Luis Alvarez

University of Las Palmas de Gran Canaria | ULPGC · Department of Computer Sciences and Systems

Image-processing transforms must satisfy a list of formal requirements. We discuss these requirements and classify them into three categories: architectural requirements like locality, recursivity and causality in the scale space, stability requirements like the comparison principle and morphological requirements, which correspond to shape-preservi...

We introduce new results connecting differential and morphological operators that provide a formal and theoretically grounded approach for stable and fast contour evolution. Contour evolution algorithms have been extensively used for boundary detection and tracking in computer vision. The standard solution based on partial differential equations an...

The main goal of this paper is the design of a novel and robust methodology for calibrating cameras from a single image in sport scenarios, such as a soccer field, or a basketball or tennis court. In these sport scenarios, the only references we use to calibrate the camera are the lines and circles delimiting the different regions. The first proble...

Exemplar-based texture synthesis methods try to emulate textures observed in our visual world. Yet the field of all possible textures (natural or not) has been little explored. Indeed, existing abstract synthesis methods focus on a single generation rule and generate a rather limited set of textures. This limitation can be overcome by combining ran...

Significance Based on a signal-processing approach, we propose a method to compute the reproduction number R t , the transmission potential of an epidemic over time. R t is estimated by minimizing a functional that enforces: 1) the ability to produce an incidence curve i t corrected of the weekly periodic bias produced by the “weekend effect,” obta...

The Covid-19 pandemic produced regional time series of incidence, hospital, ICU admission and death. The EpiInvert package estimates the incidence trend and the daily reproduction number Rt of any infectious disease, compares related time series such as incidence and death, and provides incidence forecasts. EpiInvert is an R package with the follow...

In this paper we propose a new planar shape representation, the medial branch graph representation (MBGR) which allows to easily generate, vary and edit all-new sorts of parametric shapes. Each MBGR shape is described by a collection of connected control circles organized as a graph. Each pair of connected circles generates a branch of the shape. T...

The most common reported epidemic time series in epidemiological surveillance are the daily or weekly incidence of new cases, the hospital admission count, the ICU admission count, and the death toll, which played such a prominent role in the struggle to monitor the Covid-19 pandemic. We show that pairs of such curves are related to each other by a...

The COVID-19 pandemy has created a radically new situation where most countries provide raw measurements of their daily incidence and disclose them in real time. This enables new machine learning forecast strategies where the prediction might no longer be based just on the past values of the current incidence curve, but could take advantage of obse...

The most common reported epidemic time series in epidemiological surveillance are the daily or weekly incidence of new cases, the hospital admission count, the ICU admission count, and the death toll, which played such a prominent role in the struggle to monitor the Covid-19 pandemic. We show that pairs of such curves are related to each other by a...

The COVID-19 pandemy has created a radically new situation where most countries provide raw measurements of their daily incidence and disclose them in real time. This enables new machine learning forecast strategies where the prediction might no longer be based just on the past values of the current incidence curve, but could take advantage of obse...

In this paper, we study the regularization of 3D curves connecting two points. We propose an energy-based formulation which is an extension to 3D of the geodesic active contours introduced in 2D by Caselles et al. in 1997. By minimizing this energy we try to minimize the curve length but keeping the curve close to the original one. The energy depen...

In the past two years, the COVID-19 incidence curves and reproduction number Rt have been the main metrics used by policy makers and journalists to monitor the spread of this global pandemic. However, these metrics are not always reliable in the short term, because of a combination of delay in detection, administrative delays and random noise. In t...

The sanitary crisis of the past two years has focused the public’s attention on quantitative indicators of the spread of the COVID-19 pandemic. The daily reproduction number R t , defined by the average number of new infections caused by a single infected individual at time t , is one of the best metrics for estimating the epidemic trend. In this p...

The automatic synthesis of abstract textures is, to some extent, feasible. As evidenced by abstract art theoreticians, one can think of an abstract picture as a tree of elementary shapes interacting according to a short list of compositional laws such as occlusion, exclusion and bordering, and by rendering rules such as transparency, tessellation a...

