Montage non linéaire | EPFL Graph Search

vignette|Capture d'écran du logiciel de montage non linéaire Pitivi En vidéo et en audio, un système de montage non linéaire permet un accès direct (dit parfois aléatoire) sur les images ou les sons sources captés. C'est un montage non destructif. Principe Le montage non linéaire pour les films et la postproduction vidéo permet d'accéder directement, grâce à un ordinateur, à toute image d'une séquence vidéo ou tout passage d'une piste numérique sans va-et-vient à travers des passages intermédiaires. Semblable à la technique de « couper-coller » utilisée dans le montage de film photochimique, il permet d'ajouter au passage plusieurs effets vidéo tels l'effet sépia ou le ralenti. Montage vidéo Références Voir aussi Articles connexes Liens externes Catégorie:Vidéo numérique Catégorie:Montage

Maintainable Online Classes

Alexandre David Koo

This project was initialized by Professor Prandoni from EPFL in the context of a bachelor project entitled "Maintainable Online Coursese". The motivations for this project was the lack of intuitive slideshow annotation software considering the current boom of online classes, most notably the MOOCs. One of the major limitations of current options are the fact that annotations and video tracks are interwoven, giving very little editing power after a recording has been done. Such editing could come in handy to correct mistakes or update a video lecture. In this application, we attempt to seperate out the two worlds of annotations and slide transitions(video) and experiement to see if such a setup could be beneficial to the workflow. In order to achieve this result, the approach has been to carefully and precisely register every slide transition and pen stroke timing, to then be able to succesfully replay the session but with full modularity when it comes to editing and modifications.

2016

Numerical study of transitions in Taylor-Couette flow

This thesis is divided into two parts. In the first one the creation of a complete numerical tool, from the mesh generation to the data treatment, is presented. However time consuming this part has been, this is not the most important one. The second and more important part is concerned with the description and physical interpretation of the results obtained with the numerical tool previously developed. The numerical tool, which exists in its quasi-final version since mid-1997, fully satisfies the following requirements. It is specifically designed to study the flow in the annular space between coaxial, differentially-rotating cylinders of finite length. The spectral element method is used for the space discretization. The study of transition requires the high accuracy warranted by this type of method. The numerical scheme has also to be efficient. This requirement is satisfied, due to the fully-explicit time scheme adopted, where both diffusive and non-linear terms of the Navier-Stokes equations are treated explicitly, and the direct inversion of the pseudo-Laplacian matrix applied to the pressure. This inversion is performed in the most efficient way with a fast diagonalization technique. In the Reynolds numbers range we are interested in, the time-step limitation due to the linear viscous term is only slightly more stringent than the one due to the non-linear term. The last requirement that has been fulfilled has been to design as simple a code as possible. The entire code is constructed from a number of well-known algorithms, fitted together to enhance efficiency. However, the way we regularize the boundary conditions is new. It represents the physics more precisely than Tavener et al. [65]. There is a second original feature in our code, which is linked to the time scheme. We derived our time discretization from the scheme of Gavrilakis et al. [31]. Their scheme cannot be applied to cylindrical coordinates as it is. We thus had to modify it. The second part of the thesis consists of the numerical study of the first transitions of the Taylor-Couette flow in a finite-length geometry. The aspect ratio between the length of the cylinders and the gap between them has been chosen equal to twelve; this is small enough for the effects of the upper and lower boundaries of the flow to be significant. It is believed that these end-effects play a non-negligible role in the transition of the flow. On the other hand, the aspect ratio is large enough to make qualitative comparisons with the infinite-length case, which has been studied extensively both theoretically and numerically. The transition process depends on a relatively large number of parameters. Our investigation focuses on the case where the cylinders rotate in opposite directions. The study of counter-rotating Taylor-Couette flow for a large but finite aspect ratio is the main originality of this thesis. Furthermore, the physical mechanism of the appearance of the second bifurcation in classical Taylor-Couette flow, where only the inner cylinder is rotating, is described in the light of the non-linear interaction between velocity and vorticity.

EPFL1999

Non-linear prediction: a parsimonious family of predictors

The analysis of an observed univariate time series is often undertaken in order to get a prediction of a future event. With this purpose one can fix a class of predictors from which the optimal one will be identified and estimated. The more simple and common choice is the linear family, that is linear combinations of the lags of the series. However, it is well known that considering non-linearities in the lags may improve the prediction. We introduce in this paper a class of non-linear predictors based on polynomials and neural network methodology. These predictors have both the advantages of being relatively simple to identify and of introducing non-linearity without increasing the number of estimated parameters by much compared to linear predictors

1995

Numerical study of transitions in Taylor-Couette flow

EPFL1999