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Concept

Texture (image de synthèse)

Résumé

Dans le domaine de la , une texture est une image en deux dimensions (2D) que l'on va appliquer sur une surface (2D) ou un volume en trois dimensions (3D) de manière à habiller cette surface ou ce volume. En simplifiant, on peut l'assimiler à un papier peint très plastique et déformable que l'on applique en 3D en spécifiant la transformation géométrique que subit chaque pixel du papier pour s'appliquer sur l'élément 3D. Le pixel ainsi manipulé en 3D est appelé texel. Une texture peut être définie de façon analytique par un algorithme (texture procédurale) ou être constituée d'un tableau de pixels (une image bitmap par exemple). Les textures sont utilisées pour enrichir le rendu photoréaliste des volumes. Il est souvent nécessaire pour des raisons visuelles de recourir à des algorithmes de filtrage (filtrage bilinéaire par exemple), ou à des textures préfiltrées (mipmap) permettant de lisser l'image, de façon à atténuer les artefacts liés au grossissement linéaire des pixels. Le pre

Source officielle

https://fr.wikipedia.org/wiki/Texture_(image_de_synth%C3%A8se)

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Publications associées (82)

This thesis investigates advanced signal processing concepts and their application to geometric processing and transformations of images and volumes. In the first part, we discuss the class of transformations that project volume data onto a plane using parallel line integrals; it is called the X-ray transform. In computer tomography (CT) the problem is to reconstruct the volume from these projections. We consider a basic setup with parallel projection and a geometry model in which the CT scanner rotates around one main axis of the volume. In this case, the problem is separable and reduces to the reconstruction of parallel images (slices of the volume). Each image is reconstructible from a series of 1D projections taken at different angular positions. The standard reconstruction algorithm is the filtered back-projection (FBP). We propose an alternative discretization of the Radon transform and of its inverse that is based on least-squares approximation and the convolution of splines. It improves the quality of the transform significantly. Next we discuss volume rendering based on the X-ray transform. The volume is represented by a multiresolution wavelet decomposition. The wavelets are projected onto an adaptive multiresolution 2D grid. The multiresolution grid allows to speed up the rendering process especially at coarse scales. In the second part of the thesis, we discuss transformations that warp images. In computer graphics, this is called texture mapping. Simple warps, such as shear, rotation, or zoom, can be computed by least-squares sampling, e.g. again with convolutions of splines. For more general warps there is not an easy continuous solution, if an analytical one exists at all. After a review of existing texture mapping methods, we propose a novel recursive one, which minimizes information loss. For this purpose, the texture is reconstructed from the mapped image and compared to the original texture. This algorithm outperforms the existing methods in terms of quality, but its complexity can be very high. A multiresolution version of the algorithm allows to keep the storage requirements and computational complexity within acceptable range. Fast transmission of textures and 3D models over communication links requires low bitrate and progressive compression. We can achieve very low bitrate if we code textures only with the information necessary for a given view of the 3D scene. If the view changes, the missing information will be transmitted. This results in a progressive bitstream for an animated 3D scene. In contrast to recursive texture mapping, we do not back-project the texture, but come up with a heuristic that predicts the information loss. This concept is called view-dependent scalability and we show how to apply it on DCT-based (as part of MPEG-4) and wavelet-based coders. Last, we inspect the question of how to balance the bit budget of a jointly coded mesh and texture for the progressive and view-dependent transmission of a 3D model. By exhaustive search, we find the rate-distortion optimal path. By marginal analysis we find a close solution but at much lower costs (only two evaluated frames per step as compared to a full search).

