Romuald Pépion
University of Nantes
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Featured researches published by Romuald Pépion.
electronic imaging | 2009
David M. Rouse; Romuald Pépion; Sheila S. Hemami; Patrick Le Callet
Present quality assessment (QA) algorithms aim to generate scores for natural images consistent with subjective scores for the quality assessment task. For the quality assessment task, human observers evaluate a natural image based on its perceptual resemblance to a reference. Natural images communicate useful information to humans, and this paper investigates the utility assessment task, where human observers evaluate the usefulness of a natural image as a surrogate for a reference. Current QA algorithms implicitly assess utility insofar as an image that exhibits strong perceptual resemblance to a reference is also of high utility. However, a perceived quality score is not a proxy for a perceived utility score: a decrease in perceived quality may not affect the perceived utility. Two experiments are conducted to investigate the relationship between the quality assessment and utility assessment tasks. The results from these experiments provide evidence that any algorithm optimized to predict perceived quality scores cannot immediately predict perceived utility scores. Several QA algorithms are evaluated in terms of their ability to predict subjective scores for the quality and utility assessment tasks. Among the QA algorithms evaluated, the visual information fidelity (VIF) criterion, which is frequently reported to provide the highest correlation with perceived quality, predicted both perceived quality and utility scores reasonably. The consistent performance of VIF for both the tasks raised suspicions in light of the evidence from the psychophysical experiments. A thorough analysis of VIF revealed that it artificially emphasizes evaluations at finer image scales (i.e., higher spatial frequencies) over those at coarser image scales (i.e., lower spatial frequencies). A modified implementation of VIF, denoted VIF*, is presented that provides statistically significant improvement over VIF for the quality assessment task and statistically worse performance for the utility assessment task. A novel utility assessment algorithm, referred to as the natural image contour evaluation (NICE), is introduced that conducts a comparison of the contours of a test image to those of a reference image across multiple image scales to score the test image. NICE demonstrates a viable departure from traditional QA algorithms that incorporate energy-based approaches and is capable of predicting perceived utility scores.
visual communications and image processing | 2010
Ulrich Engelke; Romuald Pépion; Patrick Le Callet; Hans-Jürgen Zepernick
In this paper, distortions caused by packet loss during video transmission are evaluated with respect to their perceived annoyance. In this respect, the impact of visual saliency on the level of annoyance is of particular interest, as regions and objects in a video frame are typically not of equal importance to the viewer. For this purpose, gaze patterns from a task free eye tracking experiment were utilised to identify salient regions in a number of videos. Packet loss was then introduced into the bit stream such as that the corresponding distortions appear either in a salient region or in a non-salient region. A subjective experiment was then conducted in which human observers rated the annoyance of the distortions in the videos. The outcomes show a strong tendency that distortions in a salient region are indeed perceived as much more annoying as compared to distortions in the non-salient region. The saliency of the distorted image content was further found to have a larger impact on the perceived annoyance as compared to the distortion duration. The findings of this work are considered to be of great use to improve prediction performance of video quality metrics in the context of transmission errors.
Proceedings of SPIE | 2010
David M. Rouse; Romuald Pépion; Patrick Le Callet; Sheila S. Hemami
The merit of an objective quality estimator for either still images or video is gauged by its ability to accurately estimate the perceived quality scores of a collection of stimuli. Encounters with radically different distortion types that arise in novel media representations require that researchers collect perceived quality scores representative of these new distortions to confidently evaluate a candidate objective quality estimator. Two common methods used to collect perceived quality scores are absolute categorical rating (ACR)1 and subjective assessment for video quality (SAMVIQ).2, 3 The choice of a particular test method affects the accuracy and reliability of the data collected. An awareness of the potential benefits and/or costs attributed to the ACR and SAMVIQ test methods can guide researchers to choose the more suitable method for a particular application. This paper investigates the tradeoffs of these two subjective testing methods using three different subjective databases that have scores corresponding to each method. The subjective databases contain either still-images or video sequences. This paper has the following organization: Section 2 summarizes the two test methods compared in this paper, ACR and SAMVIQ. Section 3 summarizes the content of the three subjective databases used to evaluate the two test methods. An analysis of the ACR and SAMVIQ test methods is presented in Section 4. Section 5 concludes this paper.
international conference on image processing | 2011
Emilie Bosc; Martin Köppel; Romuald Pépion; Muriel Pressigout; Luce Morin; Patrick Ndjiki-Nya; P. Le Callet
This paper addresses the problem of evaluating virtual view synthesized images in the multi-view video context. As a matter of fact, view synthesis brings new types of distortion. The question refers to the ability of the traditional used objective metrics to assess synthesized views quality, considering the new types of artifacts. The experiments conducted to determine their reliability consist in assessing seven different view synthesis algorithms. Subjective and objective measurements have been performed. Results show that the most commonly used objective metrics can be far from human judgment depending on the artifact to deal with.
quality of multimedia experience | 2013
Margaret H. Pinson; Christian Schmidmer; Lucjan Janowski; Romuald Pépion; Quan Huynh-Thu; Phillip Corriveau; Audrey C. Younkin; Patrick Le Callet; Marcus Barkowsky; William Ingram
In 2011, the Video Quality Experts Group (VQEG) ran subjects through the same audiovisual subjective test at six different international laboratories. That small dataset is now publically available for research and development purposes.
