François Paris
email: f.paris@brgm.fr

Sophie Lecacheux
email: s.lecacheux@brgm.fr

Rodrigo Pedreros
email: r.pedreros@brgm.fr

Jérémy Rohmer
email: j.rohmer@brgm.fr

Sophie Sauvagnargues
email: sophie.sauvagnargues@mines-ales.fr

Pierre-Alain Ayral
email: pierre-alain.ayral@mines-ales.fr

Florian Tena-Chollet
email: florian.tena-chollet@mines-ales.fr

Lgei

François Bonnardot
email: francois.bonnardot@meteo.fr

Hubert Quetelard
email: hubert.quetelard@meteo.fr

A FRAMEWORK TO DESIGN A MUNICIPAL DECISION SUPPORT TOOL FOR CYCLONE-INDUCED MARINE FLOODING EMERGENCY MANAGEMENT AT REUNION ISLAND

Keywords: Coastal hydrodynamic modelling, wave overtopping, marine flooding mapping, decision-making

Marine flooding by wave overtopping is quite regular at Reunion Island, in particular during cyclonic events. Given the limited geographic scale of this phenomenon, the municipalities are often at the forefront to assure the protection of the population. Thus, there is a crucial need for the municipalities to have flexible and practical tools at their disposal to interpret the marine forecasts in terms of flooding impact on their territory and enhance their capacity to anticipate and organize their operational response. Here, we propose a framework to design a dedicated GIS-based decision support tool (the Graduated Intervention Plan) based on feedback analysis, field observations and state-of-the-art coastal hydrodynamic modelling. The method notably relies on a pre-calculated database of 500 synthetic events simulated with a complete chain of hydrodynamic models. The analysis of the relationship between marine conditions, overtopping discharge volumes and water propagation processes leads to the constitution of (1) a set of flooding maps delimiting the spatial extent of exposed areas given increasing levels of marine conditions and field observations (2) a list of operational actions associated to each level (roads closure, etc.). The whole process was conducted in a collaborative manner with the municipal services of Sainte-Suzanne city (northern coast of Reunion Island) and the final tool is included in a multi-hazard decision support tool taking into account river flooding and runoff. It was tested during a crisis simulation exercise with very good feedbacks.

INTRODUCTION

In a context of changing climate, the perspective of increasing cyclone-induced marine flooding frequency or intensity is a matter of concern, especially when the growing urbanization rises the proportion of people living in low-lying coastal regions. The development of efficient early warning systems [START_REF] Lumbroso | Enhancing resilience to coastal flooding from severe storms in the USA: international lessons[END_REF] and dedicated crisis management strategies is complementary to prevention and preparedness measures. Given the limited geographic scale of marine flooding events, the municipalities are often at the forefront to assure the protection of the population. Thus, they have a crucial need of dedicated and practical tools to understand the ongoing physical processes and enhance their capacity to anticipate.

Operational forecast systems of cyclone-induced waves and water levels are now fairly standards since the 90' [START_REF] Jelesnianski | SLOSH: Sea, Lake and Overland Surges from Hurricanes[END_REF]Daniel et al., 2009, etc.) and current efforts of operational agencies aim to improve the forecast accuracy of probabilistic sea water levels by coupling tide, storm surge, waves and induced wave setup models [START_REF] Taylor | Probabilistic guidance for hurricane storm surge[END_REF][START_REF] Davis | Probabilistic simulation of storm surge and inundation in a limited resource environment[END_REF]etc.). On the other hand, the operational forecast of marine flooding still poses different challenges. Actually, the numerical simulation of the coastal flooding (overflowing plus wave overtopping) at a resolution useful for flood hazard assessment is still hampered by high computation time (Nicolae [START_REF] Lerma | Highresolution marine flood modelling coupling overflow and overtopping processes: framing the hazard based on historical and statistical approaches[END_REF] despite ongoing efforts to develop new generations of efficient phase-resolving models [START_REF] Filippini | UHAINA: A parallel high performance unstructured near-shore wave model[END_REF]. Promising approaches rely on the analysis of existing databases of precalculated high-fidelity simulations to derive statistical models (or "surrogates") that are able to estimate quickly the marine conditions or flooding extent directly from the storm parameters. Existing techniques rely on kriging, artificial neural networks or Bayesian networks. In cyclonic-prone areas, [START_REF] Rohmer | Early warning system for cycloneinduced wave overtopping aided by a suite of random forest approaches[END_REF] proposed a method to derive overtopping characteristics (timing and volume) thanks to a random forest technique. This kind of sophisticated tool is dedicated to expert users but is not directly usable by municipal services, especially for small cities with limited resources as often encountered in tropical islands like La Reunion.

