A nonlinear static approach for curve editing

International audienceThis paper introduces a method for interactively editing planar curves subject to positional and rotational constraints. We regard editing as a static deformation problem but our treatment differs from standard finite element methods in the sense that the interpolation is based on the deformation modes rather than the classic shape functions. A careful choice of these modes allows capturing the deformation behavior of the individual curve segments, and devising the underlying mathematical model from simple and tractable physical considerations. In order to correctly handle arbitrary user input (e.g. dragging vertices in a fast and excessive manner), our approach operates in the nonlinear regime. The arising geometric nonlinearities are addressed effectively through the modal representation without requiring complicated fitting strategies. In this way, we circumvent commonly encountered locking and stability issues while conveying a natural sense of flexibility of the shape at hand. Experiments on various editing scenarios including closed and nonsmooth curves demonstrate the robustness of the proposed approach

Numerical and variational aspects of mesh parameterization and editing

A surface parameterization is a smooth one-to-one mapping between the surface and a parametric domain. Typically, surfaces with disk topology are mapped onto the plane and genus-zero surfaces onto the sphere. As any attempt to flatten a non-trivial surface onto the plane will inevitably induce a certain amount of distortion, the main concern of research on this topic is to minimize parametric distortion. This thesis aims at presenting a balanced blend of mathematical rigor and engineering intuition to address the challenges raised by the mesh parameterization problem. We study the numerical aspects of mesh parameterization in the light of parallel developments in both mathematics and engineering. Furthermore, we introduce the concept of quasi-harmonic maps for reducing distortion in the fixed boundary case and extend it to both the free boundary and the spherical case. Thinking of parameterization in a more general sense as the construction of one or several scalar fields on a surface, we explore the potential of this construction for mesh deformation and surface matching. We propose an \u27;on-surface parameterization\u27; for guiding the deformation process and performing surface matching. A direct harmonic interpolation in the quaternion domain is also shown to give promising results for deformation transfer.Eine Flächenparameterisierung ist eine globale bijektive Abbildung zwischen der Fläche und einem zugehörigen parametrischen Gebiet. Gewöhnlich werden Flächen mit scheibenförmiger Topologie auf eine Kreisscheibe und Flächen mit Genus Null auf eine Sphäre abgebildet. Das Hauptinteresse der Forschung an diesem Thema ist die Minimierung der parametrischen Verzerrung, die unweigerlich bei jedem Versuch, eine nicht triviale Fläche über einer Ebene zu parameterisieren, erzeugt wird. Diese Arbeit strebt zur Behandlung des Parametrisierungsproblems eine ausgeglichene Mischung zwischen mathematischer Präzision und ingenieurwissenschaftlicher Intuition an. Wir behandeln dabei die numerischen Aspekte des Parameterisierungsproblems im Hinblick auf die aktuellen parallelen Entwicklungen in der Mathematik und den Ingenieurwissenschaften. Weiterhin führen wir das Konzept der quasi-harmonischen Abbildungen ein, um die Verzerrung bei gegebenen Randbedingungen zu verringern. Anschließend verallgemeinern wir dieses Konzept auf den sphärischen Fall und auf den Fall mit freien Randbedingungen. Durch allgemeinere Betrachtung der Parameterisierung als Konstruktion eines oder mehrerer skalarer Felder auf einer Fläche ergibt sich ein neuer Ansatz zur Netzdeformation und der Erzeugung von Flächenkorrespondenzen. Wir stellen eine \u27;on-surface parameterization\u27; vor, welche den Deformationsprozess leitet und Flächenkorrespondenzen erstellt. Darüber hinaus zeigt eine direkte harmonische Interpolation in der Domäne der Quaternionen auch vielversprechende Resultate für die Übertragung von Deformationen