We extended an existing web-based tool called Studierfenster (http://studierfenster.icg.tugraz.at/) for this contribution, which was built by students from TU Graz, with a semi-automatic aortic centerline calculation functionality. Studierfenster is a tool that renders three-dimensional volumes, defined by the user, and allows the user to perform m...

This paper presents the details of the flux-ordered thinning algorithm, which we refer to as theHamilton-Jacobi Skeleton (HJS). It computes the skeleton of any binary 2D shape. It is basedon the observation that the skeleton points have low average outward flux of the gradient of thedistance transform. The algorithm starts by computing the distance...

We use an exponential growth model to analyze the first wave of the COVID-19 pandemic in South Korea, Italy, Spain, France, Germany, the United Kingdom, the USA and the New-York state. This model uses the number of officially reported patients tested positive and deaths to estimate an infected hindcast of the cumulative number of patients who later...

The COVID-19 pandemic has undergone rapid changes in its infection rates driven by governmental measures. The reproduction number R ( t ) indicates the number of cases generated by an infected person at time t and is a key indicator of the spread of an epidemic. The EpiEstim method is the most widely accepted method for estimating R ( t ). Here, we...

Computed tomography angiography (CTA) is one of the most common vascular imaging modalities. However, for clinical use, it still requires laborious manual analysis. This study demonstrates the feasibility of a fully automated technology for the accurate detection and identification of several anatomical reference points (landmarks), commonly used i...

Executable of the structure tensor filter (64bit Microsoft Windows executable - tested under Windows 10). For more information, please check www.ctim.es/demo111/

Matlab version of the SDNLM filter (tested on Microsoft Windows platform with Matlab2015b). For more information, please check www.ctim.es/demo111/

Linux executable to perform lens distortion correction. See examples on the scripts run_example1_image.sh (for images) and run_example2_video.sh (for videos). For more information, please check www.ctim.es/demo112 or the readme file included with the package.

In this letter, we propose a new despeckling filter for fully polarimetric synthetic aperture radar (PolSAR) images defined by 3x 3 complex Wishart distributions. We first generalize the well-known structure tensor to deal with PolSAR data which allows to efficiently measure the dominant direction and contrast of edges. The generalization includes...

In this work we present an application of variational techniques to the smoothing and registration of aorta centerlines. We assume that a 3D segmentation of the aorta lumen and an initial estimation of the aorta centerline are available. The centerline smoothing technique aims to maximize the distance of the centerline to the boundary of the aorta...

In this letter, we propose a supervised method for the classification of fully polarimetric synthetic aperture radar (PolSAR) images based on active contour models. We use an "a priori" estimation, obtained from training data, of the complex Wishart distributions of the different types of regions in the image (for instance, water, crops, grass, for...

Purpose The shape and size of the aortic lumen can be associated with several aortic diseases. Automated computer segmentation can provide a mechanism for extracting the main features of the aorta that may be used as a diagnostic aid for physicians. This article presents a new fully automated algorithm to extract the aorta geometry for either norma...

This work presents an automatic method to segment the aortic lumen in computed tomography scans by combining an ellipse-based structure of the artery and an active contour model. The general shape of the aorta is first estimated by adapting the contour of its cross-sections to ellipses oriented in the direction orthogonal to the course of the vesse...

In this paper we propose a multiscale parametric snake model for ellipse motion estimation across a sequence of images. We use a robust ellipse parameterization based on the geometry of the intersection of a cylinder and a plane. The ellipse parameters are optimized in each frame by searching for local minima of the snake model energy including tem...

In this paper we deal with the problem of circle tracking across an image sequence. We propose an active contour model based on a new energy. The center and radius of the circle is optimized in each frame by looking for local minima of such energy. The energy estimation does not require edge extraction, it uses the image convolution with a Gaussian...

Implementation of the proposed technique (for Microsoft Windows platform). For more information, please check www.ctim.es/demo110/index2.html

In this paper we study a geometric partial differential equation including a forcing term. This equation defines a hypersurface evolution that we use for level set regularization. We study the shape of the radial solutions of the equation and some qualitative properties about the level set propagations. We show that under a suitable choice of the f...