Journal of The Optical Society of America A-optics Image Science and Vision | 2011
David M. Rouse; Sheila S. Hemami; Romuald Pépion; Patrick Le Callet
Quality estimators aspire to quantify the perceptual resemblance, but not the usefulness, of a distorted image when compared to a reference natural image. However, humans can successfully accomplish tasks (e.g., object identification) using visibly distorted images that are not necessarily of high quality. A suite of novel subjective experiments reveals that quality does not accurately predict utility (i.e., usefulness). Thus, even accurate quality estimators cannot accurately estimate utility. In the absence of utility estimators, leading quality estimators are assessed as both quality and utility estimators and dismantled to understand those image characteristics that distinguish utility from quality. A newly proposed utility estimator demonstrates that a measure of contour degradation is sufficient to accurately estimate utility and is argued to be compatible with shape-based theories of object perception.
Proceedings of SPIE | 2010
Yohann Pitrey; Marcus Barkowsky; Patrick Le Callet; Romuald Pépion
Scalable Video Coding (SVC) provides a way to encapsulate several video layers with increasing quality and resolution in a single bitstream. Thus it is particularly adapted to address heterogeneous networks and a wide variety of decoding devices. In this paper, we evaluate the interest of SVC in a different context, which is error concealment after transmission on networks subject to packet loss. The encoded scalable video streams contain two layers with different spatial and temporal resolutions designed for mobile video communications with medium size and average to low bitrates. The main idea is to use the base layer to conceal errors in the higher layers if they are corrupted or lost. The base layer is first upscaled either spatially or temporally to reach the same resolution as the layer to conceal. Two error-concealment techniques using the base layer are then proposed for the MPEG-4 SVC standard, involving frame-level concealment and pixel-level concealment. These techniques are compared to the upscaled base layer as well as to a classical single-layer MPEG- 4 AVC/H.264 error-concealment technique. The comparison is carried out through a subjective experiment, in order to evaluate the Quality-of-Experience of the proposed techniques. We study several scenarios involving various bitrates and resolutions for the base layer of the SVC streams. The results show that SVC-based error concealment can provide significantly higher visual quality than single-layer-based techniques. Moreover, we demonstrate that the resolution and bitrate of the base layer have a strong impact on the perceived quality of the concealment.
Proceedings of SPIE | 2012
Manish Narwaria; Matthieu Perreira Da Silva; Patrick Le Callet; Romuald Pépion
High Dynamic Range (HDR) images/videos require the use of a tone mapping operator (TMO) when visualized on Low Dynamic Range (LDR) displays. From an artistic intention point of view, TMOs are not necessarily transparent and might induce different behavior to view the content. In this paper, we investigate and quantify how TMOs modify visual attention (VA). To that end both objective and subjective tests in the form of eye-tracking experiments have been conducted on several still image content that have been processed by 11 different TMOs. Our studies confirm that TMOs can indeed modify human attention and fixation behavior significantly. Therefore our studies suggest that VA needs consideration for evaluating the overall perceptual impact of TMOs on HDR content. Since the existing studies so far have only considered the quality or aesthetic appeal angle, this study brings in a new perspective regarding the importance of VA in HDR content processing for visualization on LDR displays.
quality of multimedia experience | 2014
Yuming Fang; Junle Wang; Jing Li; Romuald Pépion; Patrick Le Callet
We present a large-scale eye tracking database for stereo-scopic video. A set of participants were involved in this eye tracking experiment. The human fixation maps were created as the ground truth for stereoscopic video from the gaze data from participants. To the best of our knowledge, this is the first large-scale eye tracking database of visual attention modeling for stereoscopic video. The details of the processing operations and properties of the database are described in this paper.
Proceedings of SPIE | 2011
Emilie Bosc; Romuald Pépion; Patrick Le Callet; Martin Köppel; Patrick Ndjiki-Nya; Luce Morin; Muriel Pressigout
This paper considers the reliability of usual assessment methods when evaluating virtual synthesized views in the multiview video context. Virtual views are generated from Depth Image Based Rendering (DIBR) algorithms. Because DIBR algorithms involve geometric transformations, new types of artifacts come up. The question regards the ability of commonly used methods to deal with such artifacts. This paper investigates how correlated usual metrics are to human judgment. The experiments consist in assessing seven different view synthesis algorithms by subjective and objective methods. Three different 3D video sequences are used in the tests. Resulting virtual synthesized sequences are assessed through objective metrics and subjective protocols. Results show that usual objective metrics can fail assessing synthesized views, in the sense of human judgment.
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Institut de Recherche en Communications et Cybernétique de Nantes
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