69,200 research outputs found
Progressive Transient Photon Beams
In this work we introduce a novel algorithm for transient rendering in
participating media. Our method is consistent, robust, and is able to generate
animations of time-resolved light transport featuring complex caustic light
paths in media. We base our method on the observation that the spatial
continuity provides an increased coverage of the temporal domain, and
generalize photon beams to transient-state. We extend the beam steady-state
radiance estimates to include the temporal domain. Then, we develop a
progressive version of spatio-temporal density estimations, that converges to
the correct solution with finite memory requirements by iteratively averaging
several realizations of independent renders with a progressively reduced kernel
bandwidth. We derive the optimal convergence rates accounting for space and
time kernels, and demonstrate our method against previous consistent transient
rendering methods for participating media
WLC39-1: Transient Analysis for Wireless Power Control
Power control mitigates interference and maintains required QoS levels in cellular wireless networks. An important class of distributed power control (DPC) was proposed by Foschini and Miljanic in 1993, with many variants developed since. Almost all related work focuses on the equilibrium and asymptotic convergence properties. However, for many applications transient behavior is more important. If a link's SIR drops below a critical threshold for too long, the connections over this link will be dropped, rendering the entire concept of equilibrium resource allocation meaningless. This paper proposes a systematic approach to the analysis of transient properties of DPC algorithms, in particular Foschini-Miljanic, based on tools from control theory. Analytically, we present a sufficient condition to ensure that after links reach their minimum SIR levels, their SIR requirements can be guaranteed for future time steps. Computationally, we pose this problem as verifying the invariance of certain regions in the SIR space, which for the basic DPC algorithm can be cast as a Linear Program (LP). Furthermore, using insights gained from the analysis, we propose a preliminary design framework for new iterative power control schemes
A minimization principle for the description of time-dependent modes associated with transient instabilities
We introduce a minimization formulation for the determination of a
finite-dimensional, time-dependent, orthonormal basis that captures directions
of the phase space associated with transient instabilities. While these
instabilities have finite lifetime they can play a crucial role by either
altering the system dynamics through the activation of other instabilities, or
by creating sudden nonlinear energy transfers that lead to extreme responses.
However, their essentially transient character makes their description a
particularly challenging task. We develop a minimization framework that focuses
on the optimal approximation of the system dynamics in the neighborhood of the
system state. This minimization formulation results in differential equations
that evolve a time-dependent basis so that it optimally approximates the most
unstable directions. We demonstrate the capability of the method for two
families of problems: i) linear systems including the advection-diffusion
operator in a strongly non-normal regime as well as the Orr-Sommerfeld/Squire
operator, and ii) nonlinear problems including a low-dimensional system with
transient instabilities and the vertical jet in crossflow. We demonstrate that
the time-dependent subspace captures the strongly transient non-normal energy
growth (in the short time regime), while for longer times the modes capture the
expected asymptotic behavior
The Sound Manifesto
Computing practice today depends on visual output to drive almost all user
interaction. Other senses, such as audition, may be totally neglected, or used
tangentially, or used in highly restricted specialized ways. We have excellent
audio rendering through D-A conversion, but we lack rich general facilities for
modeling and manipulating sound comparable in quality and flexibility to
graphics. We need co-ordinated research in several disciplines to improve the
use of sound as an interactive information channel.
Incremental and separate improvements in synthesis, analysis, speech
processing, audiology, acoustics, music, etc. will not alone produce the
radical progress that we seek in sonic practice. We also need to create a new
central topic of study in digital audio research. The new topic will assimilate
the contributions of different disciplines on a common foundation. The key
central concept that we lack is sound as a general-purpose information channel.
We must investigate the structure of this information channel, which is driven
by the co-operative development of auditory perception and physical sound
production. Particular audible encodings, such as speech and music, illuminate
sonic information by example, but they are no more sufficient for a
characterization than typography is sufficient for a characterization of visual
information.Comment: To appear in the conference on Critical Technologies for the Future
of Computing, part of SPIE's International Symposium on Optical Science and
Technology, 30 July to 4 August 2000, San Diego, C
VIOLA - A multi-purpose and web-based visualization tool for neuronal-network simulation output
Neuronal network models and corresponding computer simulations are invaluable
tools to aid the interpretation of the relationship between neuron properties,
connectivity and measured activity in cortical tissue. Spatiotemporal patterns
of activity propagating across the cortical surface as observed experimentally
can for example be described by neuronal network models with layered geometry
and distance-dependent connectivity. The interpretation of the resulting stream
of multi-modal and multi-dimensional simulation data calls for integrating
interactive visualization steps into existing simulation-analysis workflows.