The analysis of the crisis management organization of Sainte-Suzanne city (North of Reunion Island), as well as the lessons learnt from crisis simulation exercises [START_REF] Sauvagnargues | Méthodologie d'appui aux communes pour la gestion de crise des inondations fluviales et côtières : évaluation objective des plans communaux de sauvegarde (PCS) et organisation d'exercices de gestion de crise[END_REF], demonstrated the need for the members of the municipal crisis unit to have pragmatic tools linking the weather and marine forecasts with potential impacts on land and appropriate operational responses [START_REF] Sauvagnargues | Impact des Evènements Météorologiques Dangereux sur la gestion de crise des communes : application aux PCS des communes de l'île de la Réunion[END_REF]. The Graduated Intervention Plans (GIPs) are cartographic documents used to structure the operational response of municipal crisis units in different levels of gravity regarding various risks presents on the territory. To be operational, this cartographic support relies on a dual approach combining: (1) capitalized experience within each territory studied in terms of impact of hazards (practices, feedback, archives, etc.) (2) the representation of the spatial extent of potentially impacted areas, structured in gravity levels and built on modelling results and/or existing knowledge. If this type of tool is widely used for river flooding [START_REF] Ayral | Elaboration du Plan Communal de Sauvegarde Multirisques de la ville d'Alès : Retours d'expérience[END_REF][START_REF] Sauvagnargues | Les outils et méthodes de la gestion de crise « inondation » : la protection des personnes, des biens et de l'environnement[END_REF], to date and to the authors' best knowledge, it was not yet applied for marine flooding in France or in the Overseas Territories. The difficulty in the case of marine flooding is the scarcity of historical events (especially for cyclones) and the fact that the physical processes are governed by multiple parameters (waves, tide and storm surge). This last aspect complicates the determination of the criteria of transition between different levels of gravity, especially for non-expert users.

The objective of this study is to present a method to produce GIPs dedicated to cyclone-induced marine flooding for the pilot site of Sainte-Suzanne city at Reunion Island. It is based on both feedback analysis, field observations and state-of-the-art coastal hydrodynamic modelling. The method notably relies on a pre-calculated database of numerous synthetic events simulated with a complete chain of models. The analysis of the relationship between marine conditions, overtopping discharge volumes and flooded areas leads to the constitution of marine flooding maps, delimiting the extent of impacted areas given different increasing levels of marine conditions, and coupled with a list of operational actions associated to each level. The criteria of transition between the different levels connect operational marine forecasts with field observations and historical benchmarks, thus enabling different levels of use and anticipation. This tool was tested and validated during a crisis simulation exercise on site.

The paper is organized as follows. Section 2 presents the study site of Sainte-Suzanne city. Section 3 describes the complete chain of models and the constitution of the synthetic database. Section 4 explains the principles of the GIPs, i.e. the building of marine flooding maps, the determination of associated operational actions with the collaboration of the municipal services, and the test of the GIP during a crisis exercise on site. Finally, section 5 gives the conclusions and perspectives to adapt the method for other territories.

STUDY SITE: PROCESSES AND CRISIS MANAGEMENT PRACTICES

Configuration and marine flooding processes

Reunion Island is a French Overseas Department located in the Indian Ocean, east of Madagascar (see inset in Fig. 1). Due to the mountainous nature of the island, 80% of the population is concentrated near the coastline, thereby resulting in high vulnerability to tropical cyclones. The tide is micro-tidal, with a maximum tidal range of approximately 60 cm (at the tide gauge Pointe-des-Galets, French Hydrographic Service 2014). The lack of a continental shelf around the island reduces the generation of atmospheric storm surges that remain moderate and localized near the cyclone's eye. This setting, associated with the high topography of sea fronts prevents the coastal areas from marine flooding by overflowing. In contrast, it increases the potential impact of waves that are not dissipated before reaching the coast (except in the few reef zones) and whose breaking on the steep slopes may generate considerable overtopping. Thus, the main drivers of cyclone-induced marine flooding at Reunion Island are the energetic (high and/or long) waves. However, the variations of seawater levels may facilitate wave overtopping. Current operational forecasts at Reunion Island provide nearshore waves characteristics (significant wave height: Hs, pic period: Tp, and pic direction: Dp) but no seawater levels (SWL) yet.