EPFL2001

Chargement

Large Steps in Inverse Rendering of Geometry

Wenzel Alban Jakob, Baptiste Alexandre Marie Philippe Claude Nicolet

Inverse reconstruction from images is a central problem in many scientific and engineering disciplines. Recent progress on differentiable rendering has led to methods that can efficiently differentiate the full process of image formation with respect to millions of parameters to solve such problems via gradient-based optimization. At the same time, the availability of cheap derivatives does not necessarily make an inverse problem easy to solve. Mesh-based representations remain a particular source of irritation: an adverse gradient step involving vertex positions could turn parts of the mesh inside-out, introduce numerous local self-intersections, or lead to inadequate usage of the vertex budget due to distortion. These types of issues are often irrecoverable in the sense that subsequent optimization steps will further exacerbate them. In other words, the optimization lacks robustness due to an objective function with substantial non-convexity. Such robustness issues are commonly mitigated by imposing additional regularization, typically in the form of Laplacian energies that quantify and improve the smoothness of the current iterate. However, regularization introduces its own set of problems: solutions must now compromise between solving the problem and being smooth. Furthermore, gradient steps involving a Laplacian energy resemble Jacobi's iterative method for solving linear equations that is known for its exceptionally slow convergence. We propose a simple and practical alternative that casts differentiable rendering into the framework of preconditioned gradient descent. Our preconditioner biases gradient steps towards smooth solutions without requiring the final solution to be smooth. In contrast to Jacobi-style iteration, each gradient step propagates information among all variables, enabling convergence using fewer and larger steps. Our method is not restricted to meshes and can also accelerate the reconstruction of other representations, where smooth solutions are generally expected. We demonstrate its superior performance in the context of geometric optimization and texture reconstruction.

ASSOC COMPUTING MACHINERY2021

Chargement

Numerical simulation of a sailing boat

Matteo Lombardi

In this work, various aspects concerning the numerical simulation of a sailing boat are investigated. The attention is focused on simulation of the free-surface, the fluid-structure interaction (FSI) between wind and sails, and the dynamics of the whole boat. Some preliminary work on shape optimization is also presented. Free-surface simulations are carried out both on classical academic benchmark problems and for the prediction of the wave pattern around racing yachts. The comparison with experimental results and numerical data obtained with other CFD codes has proven the validity of the proposed set-up. In this framework, the Level Set method has been implemented and validated as a possible solution to the problem of interface diffusion arising in planing conditions with the standard Volume of Fluid approach. The FSI problem governing the interaction between wind and sails is solved via a strongly coupled segregated approach based on the standard Dirichlet-Neumann coupling. The structural solution is based on a MITC4 finite-element shell code. The fluid and structural meshes are non-conforming and the exchange of information at the interface is obtained via radial basis functions (RBF) interpolation. The fluid mesh motion is accomplished either through the mapping generated using the radial basis functions or via a method based on the inverse distance weighting (IDW) interpolations. The attention is paid to the methodological aspects of this complex problem and to the analysis of the numerical results. The fluid-structure interaction simulations of one and two sail configurations, with different trimmings, are presented; both steady and transient simulations are performed. The results obtained are very encouraging and show the potential of the proposed model. The dynamic motion of the Series 60 hull and the Alinghi’s AC 32 monohull complete of bulb, keel and sails have also been investigated. For the latter in particular, the free sink, trim and roll case has proven to be particularly interesting, with the boat rolling considerably on the side due to the aerodynamic loads exerted by the wind on the sails. Here, the whole boat has been considered as a rigid body but the integration of the FSI module for the sails into the full boat dynamic system is already under development. Finally, a preliminary analysis of shape optimization techniques applied to sailing boat elements have been investigated: in particular the attention has been focused on the drag minimization of (pseudo) bulb geometries under the constraint of fixed volume/fixed righting moment. The shape parametrization have been achieved either using directly the surface element nodes or via the Free Form Deformation (FFD) technique while the optimization algorithm has been based either on the solution of the adjoint Navier-Stokes equations or via pseudo finite-difference algorithms. The algorithm and developments mentioned have been implemented in a common open-source framework, the library OpenFOAM®.

EPFL2012

Chargement

Afficher plus

Large Steps in Inverse Rendering of Geometry

Wenzel Alban Jakob, Baptiste Alexandre Marie Philippe Claude Nicolet

ASSOC COMPUTING MACHINERY2021

EPFL2001

Numerical simulation of a sailing boat

Matteo Lombardi

EPFL2012