53,195 research outputs found
The Iray Light Transport Simulation and Rendering System
While ray tracing has become increasingly common and path tracing is well
understood by now, a major challenge lies in crafting an easy-to-use and
efficient system implementing these technologies. Following a purely
physically-based paradigm while still allowing for artistic workflows, the Iray
light transport simulation and rendering system allows for rendering complex
scenes by the push of a button and thus makes accurate light transport
simulation widely available. In this document we discuss the challenges and
implementation choices that follow from our primary design decisions,
demonstrating that such a rendering system can be made a practical, scalable,
and efficient real-world application that has been adopted by various companies
across many fields and is in use by many industry professionals today
Ray Tracing And Global Illumination
In order to represent real-world images with a computer, a program has to relate three-dimensional images on a two-dimensional monitor screen. Several ways of doing this exist with varying degrees of realism. One of the most successful methods can be grouped in a screen-to-world method of viewing, which is also known as ray-tracing. This computer graphics technology simulates light rays within a 3D environment. Since light rays have predictable physical properties, the raytracing algorithm can attempt to calculate the exact coloring of each ray/object intersection at any given pixel. Advanced levels of ray tracing allow light rays to bounce from object to object, mimicking what they do in real life. Local illumination represents the basic form of ray tracing. It only takes into account the relationship between light sources and a single object, but does not consider the effects that result from the presence of multiple objects. For instance, a light source can be intersected by another surface and therefore be obscured to any point behind that surface. Similarly, light can be contributed not by a light source, but by a reflection of light from some other object. The local illumination model does not visually show this reflection of light. Therefore, special techniques have to be used to represent these effects. In real life there are often multiple sources of light and multiple reflecting objects that interact with each other in many ways. Global illumination, the more advanced form of ray tracing, adds to the local model by reflecting light from surrounding surfaces to the object. A global illumination model is more comprehensive, more physically correct, and it produces more realistic images.Ray tracing is an essential subject when it comes to computer graphics. It combines issues of efficiency and realism, thus finding a favorable balance of the time and effort involved to make realistic three dimensional images. In the process of researching the many different ways of implementing a ray tracer, the study began with local illumination and graduated to global illumination, using some pre-established techniques and the development of new techniques
General Relativistic Ray-Tracing Method for Estimating the Energy and Momentum Deposition by Neutrino Pair Annihilation in Collapsars
Bearing in mind the application to the collapsar models of gamma-ray bursts
(GRBs), we develop a numerical scheme and code for estimating the deposition of
energy and momentum due to the neutrino pair annihilation () in the vicinity of accretion tori around a Kerr
black hole. Our code is designed to solve the general relativistic neutrino
transfer by a ray-tracing method. To solve the collisional Boltzmann equation
in curved spacetime, we numerically integrate the so-called rendering equation
along the null geodesics. For the neutrino opacity, the charged-current
-processes are taken into account, which are dominant in the vicinity of
the accretion tori. The numerical accuracy of the developed code is
certificated by several tests, in which we show comparisons with the
corresponding analytic solutions. Based on the hydrodynamical data in our
collapsar simulation, we estimate the annihilation rates in a post-processing
manner. Increasing the Kerr parameter from 0 to 1, it is found that the general
relativistic effect can increase the local energy deposition rate by about one
order of magnitude, and the net energy deposition rate by several tens of
percents. After the accretion disk settles into a stationary state (typically
later than s from the onset of gravitational collapse), we point out
that the neutrino-heating timescale in the vicinity of the polar funnel region
can be shorter than the dynamical timescale. Our results suggest the neutrino
pair annihilation has a potential importance equal to the conventional
magnetohydrodynamic mechanism for igniting the GRB fireballs.Comment: 33 pages, 15 figures, accepted to the Ap
Geant4 simulations of soft proton scattering in X-ray optics. A tentative validation using laboratory measurements
Low energy protons (< 300 keV) can enter the field of view of X-ray space
telescopes, scatter at small incident angles, and deposit energy on the
detector, causing intense background flares at the focal plane or in the most
extreme cases, damaging the X-ray detector. A correct modelization of the
physics process responsible for the grazing angle scattering processes is
mandatory to evaluate the impact of such events on the performance of future
X-ray telescopes as the ESA ATHENA mission. For the first time the Remizovich
model, in the approximation of no energy losses, is implemented top of the
Geant4 release 10.2. Both the new scattering physics and the built-in Coulomb
scattering are used to reproduce the latest experimental results on grazing
angle proton scattering. At 250 keV multiple scattering delivers large proton
angles and it is not consistent with the observation. Among the tested models,
the single scattering seems to better reproduce the scattering efficiency at
the three energies but energy loss obtained at small scattering angles is
significantly lower than the experimental values. In general, the energy losses
obtained in the experiment are higher than what obtained by the simulation. The
experimental data are not completely representative of the soft proton
scattering experienced by current X-ray telescopes because of the lack of
measurements at low energies (< 200 keV) and small reflection angles, so we are
not able to address any of the tested models as the one that can certainly
reproduce the scattering behavior of low energy protons expected for the ATHENA
mission. We can, however, discard multiple scattering as the model able to
reproduce soft proton funneling, and affirm that Coulomb single scattering can
represent, until further measurements, the best approximation of the proton
scattered angular distribution at the exit of X-ray optics.Comment: submitted to Experimental Astronom
Overcoming the Challenges Associated with Image-based Mapping of Small Bodies in Preparation for the OSIRIS-REx Mission to (101955) Bennu
The OSIRIS-REx Asteroid Sample Return Mission is the third mission in NASA's
New Frontiers Program and is the first U.S. mission to return samples from an
asteroid to Earth. The most important decision ahead of the OSIRIS-REx team is
the selection of a prime sample-site on the surface of asteroid (101955) Bennu.
Mission success hinges on identifying a site that is safe and has regolith that
can readily be ingested by the spacecraft's sampling mechanism. To inform this
mission-critical decision, the surface of Bennu is mapped using the OSIRIS-REx
Camera Suite and the images are used to develop several foundational data
products. Acquiring the necessary inputs to these data products requires
observational strategies that are defined specifically to overcome the
challenges associated with mapping a small irregular body. We present these
strategies in the context of assessing candidate sample-sites at Bennu
according to a framework of decisions regarding the relative safety,
sampleability, and scientific value across the asteroid's surface. To create
data products that aid these assessments, we describe the best practices
developed by the OSIRIS-REx team for image-based mapping of irregular small
bodies. We emphasize the importance of using 3D shape models and the ability to
work in body-fixed rectangular coordinates when dealing with planetary surfaces
that cannot be uniquely addressed by body-fixed latitude and longitude.Comment: 31 pages, 10 figures, 2 table
A fast GPU Monte Carlo Radiative Heat Transfer Implementation for Coupling with Direct Numerical Simulation
We implemented a fast Reciprocal Monte Carlo algorithm, to accurately solve
radiative heat transfer in turbulent flows of non-grey participating media that
can be coupled to fully resolved turbulent flows, namely to Direct Numerical
Simulation (DNS). The spectrally varying absorption coefficient is treated in a
narrow-band fashion with a correlated-k distribution. The implementation is
verified with analytical solutions and validated with results from literature
and line-by-line Monte Carlo computations. The method is implemented on GPU
with a thorough attention to memory transfer and computational efficiency. The
bottlenecks that dominate the computational expenses are addressed and several
techniques are proposed to optimize the GPU execution. By implementing the
proposed algorithmic accelerations, a speed-up of up to 3 orders of magnitude
can be achieved, while maintaining the same accuracy
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