AlSub: Fully Parallel and Modular Subdivision

In recent years, mesh subdivision---the process of forging smooth free-form surfaces from coarse polygonal meshes---has become an indispensable production instrument. Although subdivision performance is crucial during simulation, animation and rendering, state-of-the-art approaches still rely on serial implementations for complex parts of the subdivision process. Therefore, they often fail to harness the power of modern parallel devices, like the graphics processing unit (GPU), for large parts of the algorithm and must resort to time-consuming serial preprocessing. In this paper, we show that a complete parallelization of the subdivision process for modern architectures is possible. Building on sparse matrix linear algebra, we show how to structure the complete subdivision process into a sequence of algebra operations. By restructuring and grouping these operations, we adapt the process for different use cases, such as regular subdivision of dynamic meshes, uniform subdivision for immutable topology, and feature-adaptive subdivision for efficient rendering of animated models. As the same machinery is used for all use cases, identical subdivision results are achieved in all parts of the production pipeline. As a second contribution, we show how these linear algebra formulations can effectively be translated into efficient GPU kernels. Applying our strategies to $\sqrt{3}$, Loop and Catmull-Clark subdivision shows significant speedups of our approach compared to state-of-the-art solutions, while we completely avoid serial preprocessing.Comment: Changed structure Added content Improved description

Distortion driven variational multi-view reconstruction

International audienceThis paper revisits variational multi-view stereo and identifies two issues pertaining to matching and view merging: i) regions with low visibility and relatively high depth variation are only resolved by the sole regularizer contribution. This often induces wrong matches which tend to bleed into neighboring regions, and more importantly distort nearby features. ii) small matching errors can lead to overlapping surface layers which cannot be easily addressed by standard outlier removal techniques. In both scenarios, we rely on the analysis of the distortion of spatial and planar maps in order to improve the quality of the reconstruction. At the matching level, an anisotropic diffusion driven by spatial grid distortion is proposed to steer grid lines away from those problematic regions. At the merging level, advantage is taken of Lambert's cosine law to favor contributions from image areas where the cosine angle between the surface normal and the line of sight is maximal. Tests on standard benchmarks suggest a good blend between computational efficiency, ease of implementation, and reconstruction quality

Layered Fields for Natural Tessellations on Surfaces

Mimicking natural tessellation patterns is a fascinating multi-disciplinary problem. Geometric methods aiming at reproducing such partitions on surface meshes are commonly based on the Voronoi model and its variants, and are often faced with challenging issues such as metric estimation, geometric, topological complications, and most critically parallelization. In this paper, we introduce an alternate model which may be of value for resolving these issues. We drop the assumption that regions need to be separated by lines. Instead, we regard region boundaries as narrow bands and we model the partition as a set of smooth functions layered over the surface. Given an initial set of seeds or regions, the partition emerges as the solution of a time dependent set of partial differential equations describing concurrently evolving fronts on the surface. Our solution does not require geodesic estimation, elaborate numerical solvers, or complicated bookkeeping data structures. The cost per time-iteration is dominated by the multiplication and addition of two sparse matrices. Extension of our approach in a Lloyd's algorithm fashion can be easily achieved and the extraction of the dual mesh can be conveniently preformed in parallel through matrix algebra. As our approach relies mainly on basic linear algebra kernels, it lends itself to efficient implementation on modern graphics hardware.Comment: Natural tessellations, surface fields, Voronoi diagrams, Lloyd's algorith

Convex Boundary Angle

Angle Based Flattening is a robust parameterization method that finds a quasi-conformal mapping by solving a non-linear optimization problem. We take advantage of a characterization of convex planar drawings of triconnected graphs to introduce new boundary constraints. This prevents boundary intersections and avoids post-processing of the parameterized mesh. We present a simple transformation to effectively relax the constrained minimization problem, which improves the convergence of the optimization method. As a natural extension, we discuss the construction of Delaunay flat meshes. This may further enhance the quality of the resulting parameterization

Discrete tensorial quasiharmonic maps

We introduce new linear operators for surface parameterization. Given an initial mapping from the parametric plane onto a surface mesh, we establish a secondary map from the plane onto itself that mimics the initial one. The resulting low-distortion parameterization is smooth as it stems from solving a quasi-harmonic equation. Our parameterization method is robust and independent of (the quality of) the initial map. 1

Abstract Efficient Iterative Solvers for Angle Based Flattening

In this paper, we derive an efficient approach for solving the optimization problem which arises in Angle Based Flattening. As the size of system matrix associated with the ABF blows up to a size approximately five times the number of faces, finding a solution for reasonably sized triangular meshes becomes a challenging problem. We propose two iterative approaches to overcome these limitations. The first approach is based on the fact that decoupling the system yields a much easier matrix equation. This enables an iterative approach for carrying out the computations. The second approach is based on the analysis of saddle point problems. We derive a modified Uzawa algorithm for efficiently addressing the numerical optimization problem.