14,329 research outputs found
Parametrization of stochastic inputs using generative adversarial networks with application in geology
We investigate artificial neural networks as a parametrization tool for
stochastic inputs in numerical simulations. We address parametrization from the
point of view of emulating the data generating process, instead of explicitly
constructing a parametric form to preserve predefined statistics of the data.
This is done by training a neural network to generate samples from the data
distribution using a recent deep learning technique called generative
adversarial networks. By emulating the data generating process, the relevant
statistics of the data are replicated. The method is assessed in subsurface
flow problems, where effective parametrization of underground properties such
as permeability is important due to the high dimensionality and presence of
high spatial correlations. We experiment with realizations of binary
channelized subsurface permeability and perform uncertainty quantification and
parameter estimation. Results show that the parametrization using generative
adversarial networks is very effective in preserving visual realism as well as
high order statistics of the flow responses, while achieving a dimensionality
reduction of two orders of magnitude
Volumetric Particle Tracking Velocimetry (PTV) Uncertainty Quantification
We introduce the first comprehensive approach to determine the uncertainty in
volumetric Particle Tracking Velocimetry (PTV) measurements. Volumetric PTV is
a state-of-the-art non-invasive flow measurement technique, which measures the
velocity field by recording successive snapshots of the tracer particle motion
using a multi-camera set-up. The measurement chain involves reconstructing the
three-dimensional particle positions by a triangulation process using the
calibrated camera mapping functions. The non-linear combination of the
elemental error sources during the iterative self-calibration correction and
particle reconstruction steps increases the complexity of the task. Here, we
first estimate the uncertainty in the particle image location, which we model
as a combination of the particle position estimation uncertainty and the
reprojection error uncertainty. The latter is obtained by a gaussian fit to the
histogram of disparity estimates within a sub-volume. Next, we determine the
uncertainty in the camera calibration coefficients. As a final step the
previous two uncertainties are combined using an uncertainty propagation
through the volumetric reconstruction process. The uncertainty in the velocity
vector is directly obtained as a function of the reconstructed particle
position uncertainty. The framework is tested with synthetic vortex ring
images. The results show good agreement between the predicted and the expected
RMS uncertainty values. The prediction is consistent for seeding densities
tested in the range of 0.01 to 0.1 particles per pixel. Finally, the
methodology is also successfully validated for an experimental test case of
laminar pipe flow velocity profile measurement where the predicted uncertainty
is within 17% of the RMS error value
An Alarm System For Segmentation Algorithm Based On Shape Model
It is usually hard for a learning system to predict correctly on rare events
that never occur in the training data, and there is no exception for
segmentation algorithms. Meanwhile, manual inspection of each case to locate
the failures becomes infeasible due to the trend of large data scale and
limited human resource. Therefore, we build an alarm system that will set off
alerts when the segmentation result is possibly unsatisfactory, assuming no
corresponding ground truth mask is provided. One plausible solution is to
project the segmentation results into a low dimensional feature space; then
learn classifiers/regressors to predict their qualities. Motivated by this, in
this paper, we learn a feature space using the shape information which is a
strong prior shared among different datasets and robust to the appearance
variation of input data.The shape feature is captured using a Variational
Auto-Encoder (VAE) network that trained with only the ground truth masks.
During testing, the segmentation results with bad shapes shall not fit the
shape prior well, resulting in large loss values. Thus, the VAE is able to
evaluate the quality of segmentation result on unseen data, without using
ground truth. Finally, we learn a regressor in the one-dimensional feature
space to predict the qualities of segmentation results. Our alarm system is
evaluated on several recent state-of-art segmentation algorithms for 3D medical
segmentation tasks. Compared with other standard quality assessment methods,
our system consistently provides more reliable prediction on the qualities of
segmentation results.Comment: Accepted to ICCV 2019 (10 pages, 4 figures
Bayes' Rays: Uncertainty Quantification for Neural Radiance Fields
Neural Radiance Fields (NeRFs) have shown promise in applications like view
synthesis and depth estimation, but learning from multiview images faces
inherent uncertainties. Current methods to quantify them are either heuristic
or computationally demanding. We introduce BayesRays, a post-hoc framework to
evaluate uncertainty in any pre-trained NeRF without modifying the training
process. Our method establishes a volumetric uncertainty field using spatial
perturbations and a Bayesian Laplace approximation. We derive our algorithm
statistically and show its superior performance in key metrics and
applications. Additional results available at: https://bayesrays.github.io
Nonparametric Uncertainty Quantification for Stochastic Gradient Flows
This paper presents a nonparametric statistical modeling method for
quantifying uncertainty in stochastic gradient systems with isotropic
diffusion. The central idea is to apply the diffusion maps algorithm to a
training data set to produce a stochastic matrix whose generator is a discrete
approximation to the backward Kolmogorov operator of the underlying dynamics.
The eigenvectors of this stochastic matrix, which we will refer to as the
diffusion coordinates, are discrete approximations to the eigenfunctions of the
Kolmogorov operator and form an orthonormal basis for functions defined on the
data set. Using this basis, we consider the projection of three uncertainty
quantification (UQ) problems (prediction, filtering, and response) into the
diffusion coordinates. In these coordinates, the nonlinear prediction and
response problems reduce to solving systems of infinite-dimensional linear
ordinary differential equations. Similarly, the continuous-time nonlinear
filtering problem reduces to solving a system of infinite-dimensional linear
stochastic differential equations. Solving the UQ problems then reduces to
solving the corresponding truncated linear systems in finitely many diffusion
coordinates. By solving these systems we give a model-free algorithm for UQ on
gradient flow systems with isotropic diffusion. We numerically verify these
algorithms on a 1-dimensional linear gradient flow system where the analytic
solutions of the UQ problems are known. We also apply the algorithm to a
chaotically forced nonlinear gradient flow system which is known to be well
approximated as a stochastically forced gradient flow.Comment: Find the associated videos at: http://personal.psu.edu/thb11
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