4,422 research outputs found
Tracing Analytic Ray Curves for Light and Sound Propagation in Non-Linear Media
The physical world consists of spatially varying media, such as the atmosphere and the ocean, in which light and sound propagates along non-linear trajectories. This presents a challenge to existing ray-tracing based methods, which are widely adopted to simulate propagation due to their efficiency and flexibility, but assume linear rays. We present a novel algorithm that traces analytic ray curves computed from local media gradients, and utilizes the closed-form solutions of both the intersections of the ray curves with planar surfaces, and the travel distance. By constructing an adaptive unstructured mesh, our algorithm is able to model general media profiles that vary in three dimensions with complex boundaries consisting of terrains and other scene objects such as buildings. Our analytic ray curve tracer with the adaptive mesh improves the efficiency considerably over prior methods. We highlight the algorithm's application on simulation of visual and sound propagation in outdoor scenes
Generalized Paraxial Ray Trace Procedure Derived from Geodesic Deviation
Paraxial ray tracing procedures have become widely accepted techniques for
acoustic models in seismology and underwater acoustics. To date a generic form
of these procedures including fluid motion and time dependence has not appeared
in the literature. A detailed investigation of the characteristic curves of the
equations of hydrodynamics allows for an immediate generalization of the
procedure to be extracted from the equation form geodesic deviation. The
general paraxial ray trace equations serve as an ideal supplement to ordinary
ray tracing in predicting the deformation of acoustic beams in random
environments. The general procedure is derived in terms of affine
parameterization and in a coordinate time parameterization ideal for
application to physical acoustic ray propagation. The formalism is applied to
layered media, where the deviation equation reduces to a second order
differential equation for a single field with a general solution in terms of a
depth integral along the ray path. Some features are illustrated through
special cases which lead to exact solutions in terms of either ordinary or
special functions.Comment: Original; 40 pages (double spaced), 1 figure Replaced version; 36
pages single spaced, 7 figures. Expanded content; Complete derivation of the
equations from the equations of hydrodynamics, introduction of an auxiliary
basis for three dimensional wave-front modeling. Typos in text and equations
correcte
A random walk model of wave propagation
This paper shows that a reasonably accurate description of propagation loss in small urban cells can be obtained with a simple stochastic model based on the theory of random walks, that accounts for only two parameters: the amount of clutter and the amount of absorption in the environment. Despite the simplifications of the model, the derived analytical solution correctly describes the smooth transition of power attenuation from an inverse square law with the distance to the transmitter, to an exponential attenuation as this distance is increased - as it is observed in practice. Our analysis suggests using a simple exponential path loss formula as an alternative to the empirical formulas that are often used for prediction. Results are validated by comparison with experimental data collected in a small urban cell
Efficient Light and Sound Propagation in Refractive Media with Analytic Ray Curve Tracer
Refractive media is ubiquitous in the natural world, and light and sound propagation in refractive media leads to characteristic visual and acoustic phenomena. Those phenomena are critical for engineering applications to simulate with high accuracy requirements, and they can add to the perceived realism and sense of immersion for training and entertainment applications. Existing methods can be roughly divided into two categories with regard to their handling of propagation in refractive media; first category of methods makes simplifying assumption about the media or entirely excludes the consideration of refraction in order to achieve efficient propagation, while the second category of methods accommodates refraction but remains computationally expensive. In this dissertation, we present algorithms that achieve efficient and scalable propagation simulation of light and sound in refractive media, handling fully general media and scene configurations. Our approaches are based on ray tracing, which traditionally assumes homogeneous media and rectilinear rays. We replace the rectilinear rays with analytic ray curves as tracing primitives, which represent closed-form trajectory solutions based on assumptions of a locally constant media gradient. For general media profiles, the media can be spatially decomposed into explicit or implicit cells, within which the media gradient can be assumed constant, leading to an analytic ray path within that cell. Ray traversal of the media can therefore proceed in segments of ray curves. The first source of speedup comes from the fact that for smooth media, a locally constant media gradient assumption tends to stay valid for a larger area than the assumption of a locally constant media property. The second source of speedup is the constant-cost intersection computation of the analytic ray curves with planar surfaces. The third source of speedup comes from making the size of each cell and therefore each ray curve segment adaptive to the magnitude of media gradient. Interactions with boundary surfaces in the scene can be efficiently handled within this framework in two alternative approaches. For static scenes, boundary surfaces can be embedded into the explicit mesh of tetrahedral cells, and the mesh can be traversed and the embedded surfaces intersected with by the analytic ray curve in a unified manner. For dynamic scenes, implicit cells are used for media traversal, and boundary surface intersections can be handled separately by constructing hierarchical acceleration structures adapted from rectilinear ray tracer. The efficient handling of boundary surfaces is the fourth source of speedup of