of the SIFT feature point extraction algorithm ensures real-time processing. An eye blink detection rate of 97% is obtained on a video dataset of 33,000 frames showing 237 blinks from 22 subjects.

Lie symmetries of a generalised non-linear Schrodinger equation. II. Exact solutions

For pt.I see ibid., vol.21, p.1493 (1988). The authors obtain group-invariant solutions of the non-linear equation i psi t+ Delta psi =a0 psi +a1 psi mod psi mod 2+a2 psi mod psi mod 4 for which the symmetry group was previously shown to be the extended Galilei group for a1a2 not=0 and the extended Galilei-similitude group for a1=0 or a2=0. They use the symmetry subgroups to reduce the equation to ordinary differential equations which are solved, whenever possible, with the help of a singularity analysis. Solutions are obtained in terms of elementary functions, Jacobi elliptic functions and Painleve transcendents.

Lie symmetries of a generalised nonlinear Schrodinger equation: I. The symmetry group and its subgroups

The symmetry group of the generalised non-linear Schrodinger equation i psi t+ Delta psi =a0 psi +a1 mod psi mod 2 psi +a2 mod psi mod 4 psi in three space dimensions is shown to be the extended Galilei group G(3), for a1a2 not=0, and the Galilei-similitude group Gd(3) (including a dilation) for a1=oor a2=0. All Lie subgroups of G(3) and Gd(3) are found. They will be used in a subsequent paper to obtain group invariant solutions of the equation.

Procedure to detect anatomical structures in optical fundus images

We present an overview of the design and test of an image processing procedure for detecting all important anatomical structures in color fundus images. These structures are the optic disk, the macula and the retinal network. The algorithm proceeds through five main steps: (1) automatic mask generation using pixels value statistics and color threshold, (2) visual image quality assessment using histogram matching and Canny edge distribution modeling, (3) optic disk localization using pyramidal decomposition, Hausdorff-based template matching and confidence assignment, (4) macula localization using pyramidal decomposition and (5) bessel network tracking using recursive dual edge tracking and connectivity recovering. The procedure has been tested on a database of about 40 color fundus images acquired from a digital non-mydriatic fundus camera. The database is composed of images of various types (macula- and optic disk-centered) and of various visual quality (with or without abnormal bright or dark regions, blurred, etc).

Symmetry classes of variable coefficient nonlinear Schrodinger equations

A variable-coefficient nonlinear Schrodinger (VCNLS) equation, involving three arbitrary complex functions of space-time (in 1 + 1 dimensions) is analysed from the point of view of its symmetries. All equations of the type studied having non-trivial Lie point symmetry groups G are identified. The symmetry group is shown to be at most five-dimensional and only when the equation is equivalent to the NLS equation itself or to a rather special complex Ginzburg-Landau equation. Lie point transformations are used to obtain solutions of specific VCNLS equations that should be of interest in nonlinear optics or other branches of physics.

Multi- spectral image resolution refinement using stationary wavelet transform

Presents a pixel-level fusion method to refine the resolution of a multi-spectral image using a high-resolution panchromatic image. Our approach is an adaptation of the ARSIS method which takes into account the higher-order statistical moments of the wavelet coefficients and allow processing of non-dyadic images.

Lie symmetries of a generalised non-linear Schrodinger equation. III. Reductions to third-order ordinary differential equations

For pt. II see ibid., vol. 22, p.469 (1989). The study of group invariant solutions of the generalised non-linear Schrodinger equation (GNLSE) is continued. It is shown that eight types of subgroups of the symmetry group lead via symmetry reduction, to third-order real ordinary differential equations, giving both the phase and the absolute value of the solution. Only two of the reductions provide to a Painleve type equation and both of them only for the cubic GNLSE. This equation is solved in terms of the fourth Painleve transcendent.