3,221 research outputs found
Sampling and Reconstruction of Shapes with Algebraic Boundaries
We present a sampling theory for a class of binary images with finite rate of
innovation (FRI). Every image in our model is the restriction of
\mathds{1}_{\{p\leq0\}} to the image plane, where \mathds{1} denotes the
indicator function and is some real bivariate polynomial. This particularly
means that the boundaries in the image form a subset of an algebraic curve with
the implicit polynomial . We show that the image parameters --i.e., the
polynomial coefficients-- satisfy a set of linear annihilation equations with
the coefficients being the image moments. The inherent sensitivity of the
moments to noise makes the reconstruction process numerically unstable and
narrows the choice of the sampling kernels to polynomial reproducing kernels.
As a remedy to these problems, we replace conventional moments with more stable
\emph{generalized moments} that are adjusted to the given sampling kernel. The
benefits are threefold: (1) it relaxes the requirements on the sampling
kernels, (2) produces annihilation equations that are robust at numerical
precision, and (3) extends the results to images with unbounded boundaries. We
further reduce the sensitivity of the reconstruction process to noise by taking
into account the sign of the polynomial at certain points, and sequentially
enforcing measurement consistency. We consider various numerical experiments to
demonstrate the performance of our algorithm in reconstructing binary images,
including low to moderate noise levels and a range of realistic sampling
kernels.Comment: 12 pages, 14 figure
T4DT: Tensorizing Time for Learning Temporal 3D Visual Data
Unlike 2D raster images, there is no single dominant representation for 3D
visual data processing. Different formats like point clouds, meshes, or
implicit functions each have their strengths and weaknesses. Still, grid
representations such as signed distance functions have attractive properties
also in 3D. In particular, they offer constant-time random access and are
eminently suitable for modern machine learning. Unfortunately, the storage size
of a grid grows exponentially with its dimension. Hence they often exceed
memory limits even at moderate resolution. This work explores various low-rank
tensor formats, including the Tucker, tensor train, and quantics tensor train
decompositions, to compress time-varying 3D data. Our method iteratively
computes, voxelizes, and compresses each frame's truncated signed distance
function and applies tensor rank truncation to condense all frames into a
single, compressed tensor that represents the entire 4D scene. We show that
low-rank tensor compression is extremely compact to store and query
time-varying signed distance functions. It significantly reduces the memory
footprint of 4D scenes while surprisingly preserving their geometric quality.
Unlike existing iterative learning-based approaches like DeepSDF and NeRF, our
method uses a closed-form algorithm with theoretical guarantees
TT-NF: Tensor Train Neural Fields
Learning neural fields has been an active topic in deep learning research,
focusing, among other issues, on finding more compact and easy-to-fit
representations. In this paper, we introduce a novel low-rank representation
termed Tensor Train Neural Fields (TT-NF) for learning neural fields on dense
regular grids and efficient methods for sampling from them. Our representation
is a TT parameterization of the neural field, trained with backpropagation to
minimize a non-convex objective. We analyze the effect of low-rank compression
on the downstream task quality metrics in two settings. First, we demonstrate
the efficiency of our method in a sandbox task of tensor denoising, which
admits comparison with SVD-based schemes designed to minimize reconstruction
error. Furthermore, we apply the proposed approach to Neural Radiance Fields,
where the low-rank structure of the field corresponding to the best quality can
be discovered only through learning.Comment: Preprint, under revie
PyFR: An Open Source Framework for Solving Advection-Diffusion Type Problems on Streaming Architectures using the Flux Reconstruction Approach
High-order numerical methods for unstructured grids combine the superior
accuracy of high-order spectral or finite difference methods with the geometric
flexibility of low-order finite volume or finite element schemes. The Flux
Reconstruction (FR) approach unifies various high-order schemes for
unstructured grids within a single framework. Additionally, the FR approach
exhibits a significant degree of element locality, and is thus able to run
efficiently on modern streaming architectures, such as Graphical Processing
Units (GPUs). The aforementioned properties of FR mean it offers a promising
route to performing affordable, and hence industrially relevant,
scale-resolving simulations of hitherto intractable unsteady flows within the
vicinity of real-world engineering geometries. In this paper we present PyFR,
an open-source Python based framework for solving advection-diffusion type
problems on streaming architectures using the FR approach. The framework is
designed to solve a range of governing systems on mixed unstructured grids
containing various element types. It is also designed to target a range of
hardware platforms via use of an in-built domain specific language based on the
Mako templating engine. The current release of PyFR is able to solve the
compressible Euler and Navier-Stokes equations on grids of quadrilateral and
triangular elements in two dimensions, and hexahedral elements in three
dimensions, targeting clusters of CPUs, and NVIDIA GPUs. Results are presented
for various benchmark flow problems, single-node performance is discussed, and
scalability of the code is demonstrated on up to 104 NVIDIA M2090 GPUs. The
software is freely available under a 3-Clause New Style BSD license (see
www.pyfr.org)
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