Sainte-Suzanne city is located along the pebble coast in the northeast part of the island (see

Crises management practices

In France, the responsibilities of crisis management are shared between Ministries, Zones, Departments and Municipalities. At local level, the Municipal Safeguard Plan (PCS) is a planning and organization instrument designed to anticipate dangerous situations in order to ensure the protection and safety of the population. It aims at preparing the actors involved in the crisis management to reduce uncertainties and improvised actions as much as possible (DDSC, 2005). The PCS of Sainte-Suzanne includes the description and mapping of the hazards (when possible), the alert levels (when available), the instructions given by the authorities to the population and the general organization of the crisis management cell (organization of the Communal Command Post or PCC, reflex forms, resources as emergency shelters, materials, etc.). The analysis conducted by [START_REF] Sauvagnargues | Impact des Evènements Météorologiques Dangereux sur la gestion de crise des communes : application aux PCS des communes de l'île de la Réunion[END_REF] highlighted that the characteristic of marine conditions leading to wave overtopping and the corresponding exposed areas were not well known. The current practice of the municipal services consists in sending employees to observe different strategic locations of the city centre during an event in order to extrapolate a "mental map" of the marine flooding based on their knowledge of the territory and the memory of past events. This practice may be efficient but it is not sustainable as it only relies on personal experiences and skills. More generally, there is also a need to enhance the evacuation modalities, notably through (1) the knowledge of the amount of inhabitants per quarters or exposed areas (2) the organization of the alert of the population (3) the identification of the level of exposure of the emergency shelters.

MARINE FLOODING MODELING AND CONSTITUTION OF THE DATABASE

Modelling chain

Coastal waves and water levels

The wave model (see Fig. 2A) is a combination of a two-way nested Wavewatch III modelling framework [START_REF] Tolman | User manual and system documentation of WAVEWATCH-III version 4[END_REF]; hereinafter denoted WW3) enabling the offshore waves generation and propagation to Reunion Island coastlines. The version 4.18 of WW3 is used with the source term package described by [START_REF] Ardhuin | Semiempirical dissipation source functions for ocean waves. Part I: Definition, calibration, and validation[END_REF]. The first grid (R1) covers a large part of the South Indian Ocean with a regular resolution of 0.1° while (R2), centered on Reunion island, is composed of finite elements whose resolution reach about 300 m at the coast. The regional hydrodynamics (seawater level and currents) are computed with MARS [START_REF] Lazure | An external-internal mode coupling for a 3D hydrodynamical model for applications at regional scale (MARS)[END_REF] on a regular grid with the same extent as R2 at a resolution of 100 m (see Fig. 2A) based on tidal components (FES2004; [START_REF] Lyard | Modelling the global ocean tides: modern insights from FES[END_REF]. We use the 2DH version of the model, which resolves the Saint-Venant equations governing horizontal free-surface flows in two dimensions, after vertical integration of the Navier Stokes equations. The bathymetric data used come from ETOPO1 with a spatial resolution of 1 minute [START_REF] Amante | ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis[END_REF] as well as the bathymetric measurements from the French Naval Hydrographic and Oceanographic Service near the island.

