39,526 research outputs found
A Survey of Ocean Simulation and Rendering Techniques in Computer Graphics
This paper presents a survey of ocean simulation and rendering methods in
computer graphics. To model and animate the ocean's surface, these methods
mainly rely on two main approaches: on the one hand, those which approximate
ocean dynamics with parametric, spectral or hybrid models and use empirical
laws from oceanographic research. We will see that this type of methods
essentially allows the simulation of ocean scenes in the deep water domain,
without breaking waves. On the other hand, physically-based methods use
Navier-Stokes Equations (NSE) to represent breaking waves and more generally
ocean surface near the shore. We also describe ocean rendering methods in
computer graphics, with a special interest in the simulation of phenomena such
as foam and spray, and light's interaction with the ocean surface
Feature based volumes for implicit intersections.
The automatic generation of volumes bounding the intersection of two implicit surfaces (isosurfaces of real functions of 3D point coordinates) or feature based volumes (FBV) is presented. Such FBVs are defined by constructive operations, function normalization and offsetting. By applying various offset operations to the intersection of two surfaces, we can obtain variations in the shape of an FBV. The resulting volume can be used as a boundary for blending operations applied to two corresponding volumes, and also for visualization of feature curves and modeling of surface based structures including microstructures
Topological correction of hypertextured implicit surfaces for ray casting
Hypertextures are a useful modelling tool in that they
can add three-dimensional detail to the surface of otherwise
smooth objects. Hypertextures can be rendered as implicit
surfaces, resulting in objects with a complex but well
defined boundary. However, representing a hypertexture as
an implicit surface often results in many small parts being
detached from the main surface, turning an object into a
disconnected set. Depending on the context, this can detract
from the realism in a scene where one usually does not
expect a solid object to have clouds of smaller objects floating around it. We present a topology correction technique, integrated in a ray casting algorithm for hypertextured implicit surfaces, that detects and removes all the surface components that have become disconnected from the main surface. Our method works with implicit surfaces that are C2 continuous and uses Morse theory to find the critical points of the surface. The method follows the separatrix lines joining the critical points to isolate disconnected components
Computerized Analysis of Magnetic Resonance Images to Study Cerebral Anatomy in Developing Neonates
The study of cerebral anatomy in developing neonates is of great importance for
the understanding of brain development during the early period of life. This
dissertation therefore focuses on three challenges in the modelling of cerebral
anatomy in neonates during brain development. The methods that have been
developed all use Magnetic Resonance Images (MRI) as source data.
To facilitate study of vascular development in the neonatal period, a set of image
analysis algorithms are developed to automatically extract and model cerebral
vessel trees. The whole process consists of cerebral vessel tracking from
automatically placed seed points, vessel tree generation, and vasculature
registration and matching. These algorithms have been tested on clinical Time-of-
Flight (TOF) MR angiographic datasets.
To facilitate study of the neonatal cortex a complete cerebral cortex segmentation
and reconstruction pipeline has been developed. Segmentation of the neonatal
cortex is not effectively done by existing algorithms designed for the adult brain
because the contrast between grey and white matter is reversed. This causes pixels
containing tissue mixtures to be incorrectly labelled by conventional methods. The
neonatal cortical segmentation method that has been developed is based on a novel
expectation-maximization (EM) method with explicit correction for mislabelled
partial volume voxels. Based on the resulting cortical segmentation, an implicit
surface evolution technique is adopted for the reconstruction of the cortex in
neonates. The performance of the method is investigated by performing a detailed
landmark study.
To facilitate study of cortical development, a cortical surface registration algorithm
for aligning the cortical surface is developed. The method first inflates extracted
cortical surfaces and then performs a non-rigid surface registration using free-form
deformations (FFDs) to remove residual alignment. Validation experiments using
data labelled by an expert observer demonstrate that the method can capture local
changes and follow the growth of specific sulcus
VolumeEVM: A new surface/volume integrated model
Volume visualization is a very active research area in the field of scien-tific
visualization. The Extreme Vertices Model (EVM) has proven to be
a complete intermediate model to visualize and manipulate volume data
using a surface rendering approach. However, the ability to integrate the
advantages of surface rendering approach with the superiority in visual exploration
of the volume rendering would actually produce a very complete
visualization and edition system for volume data. Therefore, we decided
to define an enhanced EVM-based model which incorporates the volumetric
information required to achieved a nearly direct volume visualization
technique. Thus, VolumeEVM was designed maintaining the same EVM-based
data structure plus a sorted list of density values corresponding to
the EVM-based VoIs interior voxels. A function which relates interior
voxels of the EVM with the set of densities was mandatory to be defined.
This report presents the definition of this new surface/volume integrated
model based on the well known EVM encoding and propose implementations
of the main software-based direct volume rendering techniques
through the proposed model.Postprint (published version
Evidence against the Detectability of a Hippocampal Place Code Using Functional Magnetic Resonance Imaging
Individual hippocampal neurons selectively increase their firing rates in specific spatial locations. As a population, these neurons provide a decodable representation of space that is robust against changes to sensory- and path-related cues. This neural code is sparse and distributed, theoretically rendering it undetectable with population recording methods such as functional magnetic resonance imaging (fMRI). Existing studies nonetheless report decoding spatial codes in the human hippocampus using such techniques. Here we present results from a virtual navigation experiment in humans in which we eliminated visual- and path-related confounds and statistical limitations present in existing studies, ensuring that any positive decoding results would represent a voxel-place code. Consistent with theoretical arguments derived from electrophysiological data and contrary to existing fMRI studies, our results show that although participants were fully oriented during the navigation task, there was no statistical evidence for a place code
A Vortex Method for Bi-phasic Fluids Interacting with Rigid Bodies
We present an accurate Lagrangian method based on vortex particles,
level-sets, and immersed boundary methods, for animating the interplay between
two fluids and rigid solids. We show that a vortex method is a good choice for
simulating bi-phase flow, such as liquid and gas, with a good level of realism.
Vortex particles are localized at the interfaces between the two fluids and
within the regions of high turbulence. We gain local precision and efficiency
from the stable advection permitted by the vorticity formulation. Moreover, our
numerical method straightforwardly solves the two-way coupling problem between
the fluids and animated rigid solids. This new approach is validated through
numerical comparisons with reference experiments from the computational fluid
community. We also show that the visually appealing results obtained in the CG
community can be reproduced with increased efficiency and an easier
implementation
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