Here, we present a set of interactive visualization concepts called views for
the visual analysis of activity data in topological network models, and a
corresponding reference implementation VIOLA (VIsualization Of Layer Activity).
The software is a lightweight, open-source, web-based and platform-independent
application combining and adapting modern interactive visualization paradigms,
such as coordinated multiple views, for massively parallel neurophysiological
data. For a use-case demonstration we consider spiking activity data of a
two-population, layered point-neuron network model subject to a spatially
confined excitation originating from an external population. With the multiple
coordinated views, an explorative and qualitative assessment of the
spatiotemporal features of neuronal activity can be performed upfront of a
detailed quantitative data analysis of specific aspects of the data.
Furthermore, ongoing efforts including the European Human Brain Project aim at
providing online user portals for integrated model development, simulation,
analysis and provenance tracking, wherein interactive visual analysis tools are
one component. Browser-compatible, web-technology based solutions are therefore
required. Within this scope, with VIOLA we provide a first prototype.Comment: 38 pages, 10 figures, 3 table
From Equilibrium to Steady-State Dynamics after Switch-On of Shear
A relation between equilibrium, steady-state, and waiting-time dependent
dynamical two-time correlation functions in dense glass-forming liquids subject
to homogeneous steady shear flow is discussed. The systems under study show
pronounced shear thinning, i.e., a significant speedup in their steady-state
slow relaxation as compared to equilibrium. An approximate relation that
recovers the exact limit for small waiting times is derived following the
integration through transients (ITT) approach for the nonequilibrium
Smoluchowski dynamics, and is exemplified within a schematic model in the
framework of the mode-coupling theory of the glass transition (MCT). Computer
simulation results for the tagged-particle density correlation functions
corresponding to wave vectors in the shear-gradient directions from both
event-driven stochastic dynamics of a two-dimensional hard-disk system and from
previously published Newtonian-dynamics simulations of a three-dimensional
soft-sphere mixture are analyzed and compared with the predictions of the
ITT-based approximation. Good qualitative and semi-quantitative agreement is
found. Furthermore, for short waiting times, the theoretical description of the
waiting time dependence shows excellent quantitative agreement to the
simulations. This confirms the accuracy of the central approximation used
earlier to derive fluctuation dissipation ratios (Phys. Rev. Lett. 102,
135701). For intermediate waiting times, the correlation functions decay faster
at long times than the stationary ones. This behavior is predicted by our
theory and observed in simulations.Comment: 16 pages, 12 figures, submitted to Phys Rev
Classical Inflationary and Ekpyrotic Universes in the No-Boundary Wavefunction
This paper investigates the manner in which classical universes are obtained
in the no-boundary quantum state. In this context, universes can be
characterised as classical (in a WKB sense) when the wavefunction is highly
oscillatory, i.e. when the ratio of the change in the amplitude of the
wavefunction becomes small compared to the change in the phase. In the presence
of a positive or negative exponential potential, the WKB condition is satisfied
in proportion to a factor where
is the (constant) slow-roll/fast-roll parameter and designates
the number of e-folds. Thus classicality is reached exponentially fast in ,
but only when (ekpyrosis).
Furthermore, when the potential switches off and the ekpyrotic phase goes over
into a phase of kinetic domination, the level of classicality obtained up to
that point is preserved. Similar results are obtained in a cyclic potential,
where a dark energy plateau is added. Finally, for a potential of the form
(with ), where the classical solution becomes increasingly
kinetic-dominated, there is an initial burst of classicalisation which then
quickly levels off. These results demonstrate that inflation and ekpyrosis,
which are the only dynamical mechanisms known for smoothing the universe, share
the perhaps even more fundamental property of rendering space and time
classical in the first place.Comment: 35 pages, 19 figures, v2: replaced with version to be published in
PR
- …