Marine flooding

Wave overtopping discharge is calculated with profiles of the nonhydrostatic model SWASH [START_REF] Zijlema | SWASH: an operational public domain code for simulating wave fields and rapidly varied flows in coastal waters[END_REF][START_REF] Suzuki | A numerical study on the effect of beach nourishment on wave overtopping in shallow foreshores[END_REF] at a resolution of 1 m. The profiles are determined by splitting the coastline into homogeneous segments in terms of topo-bathymetry (see Fig. 2C). For each segment, the model is forced with time series of water levels at a 50-m depth, including both seawater level variations (tide and storm surge) and waves. These time series are established by extracting the water level from MARS-2DH and the wave parameters from WW3 at 10-minute intervals and reconstituting a time series of the total water level variations at high frequencies (at 2 Hz) using the WAFO toolbox [START_REF] Brodtkorb | WAFO -a Matlab toolbox for analysis of random waves and loads[END_REF]. The model MARS-FLOOD is used to reproduce the inland propagation of overtopping. It is an extension of MARS-2DH, including extended functionalities to account for flooding processes, such as breaching and hydraulic connections [START_REF] Rohmer | Casting light on forcing and breaching scenarios that lead to marine inundation: Combining numerical simulations with a random-forest classification approach[END_REF]. The grid of MARS-FLOOD (see Fig. 2C) is centred on the city center with a regular spatial resolution of 4 m, which enables the model to properly represent streets and buildings. The topo-bathymetric data used for the SWASH and MARS-FLOOD models comes from the Litto3D program (high-resolution LIDAR data, http://professionnels.ign.fr/litto3d). Here, we apply the method described by Le [START_REF] Roy | Coastal flooding of urban areas by overtopping: dynamic modelling application to the Johanna storm (2008) in Gâvres (France)[END_REF] to construct a DTM including the buildings likely to obstruct water flow and to filter out any vegetation or cars in the raw model.

Validation with reanalyzed data

We apply this chain of models to two recent cyclones passing within 200 km from Reunion Island: Dumile in 2013 and Dina in 2002 (see Fig. 2A). Dina was the most damaging event since the early 2000s. The southwestward track passed 65 km north of the island with a maximum wind intensity of 100 kt. Dumile was less damaging, with a southward track that passed 100 km west of the island and a moderate wind intensity approximately 70 kt. To force the models, the wind fields are calculated from best-track data of Météo-France (describing time-varying track, intensity, size and shape of the cyclone) with the model MESO-NH following the method described by [START_REF] Lecacheux | Probabilistic forecast of coastal waves for flood warning applications at Reunion Island (Indian Ocean)[END_REF]. The simulation-based estimates of the seawater levels (including tide and surge) was crosschecked with data from the tide gauge Pointe-des-Galets with good fitting and errors under 10 cm (the data is not presented here). Fig. 2B shows the comparison between simulations and observations of wave heights with satellite altimetry from the CERSAT (Dina) or buoy measurements (Dumile) depending on the data availability. For both cases, the comparison is very satisfactory with less than 10% error for the peak wave heights. Concerning marine flooding, we compared the simulation results with local eyewitness accounts enabling an estimation of the extent of the inundation (Fig. 2C). For Dina, the simulated marine flooding extent agrees very well with the local eyewitness accounts. The first row of buildings was flooded and the water penetrated into the perpendicular streets to inundate the second row of buildings, the emergency centre, and the post office. For Dumile, the simulated marine flooding was slightly overestimated but remains in agreement with a limited flooding event.

Database of synthetic events

The database gathers 500 synthetic cyclones derived from historical cases (see Fig. 3). First, 125 systems analysed between 1981 and 2016 are selected. For each track, the position where the system reached the maximum intensity is placed on the centre of Reunion Island. Then, each track is shifted by randomly choosing a direction (between 0° and 355°) and a distance from the centre of the island (between 0 km and 400 km). This range of distances enables the simulation of both scenarios that are likely to generate major overtopping in Sainte-Suzanne and scenarios with low to moderate waves that do not generate overtopping. For each scenario (describing time-varying track, intensity, size and shape of the cyclone), the marine flooding of Sainte-Suzanne is simulated using the modelling method presented in section 3.1 (with the series of models Meso-NH, WW3, MARS and SWASH) considering random tide level time series and large-scale conditions. Among the 500 initial cases, 136 generated wave overtopping with an inundation of Sainte-Suzanne city.