We propose a fast incremental technique to compute the 3D geometry of the aortic lumen from a seed point located inside it. Our approach is based on the optimization of the 3D orientation of the cross-sections of the aorta. The method uses a robust ellipse estimation algorithm and an energy-based optimization technique to automatically track the ce...

In this work we propose a technique to automatically estimate circular cross-sections of the vessels in CT scans. First, a circular contour is extracted for each slice of the CT by using the Hough transform. Afterward, the locations of the circles are optimized by means of a parametric snake model, and those circles which best fit the contours of t...

Perspective and optical (lens) distortions are aberrations of very different nature that can simultaneously affect an image. Perspective distortion is caused by the position of the camera, especially when it is too close to the scene. Optical distortion is a lens aberration which causes straight lines in the scene to be projected onto the image as...

We introduce a method for corner estimation based on the affine morphological scale space (AMSS). Using some explicit known formula about corner evolution across AMSS, proven by Alvarez and Morales in 1997, we define a morphological cornerness measure based on the expected evolution of an ideal corner across AMSS. We define a new procedure to track...

In this paper we study the problem of supervised Fully PolSAR (Polarimetric Synthetic Aperture Radar) image classification. We estimate a complex Wishart model distribution for each class using training data, and we use such models to design a new classification procedure based on a diffusion-reaction equation. The method relies on simultaneously f...

The aim of this study was to assess the applicability and reliability of a single wide angle lens GoPro® camera for tracking and kinematics analysis of futsal players. An official game of a Brazilian professional team in the quarter-final round of the 2013 São Paulo futsal league was recorded by four digital video cameras (30 Hz; 720 X 480) placed...

Most techniques for camera calibration that use planar calibration patterns require the computation of a lens distortion model and a homography. Both are simultaneously refined using a bundle adjustment that minimizes the reprojection error of a collection of points when projected from the scene onto the camera sensor. These points are usually the...

We present a method for the automatic estimation of two-parameter radial distortion models, considering polynomial as well as division models. The method first detects the longest distorted lines within the image by applying the Hough transform enriched with a radial distortion parameter. From these lines, the first distortion parameter is estimate...

Accurate image classification of Fully PolSAR (Polarimetric Synthetic Aperture Radar) images is of maximum interest in monitoring remote sensing data. Such classification is a difficult task because of the presence of speckle. Diffusion-reaction systems (DRS) using stochastic distances have been recently applied to successfully despeckling and clas...

The validation of filters for SAR image denoising is mostly performed through personal choices of experimentation on a set of author-selected synthetic and true SAR images, which makes the objective elucidation of their benefits over well-established filtering methods hard. Following the example of other scientific fields, this situation can be all...

In the course of the history of art we can find artistic expressions that can be understandable in terms of geometric compositions where a number of objects are organized following some composition rules and basic geometric transformations. In this work we are interested in the formalization of such generation principles in such a way that the arti...

We propose a new method for PolSAR (Polarimetric Synthetic Aperture Radar) imagery classification based on stochastic distances in the space of random matrices obeying complex Wishart distributions. Given a collection of prototypes $\{Z_m\}_{m=1}^M$ and a stochastic distance $d(.,.)$, we classify any random matrix $X$ using two criteria in an itera...

We study the geometric flow parabolic equation and its implicit discretization which yield a family of nonlinear elliptic problems. We show that there are important differences in the study of those equations which concerns the propagation of level sets of data. Our study is based on the previous study of radially symmetric solutions of the corresp...

In this paper, we study lens distortion for still images considering two well-known distortion models: the two-parameter polynomial model and the two-parameter division model. We study the invertibility of these models, and we mathematically characterize the conditions for the distortion parameters under which the distortion model defines a one-to-...

Presentation related to the paper submitted and presented at IWOBI 2015 (Donostia, San Sebastián, País Vasco, SPAIN).

In this paper we introduce an affine invariant distance definition from a (Formula presented.) point to the boundary of a bounded shape using morphological multiscale analysis. We study the mathematical behavior of this distance by examining separately the cases of convex and non-convex shapes. We prove that the proposed distance is bounded in the...

We present a method to automatically correct the radial distortion caused by wide-angle lenses using the distorted lines generated by the projection on the image of 3D straight lines. Lens distortion is estimated by the division model using one parameter, which allows to state the problem into the Hough transform scheme by adding a distortion param...

