Video Quality Model based on a spatiotemporal features extraction for H.264-coded HDTV sequences

International audienceAs a contribution to the design of an objective quality metric in the specific context of High Definition Television (HDTV), this paper proposes a video quality evaluation model. A spatio-temporal segmentation of sequences provide features used with the bitrate to predict the subjective evaluation of the H.264-distorted sequences. In addition, subjective tests have been conducted to provide the mean observer's quality appreciation and assess the model against reality. Existing video quality algorithms have been compared to our model. They are outperformed on every performance criterion

Influence of motion on contrast perception: supra-threshold spatio-velocity CSF measurements

International audienceIn this paper, a supra-threshold spatio-velocity CSF experiment is described. It consists in a contrast matching task with a methods of limits procedure. Results enable the determination of contrast perception functions which give, for given spatial and temporal frequencies, the perceived contrast of a moving stimulus. These contrast perception functions are then used to construct supra-threshold spatio-velocity CSF. As for supra-threshold CSF in spatial domain, it can be observed that CSF shape changes from band-pass behaviour at threshold to low-pass behaviour at supra-threshold, along spatial frequencies. However, supra-threshold CSFs have a band-pass behaviour along temporal frequency has threshold one. This means that if spatial variations can be neglected above the visibility threshold, temporal ones are still of primary importance

Predicting visual fixations on video based on low-level visual features

AbstractTo what extent can a computational model of the bottom–up visual attention predict what an observer is looking at? What is the contribution of the low-level visual features in the attention deployment? To answer these questions, a new spatio-temporal computational model is proposed. This model incorporates several visual features; therefore, a fusion algorithm is required to combine the different saliency maps (achromatic, chromatic and temporal). To quantitatively assess the model performances, eye movements were recorded while naive observers viewed natural dynamic scenes. Four completing metrics have been used. In addition, predictions from the proposed model are compared to the predictions from a state of the art model [Itti’s model (Itti, L., Koch, C., & Niebur, E. (1998). A model of saliency-based visual attention for rapid scene analysis. IEEE Transactions on Pattern Analysis and Machine Intelligence 20(11), 1254–1259)] and from three non-biologically plausible models (uniform, flicker and centered models). Regardless of the metric used, the proposed model shows significant improvement over the selected benchmarking models (except the centered model). Conclusions are drawn regarding both the influence of low-level visual features over time and the central bias in an eye tracking experiment

Construction d'images miniatures avec recadrage automatique basée sur un modèle perceptuel bio-inspiré

Cet article présente un procédé de zoom automatique, destiné à adapter la taille des images pour des dispositifs d'affichage à écran de petite taille (Téléphone mobile...). L'adaptation de la taille des images s'effectue par la sélection des zones les plus intéressantes visuellement. Ces dernières sont déterminées via une approche computationnelle de modélisation de l'attention visuelle. Tout d'abord, les performances de modélisation de l'attention visuelle sont d´eduites par comparaison avec une v´erit´e terrain issue de tests oculom´etrique. Enfin, les performances qualitatives du syst`eme de vision complet, mod`ele d'attention visuelle et proc´ed´e de miniaturisation, sont pr´esent´ees

From SD to HD television: effects of H.264 distortions versus display size on quality of experience

International audienceHigh Definition Television (HDTV) is the new broadcasting system designed to take the place of Standard Definition Television (SDTV) at home in the near future. This system requires modification of many features in the broadcasting chain with an overall objective of reaching a noticeably higher quality of experience. Since broadcasters desire a high level of service acceptability, they require efficient measurements of quality of experience. The purpose of this paper is to provide such measurements concerning the noticeable artifacts in H.264 distortions over a range of display sizes and comparing HDTV to SDTV. A subjective characterization of some HDTV quality of experience aspects is proposed and the results are discussed

Which Semi-Local Visual Masking Model For Wavelet Based Image Quality Metric?

International audienceProperties and models of the Human Visual System (HVS) are the fundaments for most of efficient objective image or video quality metrics. Among HVS properties, visual masking is a sensitive issue. Many models exist in literature. Simplest models can only predict visibility threshold for very simple cue while for natural images one should consider more complex approaches such as semi-local masking. Our previous work has shown the positive impact of incorporating semi-local masking in image quality metric according to one subjective study. It is important to consolidate this work with different subjective experiments. In this paper, different visual masking models, including contrast masking and semi-local masking, are evaluated according to three subjective studies. These subjective experiments were conducted with different protocols, different types of display devices, different contents and different populations

On The Performance of Human Visual System Based Image Quality Assessment Metric Using Wavelet Domain

International audienceMost of the efficient objective image or video quality metrics are based on properties and models of the Human Visual System (HVS). This paper is dealing with two major drawbacks related to HVS properties used in such metrics applied in the DWT domain : subband decomposition and masking effect. The multi-channel behavior of the HVS can be emulated applying a perceptual subband decomposition. Ideally, this can be performed in the Fourier domain but it requires too much computation cost for many applications. Spatial transform such as DWT is a good alternative to reduce computation effort but the correspondence between the perceptual subbands and the usual wavelet ones is not straightforward. Advantages and limitations of the DWT are discussed, and compared with models based on a DFT. Visual masking is a sensitive issue. Several models exist in literature. Simplest models can only predict visibility threshold for very simple cue while for natural images one should consider more complex approaches such as entropy masking. The main issue relies on finding a revealing measure of the surround influences and an adaptation: should we use the spatial activity, the entropy, the type of texture, etc.? In this paper, different visual masking models using DWT are discussed and compared

Towards the next generation of video and image quality metrics: Impact of display, resolution, contents and visual attention in subjective assessment

International audienceTwo decades of research in video and image quality assessment has led to the design of subjective assessment protocols and objective metrics. In order to get good performances, most of research works have restricted their focus of interest on SD format or below and on distortion stemming from coding artifacts or transmission error. Considering up-coming services such as HDTV or scalable video coding, next generation of quality metric should take into account more factors that affect the end user quality of experience. In this paper, a review of factors is proposed considering subjective quality assessment. The four studied factors include display, resolution, content and visual attention. Each factor reveals open issues in quality assessment

Critère de qualité visuelle d'images couleur : application à l'évaluation réaliste de codeurs d'images

- Nous proposons un critère objectif bi variant de qualité visuelle pour l'évaluation de méthodes de codage. Le critère est basé sur les propriétés du système visuel humain de manière à obtenir la meilleure correspondance avec les jugements humains. Contrairement à d'autres critères de la littérature, nous n'utilisons aucune information sur le type de dégradations introduites par les méthodes de codage. Le critère de qualité comporte deux fonctions principales, la première construit une représentation visuelle de l'image et des erreurs perceptuelles (basée sur les résultats d'expériences de psychophysique conduites au laboratoire), la seconde effectue la combinaison de ces erreurs perceptuelles pour produire une note de qualité. Nous proposons une approche originale pour cette seconde fonction dans la mesure où nous avons montré précédemment son importance. Nous comparons les notes fournies par ce critère avec celles issues de jugements humains sur une base de données de 140 images dégradées avec trois codeurs différents : JPEG, JPEG2000, une méthode basée sur des régions d'intérêt (ROI). Les résultats obtenus montrent une très bonne corrélation entre les jugements humains et les notes fournies par notre critère, beaucoup plus importante que celles liées au rapport Signal à Bruit ou qui s'en dérivent