460 research outputs found
Curvature-based transfer functions for direct volume rendering: methods and applications
Journal ArticleDirect volume rendering of scalar fields uses a transfer function to map locally measured data properties to opacities and colors. The domain of the transfer function is typically the one-dimensional space of scalar data values. This paper advances the use of curvature information in multi-dimensional transfer functions, with a methodology for computing high-quality curvature measurements. The proposed methodology combines an implicit formulation of curvature with convolution-based reconstruction of the field. We give concrete guidelines for implementing the methodology, and illustrate the importance of choosing accurate filters for computing derivatives with convolution. Curvature-based transfer functions are shown to extend the expressivity and utility of volume rendering through contributions in three different application areas: nonphotorealistic volume rendering, surface smoothing via anisotropic diffusion, and visualization of isosurface uncertainty
FPGA-Based Multimodal Embedded Sensor System Integrating Low- and Mid-Level Vision
Motion estimation is a low-level vision task that is especially relevant due to its wide range of applications in the real world. Many of the best motion estimation algorithms include some of the features that are found in mammalians, which would demand huge computational resources and therefore are not usually available in real-time. In this paper we present a novel bioinspired sensor based on the synergy between optical flow and orthogonal variant moments. The bioinspired sensor has been designed for Very Large Scale Integration (VLSI) using properties of the mammalian cortical motion pathway. This sensor combines low-level primitives (optical flow and image moments) in order to produce a mid-level vision abstraction layer. The results are described trough experiments showing the validity of the proposed system and an analysis of the computational resources and performance of the applied algorithms
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Visual perception of solid shape from occluding contours
The relative motion of object and observer induces a motion field in the observer's visual image that is smooth everywhere except along the object's occluding contours. Thus, occluding contours and smooth motion fields can be viewed as complementary and as separate sources of information about an object's shape. I studied how the human visual system perceives solid shape from the occluding contours of rotating objects and from the smooth motion field induced by moving planar surface patches.I propose a three-stage model for the perception of solid shape from the occluding contours of a rotating object. First, the object's motion is determined. I argue that this is only possible using points of correspondence and only when the object's axis of rotation is frontoparallel. In the second stage, the motion field along the contour is used to compute relative depth and surface curvature along the rim, the contour's pre-image. Third, local shape descriptors are propagated inside the figure to yield a global percept of solid shape. To determine which shape descriptors are computed by human subjects, I used a novel task in which subjects have to discriminate between flat ellipses and solid ellipsoids with varying thickness. I found that discriminability is proportional to the inverse of radial curvature but is not proportional to Gaussian or mean curvature. Certain slants of the axis of rotation decrease discriminability. Subjects who could discriminate ellipsoids and ellipses perceived the ellipsoids' angular velocity more veridically than did subjects who could not discriminate the two.Any smooth motion field can locally be described by divergence, curl, and deformation. If the motion field is induced by a rotating plane, the amount of deformation is proportional to the plane's slant and its angular velocity. Similarly, for translating planes, deformation is proportional to slant and image motion. Slant judgments of human observers were to a first-order approximation proportional to deformation per se, that is, observers do not take object motion into account. Recent psychophysical evidence suggests that human subjects need motion discontinuities for this. Thus, contours might be necessary to correctly perceive slant from smooth motion fields
Visualization and analysis of diffusion tensor fields
technical reportThe power of medical imaging modalities to measure and characterize biological tissue is amplified by visualization and analysis methods that help researchers to see and understand the structures within their data. Diffusion tensor magnetic resonance imaging can measure microstructural properties of biological tissue, such as the coherent linear organization of white matter of the central nervous system, or the fibrous texture of muscle tissue. This dissertation describes new methods for visualizing and analyzing the salient structure of diffusion tensor datasets. Glyphs from superquadric surfaces and textures from reactiondiffusion systems facilitate inspection of data properties and trends. Fiber tractography based on vector-tensor multiplication allows major white matter pathways to be visualized. The generalization of direct volume rendering to tensor data allows large-scale structures to be shaded and rendered. Finally, a mathematical framework for analyzing the derivatives of tensor values, in terms of shape and orientation change, enables analytical shading in volume renderings, and a method of feature detection important for feature-preserving filtering of tensor fields. Together, the combination of methods enhances the ability of diffusion tensor imaging to provide insight into the local and global structure of biological tissue