Sensitivity analysis of the governing parameters

The relative importance of the marine conditions (extracted at 50m-depth in front of our site) in the is studied with respect to the log-transformed of the maximum cumulative overtopping discharge volume V (i.e. log10(V+1)). To do so, the combination of the pre-calculated V values considering the 500 synthetic tracks and their corresponding marine conditions (maximum Hs and associated Tp and SWL at the same time) are used as training data of a regression random-forest model [START_REF] Breiman | Random forests[END_REF]. Full details of this implementation is provided in [START_REF] Rohmer | Early warning system for cycloneinduced wave overtopping aided by a suite of random forest approaches[END_REF]. Using this statistical model, the importance of each governing parameter can be studied with respect to V. Fig. 4A provides the measure of variable importance using the permutation-based method of the random forest (e.g., [START_REF] Wei | Variable importance analysis: a comprehensive review[END_REF]. This clearly confirms the very high importance of the wave parameters, and more specifically of Hs, as mentioned is section 2.

The analysis is completed by the Individual Conditional Expectation plots of [START_REF] Goldstein | Peeking inside the black box: Visualizing statistical learning with plots of individual conditional expectation[END_REF] in Fig. 4B, which depicts the functional relationship between the predicted value of the logtransformed of V and maximum Hs given the values for the other variables. This clearly outlines the step-like influence of Hs, i.e. the existence of a threshold at about 5m above which the overtopping volume increases correspondingly with an increase of Hs, and a horizontal asymptote at ~10m. For Hs values under 4-5m, the other parameters, notably the water level (tide and atmospheric surge), may have a greater influence to discriminate between flooding and non-flooding events.

BUILDING THE GRADUATED INTERVENTION PLAN (GIP)

Analysis of the database and determination of graduated flooding maps

The set of synthetic events is analyzed with a coupled analytic and GIS mapping method in terms of overtopping volumes, flood spatial extent and maximum water depths on land. As the sensitivity analysis shows that the wave height is the main parameter controlling the overtopping discharge volume (and then the extension of marine flooding), we focus only on the relationship between the maximum wave height reached during the event (at 50m-depth in front of Sainte-Suzanne) and the maximum flood spatial extent on land. This choice is also motivated by (1) a concern of simplicity for the end-users who need easy-to-read tools (2) the fact that there is not yet public operational forecast of seawater levels at Reunion Island.

Data mining of the simulations enables to classify the results into different clusters depending on Hs thresholds and controlled by the topography (notably the presence of low-lying basins) and the drainage capacity of the artificial lagoon. The bivariate diagram (Fig. 5) reveals four groups of equivalent flood spatial extent associated with ranges of Hs:  The first cluster (Hsmax between 4m to 6m) corresponds to simulations leading to the first projections of water and pebbles on the waterfront, mainly on the parking of the city centre, without any flooding of the buildings.  The second cluster (Hsmax between 6m to 8m) illustrates the simulations leading to flood propagation in the Cayenne Street with low water depths.  The third cluster (Hsmax between 8m to 10m) represents the intensification of marine conditions with flows penetrating further the main streets (Cayenne, La Gare and Hippolyte Piot) and reaching the city centre. The low-lying basins are saturated with upper water depths and a part of the water is drained by the lagoon that overflows in front of the Bertin Street and Desprez neighbourhood.  The fourth cluster (Hsmax above 10m) includes the simulations whose flooding exceeds the Charles de Gaulle Streets with water depths approaching capacity storage, and water flowing toward the river Sainte-Suzanne.

Using a GIS method, we delimit the boundaries or contours of each cluster aided by the simulated flood maps and an analysis of the volume storage capacity through topographic ridges (like roads or streets and troughs). The contours are established with a safe approach to encompass all the scenarios of flooding events constituting the cluster and taking into account the possible variations of wave period and seawater level as well as the various kinetic and the duration of the events. Finally, despite the simplification of the analysis with one variable (maximum Hs), the corresponding maps are robust thanks to the large amount of realistic events constituting the dataset.

Collaborative work with the municipality and determination of the operational responses

The transposition of the flooding maps into a Graduated Intervention Plan (GIP) was conducted in close collaboration with the municipal services, notably through several on-site visits. The maps and a general outline of the action forms were provided to the municipality. From these documents, the technical services specified the different levels of the plan, and adapted the boundaries of the maps and the associated action forms to fit with the particularities of the territory and their own practices. This two-step process ensures the global relevance of the GIP from an operational point of view.