our propagation path computation. We demonstrate over two orders-of-magnitude performance improvement of our analytic ray tracing algorithms over prior methods for refractive light and sound propagation. We additionally present a complete sound-propagation simulation solution that matches the path computation efficiency achieved by the ray curve tracer. We develop efficient pressure computation algorithm based on analytic evaluations and combine our algorithm with the Gaussian beam for fast acoustic field computation. We validate the accuracy of the simulation results on published benchmarks, and we show the application of our algorithms on complex and general three-dimensional outdoor scenes. Our algorithms enable simulation scenarios that are simply not feasible with existing methods, and they have the potential of being extended and complementing other propagation methods for capability beyond handling refractive media.Doctor of Philosoph
Spatial Sound Rendering – A Survey
Simulating propagation of sound and audio rendering can improve the sense of realism and the immersion both in complex acoustic environments and dynamic virtual scenes. In studies of sound auralization, the focus has always been on room acoustics modeling, but most of the same methods are also applicable in the construction of virtual environments such as those developed to facilitate computer gaming, cognitive research, and simulated training scenarios. This paper is a review of state-of-the-art techniques that are based on acoustic principles that apply not only to real rooms but also in 3D virtual environments. The paper also highlights the need to expand the field of immersive sound in a web based browsing environment, because, despite the interest and many benefits, few developments seem to have taken place within this context. Moreover, the paper includes a list of the most effective algorithms used for modelling spatial sound propagation and reports their advantages and disadvantages. Finally, the paper emphasizes in the evaluation of these proposed works
AREPO-RT: Radiation hydrodynamics on a moving mesh
We introduce AREPO-RT, a novel radiation hydrodynamic (RHD) solver for the
unstructured moving-mesh code AREPO. Our method solves the moment-based
radiative transfer equations using the M1 closure relation. We achieve second
order convergence by using a slope limited linear spatial extrapolation and a
first order time prediction step to obtain the values of the primitive
variables on both sides of the cell interface. A Harten-Lax-Van Leer flux
function, suitably modified for moving meshes, is then used to solve the
Riemann problem at the interface. The implementation is fully conservative and
compatible with the individual timestepping scheme of AREPO. It incorporates
atomic Hydrogen (H) and Helium (He) thermochemistry, which is used to couple
the ultra-violet (UV) radiation field to the gas. Additionally, infrared
radiation is coupled to the gas under the assumption of local thermodynamic
equilibrium between the gas and the dust. We successfully apply our code to a
large number of test problems, including applications such as the expansion of
regions, radiation pressure driven outflows and the levitation
of optically thick layer of gas by trapped IR radiation. The new implementation
is suitable for studying various important astrophysical phenomena, such as the
effect of radiative feedback in driving galactic scale outflows, radiation
driven dusty winds in high redshift quasars, or simulating the reionisation
history of the Universe in a self consistent manner.Comment: v2, accepted for publication in MNRAS, changed to a Strang split
scheme to achieve second order convergenc
INVESTIGATION OF LIGHT TRANSPORT AND SCATTERING IN TURBULENT CLOUDS: SIMULATIONS AND LABORATORY MEASUREMENTS
A better understanding of light transport and scattering in turbulent clouds is needed for more accurate remote sensing, improved imaging and signal transmission through atmospheric aerosol and fog, and deeper understanding of cloud optical properties relevant to weather and climate. In this study, we investigate the impact of light scattering in clouds on two problems of atmospheric relevance.
In the first part, we examine deleterious effects of the atmosphere on remotely acquired images including signal attenuation and potential blurring due to forward-scattered light accepted by the imaging system. A prior proposed aerosol scattering model provides a method for calculating the contrast and spatial detail expected when imaging through atmospheres with significant aerosol optical depth. We compare modulation transfer functions obtained directly from images taken through a cloud chamber to those calculated from theory using measured cloud properties. We find that the significance of scattering-induced optical blurring depends sensitively on the properties of both the particles and the imaging system. The theoretical aerosol expression modulation transfer function capture the basic behavior of the system, with deviations likely a result of not accounting for broad particle size distributions.
In the second part, we investigate how clusters and voids in the spatial distributions of particles within a cloud cause light transport to deviate from the exponential extinction law. We explore both perfectly random and correlated scattering media with a Monte Carlo ray tracing program, and find that the degree of non-exponential attenuation can be characterized by the radial distribution function. Our numerical observations regarding direct, diffuse and backward radiative transfer are shown to be consistent with a previous “cloudlet” approach, providing a bridge between the analytical cloudlet model and continuous correlation function approaches. Finally, we numerically explore light propagation through turbulent clouds with polydisperse size distributions calculated by a large eddy simulation of the MTU Pi Chamber. We find that both the mean and standard deviation of direct and diffuse forward flux change when clustering exists, and make suggestions for future laboratory cloud chamber experiments to detect the presence of spatial correlation
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