331 research outputs found
Bilinear Parameterization For Differentiable Rank-Regularization
Low rank approximation is a commonly occurring problem in many computer
vision and machine learning applications. There are two common ways of
optimizing the resulting models. Either the set of matrices with a given rank
can be explicitly parametrized using a bilinear factorization, or low rank can
be implicitly enforced using regularization terms penalizing non-zero singular
values. While the former approach results in differentiable problems that can
be efficiently optimized using local quadratic approximation, the latter is
typically not differentiable (sometimes even discontinuous) and requires first
order subgradient or splitting methods. It is well known that gradient based
methods exhibit slow convergence for ill-conditioned problems.
In this paper we show how many non-differentiable regularization methods can
be reformulated into smooth objectives using bilinear parameterization. This
allows us to use standard second order methods, such as Levenberg--Marquardt
(LM) and Variable Projection (VarPro), to achieve accurate solutions for
ill-conditioned cases. We show on several real and synthetic experiments that
our second order formulation converges to substantially more accurate solutions
than competing state-of-the-art methods.Comment: 17 page
Accurate Optimization of Weighted Nuclear Norm for Non-Rigid Structure from Motion
Fitting a matrix of a given rank to data in a least squares sense can be done
very effectively using 2nd order methods such as Levenberg-Marquardt by
explicitly optimizing over a bilinear parameterization of the matrix. In
contrast, when applying more general singular value penalties, such as weighted
nuclear norm priors, direct optimization over the elements of the matrix is
typically used. Due to non-differentiability of the resulting objective
function, first order sub-gradient or splitting methods are predominantly used.
While these offer rapid iterations it is well known that they become inefficent
near the minimum due to zig-zagging and in practice one is therefore often
forced to settle for an approximate solution.
In this paper we show that more accurate results can in many cases be
achieved with 2nd order methods. Our main result shows how to construct
bilinear formulations, for a general class of regularizers including weighted
nuclear norm penalties, that are provably equivalent to the original problems.
With these formulations the regularizing function becomes twice differentiable
and 2nd order methods can be applied. We show experimentally, on a number of
structure from motion problems, that our approach outperforms state-of-the-art
methods
Towards Reliable and Accurate Global Structure-from-Motion
Reconstruction of objects or scenes from sparse point detections across multiple views is one of the most tackled problems in computer vision. Given the coordinates of 2D points tracked in multiple images, the problem consists of estimating the corresponding 3D points and cameras\u27 calibrations (intrinsic and pose), and can be solved by minimizing reprojection errors using bundle adjustment. However, given bundle adjustment\u27s nonlinear objective function and iterative nature, a good starting guess is required to converge to global minima. Global and Incremental Structure-from-Motion methods appear as ways to provide good initializations to bundle adjustment, each with different properties. While Global Structure-from-Motion has been shown to result in more accurate reconstructions compared to Incremental Structure-from-Motion, the latter has better scalability by starting with a small subset of images and sequentially adding new views, allowing reconstruction of sequences with millions of images. Additionally, both Global and Incremental Structure-from-Motion methods rely on accurate models of the scene or object, and under noisy conditions or high model uncertainty might result in poor initializations for bundle adjustment. Recently pOSE, a class of matrix factorization methods, has been proposed as an alternative to conventional Global SfM methods. These methods use VarPro - a second-order optimization method - to minimize a linear combination of an approximation of reprojection errors and a regularization term based on an affine camera model, and have been shown to converge to global minima with a high rate even when starting from random camera calibration estimations.This thesis aims at improving the reliability and accuracy of global SfM through different approaches. First, by studying conditions for global optimality of point set registration, a point cloud averaging method that can be used when (incomplete) 3D point clouds of the same scene in different coordinate systems are available. Second, by extending pOSE methods to different Structure-from-Motion problem instances, such as Non-Rigid SfM or radial distortion invariant SfM. Third and finally, by replacing the regularization term of pOSE methods with an exponential regularization on the projective depth of the 3D point estimations, resulting in a loss that achieves reconstructions with accuracy close to bundle adjustment
Tensor Regression with Applications in Neuroimaging Data Analysis
Classical regression methods treat covariates as a vector and estimate a
corresponding vector of regression coefficients. Modern applications in medical
imaging generate covariates of more complex form such as multidimensional
arrays (tensors). Traditional statistical and computational methods are proving
insufficient for analysis of these high-throughput data due to their ultrahigh
dimensionality as well as complex structure. In this article, we propose a new
family of tensor regression models that efficiently exploit the special
structure of tensor covariates. Under this framework, ultrahigh dimensionality
is reduced to a manageable level, resulting in efficient estimation and
prediction. A fast and highly scalable estimation algorithm is proposed for
maximum likelihood estimation and its associated asymptotic properties are
studied. Effectiveness of the new methods is demonstrated on both synthetic and
real MRI imaging data.Comment: 27 pages, 4 figure
- …