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If you cannot find anything more, look for something else (Bridget Fountain)
fpayan(ad)i3s.unice.fr, sebastien.schneider(ad)ifpen.fr, laurent.duval(ad)ifpen.fr
Team: Sébastien Schneider, Laurent Duval (IFP Energies nouvelles); Frédéric Payan, Marc Antonini (MediaCoding Laboratory, Université Nice-Sophia-Antipolis)
Keywords: Geology, Geometry, Multiresolution analysis, Wavelet transforms, Transform coding, mesh generation, data compression, mesh coding, upscaling, Hexahedral mesh, compression, multiresolution analysis, geometrical discontinuitiesThe 3D mesh objects for dynamic volume simulations have increasing sizes making them complex to store, manipulate and visualize. These digital objects reach the target sizes of billions of cells. They are increasingly used in the context of web applications, collaborative platforms; they therefore need to be efficiently transmitted over the network, and processed on devices with computing power and resolutions of various strength. Thus, compression of these contents becomes a critical scientific challenge with several objectives such as the efficient storage of such data, an interactive time transmission on the network, a progressive display of the content adapted to the devices (see Figure 1), or yet a random and quick access. Their compression in progressive mode allows access without decompressing the entire stored binary file, at some part of the mesh at different levels of resolution. In other words, this mode means to obtain a single binary file, encoding the entire mesh, either lossless compressed or with an allowable loss of information (reducing the initial numerical accuracy of the file) with a low compression rate (less than 10). This is the equivalent of lossless or lossy modes of the JPEG 2000 image compression standard. The aim of this thesis is to propose a new compression method for volumetric meshes able to compress both the geometry and the associated properties, while allowing a progressive decompression adapted to the display devices. Geometry of the mesh can be structured (a set of hexahedral cell with an implied topology) or unstructured (mixture of different cell types: tetrahedron, hexahedron, prism, pyramid, etc.). Following IFPEN requirements, meshes may also be perceived as 4D, with the notion of evolution in time (3D + t). Today there is very few work on the progressive compression of sequences of volumetric meshes, including properties. Also, we propose to focus this thesis on the volumetric mesh compression composed of hexahedrons or tetrahedrons, corresponding to most cases in geosciences and in combustion, and predominantly treated in the literature. We will also look at the refined mesh, that is to say the number of meshes is only increasing or decreasing in a same time sequence. From the geometry point of view, we propose from this sequence to build a "synthetic" mesh, consisting of the maximum number of cells. A less refined mesh can be treated as an instance of that mesh to which a number of cells would be degenerate or "flat", as it is done in geosciences. Each mesh in time will thus be compressed by the data of the "synthetic" compressed mesh (progressively), and auxiliary information encoding the difference of the positions of the nodes between the two meshes. The relative compression of the mesh properties from one mesh to the other remains an open issue. We could learn from so-called up-scaling approaches such as implemented in a previous patent to make predictions of properties at two successive resolution. The motivation of the proposed approach is based on the observation that the coding of the differences between nodes positions, or between properties is generally of lower amplitude than the initial values, and therefore less expensive in storage size, insofar the mesh evolves over time with some regularity. This would allow us to bring us close to the techniques used in video coding, where one is interested in coding the displacement between successive images, with full encoding of reference images at the beginning and the end of the sequence, called intra images. If this approach proved right, we could address in this thesis hybrid mesh composed of tetrahedral and hexahedral cells to go more easily from a given mesh to its refined version. The thesis will build on recent work on compression of volumetric mesh, surface mesh with attributes, as well as the skills of the MediaCoding I3S team in terms of compression. It will be the continuation of an internship (during the summer of 2014) and a post-doctoral work on the same theme, initiated at IFP Energies nouvelles in January 2015 in collaboration with I3S. Multiresolution decompositions are based on linear and non-linear wavelets.