Two-Sphere Partition Functions and Gromov-Witten Invariants
Many N=(2,2) two-dimensional nonlinear sigma models with Calabi-Yau target
spaces admit ultraviolet descriptions as N=(2,2) gauge theories (gauged linear
sigma models). We conjecture that the two-sphere partition function of such
ultraviolet gauge theories -- recently computed via localization by Benini et
al. and Doroud et al. -- yields the exact K\"ahler potential on the quantum
K\"ahler moduli space for Calabi-Yau threefold target spaces. In particular,
this allows one to compute the genus zero Gromov-Witten invariants for any such
Calabi-Yau threefold without the use of mirror symmetry. More generally, when
the infrared superconformal fixed point is used to compactify string theory,
this provides a direct method to compute the spacetime K\"ahler potential of
certain moduli (e.g., vector multiplet moduli in type IIA), exactly in
{\alpha}'. We compute these quantities for the quintic and for R{\o}dland's
Pfaffian Calabi-Yau threefold and find agreement with existing results in the
literature. We then apply our methods to a codimension four determinantal
Calabi-Yau threefold in P^7, recently given a nonabelian gauge theory
description by the present authors, for which no mirror Calabi-Yau is currently
known. We derive predictions for its Gromov-Witten invariants and verify that
our predictions satisfy nontrivial geometric checks.Comment: 25 pages + 2 appendices; v2 corrects a divisor in K\"ahler moduli
space and includes a new calculation that confirms a geometric prediction; v3
contains minor update of Gromov-Witten invariant extraction procedur
Angular variation as a monocular cue for spatial percepcion
Monocular cues are spatial sensory inputs which are picked up exclusively from one eye. They are in majority static features that
provide depth information and are extensively used in graphic art to create realistic representations of a scene. Since the spatial
information contained in these cues is picked up from the retinal image, the existence of a link between it and the theory of direct
perception can be conveniently assumed. According to this theory, spatial information of an environment is directly contained in the
optic array. Thus, this assumption makes possible the modeling of visual perception processes through computational approaches.
In this thesis, angular variation is considered as a monocular cue, and the concept of direct perception is adopted by a computer
vision approach that considers it as a suitable principle from which innovative techniques to calculate spatial information can be
developed.
The expected spatial information to be obtained from this monocular cue is the position and orientation of an object with respect to
the observer, which in computer vision is a well known field of research called 2D-3D pose estimation. In this thesis, the attempt to
establish the angular variation as a monocular cue and thus the achievement of a computational approach to direct perception is
carried out by the development of a set of pose estimation methods. Parting from conventional strategies to solve the pose
estimation problem, a first approach imposes constraint equations to relate object and image features. In this sense, two algorithms
based on a simple line rotation motion analysis were developed. These algorithms successfully provide pose information; however,
they depend strongly on scene data conditions. To overcome this limitation, a second approach inspired in the biological processes
performed by the human visual system was developed. It is based in the proper content of the image and defines a computational
approach to direct perception.
The set of developed algorithms analyzes the visual properties provided by angular variations. The aim is to gather valuable data
from which spatial information can be obtained and used to emulate a visual perception process by establishing a 2D-3D metric
relation. Since it is considered fundamental in the visual-motor coordination and consequently essential to interact with the
environment, a significant cognitive effect is produced by the application of the developed computational approach in environments
mediated by technology. In this work, this cognitive effect is demonstrated by an experimental study where a number of participants
were asked to complete an action-perception task. The main purpose of the study was to analyze the visual guided behavior in
teleoperation and the cognitive effect caused by the addition of 3D information. The results presented a significant influence of the
3D aid in the skill improvement, which showed an enhancement of the sense of presence.Las seĂąales monoculares son entradas sensoriales capturadas exclusivamente por un
solo ojo que ayudan a la percepciĂłn de distancia o espacio. Son en su mayorĂa
caracterĂsticas estĂĄticas que proveen informaciĂłn de profundidad y son muy
utilizadas en arte grĂĄfico para crear apariencias reales de una escena. Dado que la
informaciĂłn espacial contenida en dichas seĂąales son extraĂdas de la retina, la
existencia de una relaciĂłn entre esta extracciĂłn de informaciĂłn y la teorĂa de
percepciĂłn directa puede ser convenientemente asumida. De acuerdo a esta teorĂa, la
informaciĂłn espacial de todo le que vemos estĂĄ directamente contenido en el arreglo
Ăłptico. Por lo tanto, esta suposiciĂłn hace posible el modelado de procesos de
percepciĂłn visual a travĂŠs de enfoques computacionales. En esta tesis doctoral, la
variaciĂłn angular es considerada como una seĂąal monocular, y el concepto de
percepciĂłn directa adoptado por un enfoque basado en algoritmos de visiĂłn por
computador que lo consideran un principio apropiado para el desarrollo de nuevas
tĂŠcnicas de cĂĄlculo de informaciĂłn espacial.
