Discrete cosine transform | EPFL Graph Search

A discrete cosine transform (DCT) expresses a finite sequence of data points in terms of a sum of cosine functions oscillating at different frequencies. The DCT, first proposed by Nasir Ahmed in 1972, is a widely used transformation technique in signal processing and data compression. It is used in most digital media, including (such as JPEG and HEIF), digital video (such as MPEG and H.26x), digital audio (such as Dolby Digital, MP3 and AAC), digital television (such as SDTV, HDTV and VOD), digital radio (such as AAC+ and DAB+), and speech coding (such as AAC-LD, Siren and Opus). DCTs are also important to numerous other applications in science and engineering, such as digital signal processing, telecommunication devices, reducing network bandwidth usage, and spectral methods for the numerical solution of partial differential equations. The use of cosine rather than sine functions is critical for compression since fewer cosine functions are needed to approximate a typical signal, w

On Local Features for Face Verification

{W}e compare four local feature extraction techniques for the task of face verification, namely (ordered in terms of complexity): raw pixels, raw pixels with mean removal, 2D Discrete Cosine Transform (DCT) and local Principal Component Analysis (PCA). The comparison is performed in terms of discrimination ability and robustness to illumination changes. We also evaluate the effectiveness of several approaches to modifying standard feature extraction methods in order to increase performance and robustness to illumination changes. Results on the XM2VTS database suggest that when using a Gaussian Mixture Model (GMM) based classifier, the raw pixel technique provides poor discrimination and is easily affected by illumination changes; the mean removed raw pixel technique provides performance that is fairly close to 2D DCT and local PCA, but is considerably affected by illumination changes. The performance of 2D DCT and local PCA techniques is quite similar, suggesting that the 2D DCT technique is to be preferred over the local PCA technique, due to the lower complexity of the 2D DCT. Both 2D DCT and local PCA techniques are considerably more robust to illumination changes compared to the raw pixel techniques. Modifying the 2D DCT and local PCA techniques by removing the first coefficient, which is deemed to be the most affected by illumination changes, clearly enhances robustness; removing more than the first coefficient causes a noticeable reduction in performance on clean images and provides no further gains in robustness. Compared to just throwing out the first coefficient, the use of deltas can achieve a small increase in performance and robustness. Lastly, we suggest that it is more appropriate to use analysis blocks of size 8x8 (as opposed to 16x16) with 2D DCT decomposition; out of the 64 resulting coefficients, the second through to 21-st (resulting in 20 dimensional feature vectors) are the most robust to illumination changes while providing good discriminatory information.

IDIAP2004

Entre ville et jungle. Relecture d'une lisière

Paola Grimaldo

Suite à l'occupation américaine, le parc métropolitain a bénéficié d'une protection particulière pendant presque un siècle. Cette situation extraordinaire l'a placé au cœur de la capitale du Panama. Les plantes tropicales y absorbent 30 % de la pollution citadine. Pourtant, tout s'oppose entre ville et jungle, au point que la construction de l'un signifie souvent la destruction de l'autre. Cette oasis est clôturée derrière les autoroutes et grignotée par le mitage du territoire. Une vision sur le long terme peut-elle mobiliser les consciences contre des buts lucratifs immédiats? Ainsi, la connaissance et l'éducation seraient les meilleures armes pour lutter contre la pression immobilière. Ce projet propose d'inverser les tendances: comment construire de la jungle? Dans notre cas, on se place sur une cicatrice, un terrain déjà déforesté mais pas encore occupé. On rend ses terres à la nature, sauf la lisière qui protège et définit la frontière. Telle une mise en abyme, cette orée accueille des livres élaborés de bois. La culture redessine et tient la lisière. Ainsi, une bibliothèque nationale pour Panama City, cristalliserait enfin le savoir accumulé d'un pays en plein essor. Sa position met en relation les universités environnantes et son échelle fait face aux méga- structures de transport. Les citadins rencontreraient la nature dans un bâtiment au climat tempéré et naturellement ventilé. Le projet se veut le produit des environnements géographique et culturel qu'il côtoie. Il existe pour permettre à deux mondes aux langages différents de pouvoir dialoguer.

2014

Exploiting passive dynamics for robot throwing task

Amir Jafari, Robin Thandiackal

Throwing is a complex and highly dynamic task. Humans usually exploit passive dynamics of their limbs to optimize their movement and muscle activation. In order to approach human throwing, we developed a double pendulum robotic platform. To introduce passivity into the actuated joints, clutches were included in the drive train. In this paper, we demonstrate the advantage of exploiting passive dynamics in reducing the mechanical work. However, engaging and disengaging the clutches are done in discrete fashions. Therefore, we propose an optimization approach which can deal with such discontinuities. It is shown that properly engaging/disengaging the clutches can reduce the mechanical work of a throwing task. The result is compared to the solution of fully actuated double pendulum, both in simulation and experiment.

IEEE2012

On Local Features for Face Verification

IDIAP2004