Zhang's method is a widely used technique for camera cali-bration from different views of a planar calibration pattern. This pattern contains a set of squares arranged in a certain configuration. In order to calibrate the camera, the corners of the squares in the images must be matched with those in the reference model. When the images show a stron...

In sport scenarios like football or basketball, we often deal with central views where only the central circle and some additional primitives like the central line and the central point or a touch line are visible. In this paper we first characterize, from a mathematical point of view, the set of homographies that project a given ellipse into the u...

In this paper we propose a new method to automatically correct wide-angle lens distortion from the distorted lines generated by the projection on the image of 3D straight lines. We have to deal with two major problems: on the one hand, wide-angle lenses produce a strong distortion, which makes the detection of distorted lines a particularly difficu...

Computed Tomography is one of the most valuable modalities of medical imaging and is used in the diagnosis of a wide range of pathologies. In many cases, it is necessary to obtain a precise segmentation and a 3D visualization of certain organs, tissues, nodules or regions in the tomography. However, manual delimitation is a very time-consuming proc...

We propose a novel method for real-time camera motion tracking in planar view scenarios. This method relies on the geometry of a tripod, an initial estimation of camera pose for the first video frame and a primitive tracking procedure. This process uses lines and circles as primitives, which are extracted applying classification and regression tree...

Lines are one of the basic primitives used by the perceptual system to analyze and interpret a scene. Therefore, line detection is a very important issue for the robustness and flexibility of Computer Vision systems. However, in the case of images showing a significant lens distortion, standard line detection methods fail because lines are not stra...

In this work, we have designed and implemented, based on traditional board games such as the game of the Goose or Parchis, an educational software that aim to reinforce the learning of children. The idea that we are going to develop to do this is very simple: the children play with the same rules as in the traditional game but we add the functional...

We propose new mathematical models to study the variation of lens distortion models when modifying zoom setting in the case of zoom lenses. The new models are based on a polynomial approximation to account for the variation of the radial distortion parameters through the range of zoom lens settings and, on the minimization of a global error energy...

Ear image analysis is an emerging biometrie application. A method for normalizing ear images and extracting from them a set of measurable features (feature vector) that can be used to identify its owner is proposed. The identification would be made based on the comparison between the feature vector of the input image and all feature vectors of the...

The main goal of this paper is to recognize automatically camera pose from a single image of a planar view scenario. We apply this technique to sport event scenarios using as information the white lines/circles dividing the different parts of the sport court. Using these court primitives we define a loss function that we minimize to obtain the best...

This book constitutes the refereed proceedings of the 17th Iberoamerican Congress on Pattern Recognition, CIARP 2012, held in Buenos Aires, Argentina, in September 2012. The 109 papers presented, among them two tutorials and four keynotes, were carefully reviewed and selected from various submissions. The papers are organized in topical sections on...

In this paper we present new mathematical models for video sequence calibration when cameras are mounted on a tripod. One of the main novelties is that tripod rotation center and camera projection center are not supposed to be the same. The calibration is based on the geometry of the tripod and a primitive tracking procedure which uses lines and ci...

Active contours, or snakes, are computer-generated curves that move within images to find out salient image structures like object boundaries. Energy based formulations using a level set approach have been successfully used to model the snake evolution. The Euler-Lagrange equation associated to such energies yields to partial differential equations...

In this paper we revisit some classic board games like Pachisi or the Game of Gosse. The main contribution of the paper is to design and add some functionalities to the games in order to transform them in serious games, that is, in games with learning and educational purposes. To do that, at the beginning of the game, players choose one or several...

AMILab is a free software for image analysis, processing and visualization. It provides convenient visualization tools for 2D and 3D images and it is highly extensible through its own scripting language. We describe the main visualization features and the scripting language of AMILab. The software includes an automatic C++ wrapping system which per...

In this paper we address the problem of inserting virtual content in a video sequence. The method we propose uses just image informa- tion. We perform primitive tracking, camera calibration, real and vir- tual camera synchronisation and �nally rendering to insert the virtual content in the real video sequence. To simplify the calibration step we as...