La informaciĂłn espacial esperada a obtener de esta seĂąal monocular es la posiciĂłn y
orientaciĂłn de un objeto con respecto al observador, lo cual en visiĂłn por computador
es un conocido campo de investigaciĂłn llamado estimaciĂłn de la pose 2D-3D. En esta
tesis doctoral, establecer la variaciĂłn angular como seĂąal monocular y conseguir un
modelo matemĂĄtico que describa la percepciĂłn directa, se lleva a cabo mediante el
desarrollo de un grupo de mĂŠtodos de estimaciĂłn de la pose. Partiendo de estrategias
convencionales, un primer enfoque implanta restricciones geomĂŠtricas en ecuaciones
para relacionar caracterĂsticas del objeto y la imagen. En este caso, dos algoritmos
basados en el anĂĄlisis de movimientos de rotaciĂłn de una lĂnea recta fueron
desarrollados. Estos algoritmos exitosamente proveen informaciĂłn de la pose. Sin
embargo, dependen fuertemente de condiciones de la escena. Para superar esta
limitaciĂłn, un segundo enfoque inspirado en los procesos biolĂłgicos ejecutados por el
sistema visual humano fue desarrollado. EstĂĄ basado en el propio contenido de la
imagen y define un enfoque computacional a la percepciĂłn directa.
El grupo de algoritmos desarrollados analiza las propiedades visuales suministradas
por variaciones angulares. El propĂłsito principal es el de reunir datos de importancia
con los cuales la informaciĂłn espacial pueda ser obtenida y utilizada para emular
procesos de percepciĂłn visual mediante el establecimiento de relaciones mĂŠtricas 2D-
3D. Debido a que dicha relaciĂłn es considerada fundamental en la coordinaciĂłn
visuomotora y consecuentemente esencial para interactuar con lo que nos rodea, un
efecto cognitivo significativo puede ser producido por la aplicaciĂłn de mĂŠtodos de
L
estimaciĂłn de pose en entornos mediados tecnolĂłgicamente. En esta tesis doctoral, este
efecto cognitivo ha sido demostrado por un estudio experimental en el cual un nĂşmero
de participantes fueron invitados a ejecutar una tarea de acciĂłn-percepciĂłn. El
propĂłsito principal de este estudio fue el anĂĄlisis de la conducta guiada visualmente en
teleoperaciĂłn y el efecto cognitivo causado por la inclusiĂłn de informaciĂłn 3D. Los
resultados han presentado una influencia notable de la ayuda 3D en la mejora de la
habilidad, asĂ como un aumento de la sensaciĂłn de presencia
Statistics of gradient directions in natural images.
Interest in finding statistical regularities in natural images has been growing since the advent of information theory and the advancement of the efficient coding hypothesis that the human visual system is optimised to encode natural visual stimuli. In this thesis, a statistical analysis of gradient directions in an ensemble of natural images is reported. Information-theoretic measures have been used to compute the amount of dependency which exists between triples of gradient directions at separate image locations. Control experiments are performed on other image classes: phase randomized natural images, whitened natural images, and Gaussian noise images. The main results show that for an ensemble of natural images the average amount of de pendency between two and three gradient directions is the same as for an ensemble of phase randomized natural images. This result does not extend to i) the amount dependency between gradient magnitudes, ii) gradient directions at high gradient magnitude locations, or iii) individual natural images. Furthermore, no significant synergetic dependencies are found between triples of gradient directions in an ensemble natural images a synergetic dependency is an increase in dependency between a pair of gradient directions given the interaction of a third gradient direction. Additional experiments are performed to establish both the generality and specificity of the main results by studying the gradient direction dependencies of ensembles of noise (random phases) images with varying power law power spectra. The results of the additional experiments indicate that, for ensembles of images with varying power law power spectra, the amount of dependency between two and three gradient directions is determined by the ensemble's mean power spectrum rather than the phase spectrum. A framework is also presented for future work and preliminary results are provided for the dependency between second order derivative measurements (shape index) for up to 9-point configurations
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