At the end, two levels have been considered: one corresponding to the first visible impacts on the seafront parking, and a second (decomposed in 3 levels of severity) corresponding to the flooding of buildings in the city center (Fig. 6). The transition conditions between the different levels rely on both direct observations of the consequences on the field (to fit with the current practice of the services) and the thresholds of wave heights that can be inferred from the operational forecasts distributed by Météo-France (http://www.meteofrance.re). In addition, historical benchmarks of past major events are indicated in order to give a mental representation of the importance of the current event. These different keys of reading enables one to use the GIP with different levels of anticipation:

 During the early-warning period, to estimate the potential exposed sectors;  During the crisis, to spatially extrapolate the observations made on strategic monitoring points. For each level of the GIP, specific actions are declined and the information is reported on the maps: the potential impacted area, the territory's monitoring points, the road closure points, the preestablished circuit of the sound car (mobile warning As the decision and the allocation of means for the evacuation is not under the responsibility of the municipality level, this type of action is not directly reported in the GIP. However, the graduated flood maps will enable the municipality to (1) encourage the exposed population to join the emergency shelters (2) transfer precise information to the department level to better target and anticipate an official evacuation. The population count per district and exposed areas also facilitates the dimensioning of the means.

Testing the GIP during a crisis simulation

The marine flooding GIP is included within a global multi-hazard GIP articulating river flooding, runoff and marine flooding plans for cyclonic events (see Fig. 7). In order to facilitate the appropriation of the GIP developed previously, a simulation was carried out in October 2017 as part of an in-situ crisis management exercise. A complete scenario of cyclonic event was prepared for the municipality with the combination of an intense rainy event 24 hours before the arrival of the cyclone close to the coasts of Reunion Island. During the exercise, the municipality had to deal with the runoff issue while anticipating the marine flooding hazard thanks to marine. The GIP proved to be very useful and during the debriefing, several discussions emerged, notably:

 The safety and dimensioning of emergency shelters (sometimes located in flood-prone quarters) and a better anticipation for their opening.  The need to anticipate a withdrawal room for the communal crisis unit.

CONCLUSIONS

In this paper, we present a method to produce a municipal Graduated Intervention Plan (GIP) dedicated to cyclone-induced marine flooding emergency management. It relies on feedback analysis, field observations and the constitution of a precalculated database of flooding events based on stateof-the-art coastal hydrodynamic modelling. The plan couples a set of flooding maps indicating the exposed areas for increasing levels of marine conditions and field observations with a list of predetermined operational actions associated to each level.

Despite the significant effort required to produce the pre-calculated database, the use of numerous synthetic (but realistic) events enables a robust sensitivity analysis of the governing parameters leading to marine flooding to (1) simplify the interpretation and understanding of non-expert endusers (2) construct a robust set of maps taking into account various sources of uncertainties (seawater level variations, event duration, etc.). The different reading keys (wave forecast, field observations, and historical benchmarks) enables one to use the GIP with different levels of anticipation.

More generally, the whole method can easily be adapted to other territories, scales and any marine flooding processes (overflowing and overtopping) through the implementation of suitable marine flooding models validated against past events and the early involvement of stakeholders.

IGN for providing the LIDAR data. We also thank the employees of Sainte-Suzanne city, the DEAL service and the emergency services for their high level of involvement during the project and their support and instructive information.

  Fig. 1). In recent years, the city centre has been regularly affected by marine flooding, e.g., cyclones Dumile in 2013, Gamède in 2007, Dina in 2002, Clotilda in 1987, etc. Experience feedback and simulation results indicate that water discharge from wave overtopping first affects the sea-front parking area before flowing along December the 20 th Street and penetrating into the perpendicular streets Hippolyte Piot, Cayenne and La Gare (Fig. 1). Small low-lying urban areas around the emergency centre and the city-centre buildings (school and city hall) behind the parking area can be quickly flooded by more than one meter of water. The artificial lagoon lying north of the eastern side of December the 20 th Street protects the buildings situated behind (Bertin Street and Desprez neighbourhoods) and enables the evacuation of a large part of the water coming from the seafront.

Figure 1 :

 1 Figure 1: Topography of Sainte-Suzanne city derived from high-resolution LIDAR (IGN) data with main streets and critical buildings of the sea-front quarter.

