67,943 research outputs found
Fourier Analysis of Stochastic Sampling Strategies for Assessing Bias and Variance in Integration
Each pixel in a photorealistic, computer generated picture is calculated by approximately integrating all the light arriving at the pixel, from the virtual scene. A common strategy to calculate these high-dimensional integrals is to average the estimates at stochastically sampled locations. The strategy with which the sampled locations are chosen is of utmost importance in deciding the quality of the approximation, and hence rendered image.
We derive connections between the spectral properties of stochastic sampling patterns and the first and second order statistics of estimates of integration using the samples. Our equations provide insight into the assessment of stochastic sampling strategies for integration. We show that the amplitude of the expected Fourier spectrum of sampling patterns is a useful indicator of the bias when used in numerical integration. We deduce that estimator variance is directly dependent on the variance of the sampling spectrum over multiple realizations of the sampling pattern. We then analyse Gaussian jittered sampling, a simple variant of jittered sampling, that allows a smooth trade-off of bias for variance in uniform (regular grid) sampling. We verify our predictions using spectral measurement, quantitative integration experiments and qualitative comparisons of rendered images.</jats:p
Spectrum of the Laplace-Beltrami Operator and the Phase Structure of Causal Dynamical Triangulation
We propose a new method to characterize the different phases observed in the
non-perturbative numerical approach to quantum gravity known as Causal
Dynamical Triangulation. The method is based on the analysis of the eigenvalues
and the eigenvectors of the Laplace-Beltrami operator computed on the
triangulations: it generalizes previous works based on the analysis of
diffusive processes and proves capable of providing more detailed information
on the geometric properties of the triangulations. In particular, we apply the
method to the analysis of spatial slices, showing that the different phases can
be characterized by a new order parameter related to the presence or absence of
a gap in the spectrum of the Laplace-Beltrami operator, and deriving an
effective dimensionality of the slices at the different scales. We also propose
quantities derived from the spectrum that could be used to monitor the running
to the continuum limit around a suitable critical point in the phase diagram,
if any is found.Comment: 21 pages, 26 figures, 2 table
Comment of Global dynamics of biological systems
In a recent study, (Grigorov, 2006) analyzed temporal gene expression
profiles (Arbeitman et al., 2002) generated in a Drosophila experiment using
SSA in conjunction with Monte-Carlo SSA. The author (Grigorov, 2006) makes
three important claims in his article, namely:
Claim1: A new method based on the theory of nonlinear time series analysis is
used to capture the global dynamics of the fruit-fly cycle temporal gene
expression profiles.
Claim 2: Flattening of a significant part of the eigen-spectrum confirms the
hypothesis about an underly-ing high-dimensional chaotic generating process.
Claim 3: Monte-Carlo SSA can be used to establish whether a given time series
is distinguishable from any well-defined process including deterministic chaos.
In this report we present fundamental concerns with respect to the above
claims (Grigorov, 2006) in a systematic manner with simple examples. The
discussion provided especially discourages the choice of SSA for inferring
nonlinear dynamical structure form time series obtained in any biological
paradigm.Comment: 6 pages, 2 figure
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