Figure 2 :

 2 Figure 2: A) Grid extent with tracks (black dots), wind intensity (colors) and directions (arrows) of cyclones Dina and Dumile calculated with MESO-NH from best-track data. B) Comparison between simulated wave heights with observations from satellite altimetry on January the 23 rd at 06 UTC (Dina) or time-varying observations from buoy RN4 (Dumile). C) Position of the SWASH profiles and comparison between simulated flooding with eyewitness accounts.

Figure 3 :

 3 Figure 3: 500 synthetic tracks constituting the database with examples of 30 time series (presented on a normalized time index) of significant wave heights (Hs) and seawater levels (SWL) at 50m depth in front of Sainte-Suzanne city.

Figure 4 :

 4 Figure 4: A) Importance measure for each variable (Hs, Tp, SWL) derived from a random-forest model trained using the synthetic cyclones B) Functional relationship between the predicted value of the log-transformed of and Hs. The yellow line indicates the average effect of Hs.

Figure 5 :

 5 Figure 5: Right panel: Relationship between maximum coastal wave heights (at 50m-depth) with maximum flooded areas and overtopping volumes in the city centre of Sainte Suzanne. Left panel: Two examples of flood maps compared with the GIP contours.

Figure 6 :

 6 Figure 6: Map aggregating the different levels of the Graduated Intervention Plan (GIP) of Sainte Suzanne city centre with the transition criteria (field observations, Hs thresholds and benchmarks).

Figure 7 .

 7 Figure 7. Complete GIP for a cyclonic event including runoff, marine flooding and river flooding.

ACKNOWLEDGEMENTS

This work is supported by the French National Research Agency within the SPICy project (ANR -14 -CE03 -0013). The authors thank the Region of Reunion Island for providing the wave measurements, the SHOM for providing the bathymetry and the tide gauge measurements and the


Patrick Le Callet

Stéphane Pechard

Sylvain Tourancheau

Alexandre Ninassi

Dominique Barba

TOWARDS THE NEXT GENERATION OF VIDEO AND IMAGE QUALITY METRICS: IMPACT OF DISPLAY, RESOLUTION, CONTENT AND VISUAL ATTENTION IN SUBJECTIVE ASSESSMENT

Two 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.

INTRODUCTION

Among topics related to image and video processing, quality assessment is one of the most difficult task. Nevertheless, quality assessment is mandatory either to estimate performances of a compression algorithm or either to improve a given system or a whole transmission chain from the end user quality of experience point of view. Image and video quality assessment have been extensively studied during the last two decades either through subjective experiments, e.g. with human observers, or through objective metrics, e.g. with dedicated algorithms. If subjective assessment represents the ground truth, it is an annoying, time consuming approach: when running a subjective test, standard conditions must be reproduced to obtain correct, universal and reliable evaluations. Moreover, this concept of ground truth should be admitted with caution. Automatic objective metrics reliably predicting the perceived quality of images, would be a great improvement in quality assessment field.

Best examples of the research effort on quality assessment relies in the ITU recommendations and VQEG activities. A part conditions and methodologies in order to run subjective test, standard objective metrics have been released for full reference metrics in the context of SD TV. At this time, VQEG is running test plan to define full reference, reduced reference and no reference metrics for multimedia applications. Previous works and work in progress are mainly related to coding or transmitting purpose, e.g. coding artifacts and transmission errors, at a given resolution (no relation between resolution). Considering, the whole chain, quality assessment should be able to manage dependency to other technology issues. New services in relation with technology improvements could be sold at the right acceptable prices if end-user quality of experience is well identified. However, each new feature brings specific distortions but also enhancements for visual quality. New techniques of quality assessment including subjective methodology need to be developed in order to characterize each element.

Among new video services, High Definition Television (HDTV) is an excellent example of challenging issues regarding quality assessment. First, this new generation broadcasting system provides higher immersion in action and better visualization comfort. Television has always suffering from a lack of presence, immediacy and impact compared with "cinema-like" experience. The basic idea to produce psychological effects is to widen the display screen and, simultaneously, to increase considerably the resolution of the source. Previous studies [START_REF] Ichiro | Fundamental requirements for high-definition television systems[END_REF] have shown that ideal distance to watch moving pictures is 3H (4H for programs with rapid movements). The corresponding viewing angle of 20°-30° reduces considerably the sensation of presence of the display system. Because, viewing angle is higher in HDTV compared to SDTV, parafoveal vision is highly excited. It means that as display size is increasing, artifacts perception is less uniform. Second issue related to video quality and HDTV comes from the display technologies. These new materials bring specific distortions (and also enhancements for visual quality) that must be identified. Finally, due to its better rate performance, H264 coding standard is going to be used for HDTV broadcasting in Europe. This standard induces artefacts that are highly dependent on the video contents, and so impact on quality is obvious but still unknown.

In this paper, we propose a review of the impact of different factors on video quality judgement through subjective experiments.

IMPACT OF DISPLAY ON SUBJECTIVE QUALITY

LCD vs CRT

As the screen size is heightening, standard CRT displays becomes bulky and heavy. As a result CRT displays are doomed to disappear and new display technologies (LCD, PDP) are improving and will soon replace old CRT technology. It seems that LCD is more likely to succeed because of problems attached to plasma large resolution displays. However, it has been shown that subjective quality of a sequence displayed on LCD is globally lower than subjective quality of the same sequence displayed on CRT [2]. Among all the defects mentioned by 36 video expert observers, motion blur seems to be the most annoying one. This appears in sequences with rapid movements. Other shortcomings have been enumerated such as colour differences, degradations in dark areas and de-interlacing artefacts for interlaced sequences. Most of the recent improvements in LCD technology are related to motion blur minimization. A part response time compensation, a material solution to reduce motion blur is to modify the temporal aperture of the display. Different methods have been proposed, such as backlight flashing [START_REF] Kurita | Moving picture quality improvement for hold-type AM-LCDs[END_REF][START_REF] Feng | LCD motion-blur analysis, perception, and reduction using synchronized backlight flashing[END_REF], frame rate doubling [START_REF] Kurita | Moving picture quality improvement for hold-type AM-LCDs[END_REF][START_REF] Michiel | Motion blur reduction for liquid crystal displays: motioncompensated inverse filtering[END_REF], black data insertion [START_REF] Hong | Motion image enhancement of LCDs[END_REF] and motion-compensated inverse filtering [START_REF] Michiel | Motion blur reduction for liquid crystal displays: motioncompensated inverse filtering[END_REF]. From manufacturers point of view, it is very important to assess the reduction of motion blur and ideally to estimate the quality loss due to motion blur. We have conducted subjective experiments to quantify the impact of LCD motion blur on the perceived quality on LCD with respect to the perceived quality on CRT (which is considered to be the reference here). quality between CRT and LCD.

Subjective quality assessment

Protocol

Eigth sequences in 1080i format with significant movements, and for which motion blur is the main perceived defect when displayed on LCD, have been selected. Each of them contain 250 frames which correspond to 10 second duration. Tests have been performed both on a CRT and on a LCD display in a specific showroom. Lighting conditions and display parameters have been precisely measured and adjusted according to BT.500-11 and BT.710-4 ITU recommendations. The HDTV displays used were a JVC DT-V 1910CG and a Philips T370 HW01 which both can display 1080i sequences. Viewing distance was set to 3H, where H is the height of the screen.

Observers were mostly male students in their mid twenties. Every candidate is first checked for color blindness with Ishihara test and for acuity with Monoyer's plates. People with at least one error in Ishihara's test or less than 9/10 in Monoyer's test are rejected. 21 people took part in these tests in the CRT session and 19 in the LCD session.

As very little quality differences must be detected, the protocol to rate the video should allow quality discrimination. A well known stable method for this purpose is the SAMVIQ method [START_REF] Ebu | SAMVIQ -Subjective assessment methodology for video quality[END_REF], developed by France Telecom R&D and standardized by the European Broadcasting Union (EBU). Observers compare sequences (seven distorted sequences and one hidden reference) both between them and with the explicit reference. Notation scale is continuous, each score can take a value between 0 and 100. SAMVIQ is a multi stimuli continuous quality scale protocol. It provides a precise and reliable [START_REF] Blin | New quality evaluation method suited to multimedia context: Samviq[END_REF] measure of the subjective video quality which can be compared directly to the reference.

Results

Mean opinion scores (MOS) of observers for the eight reference sequences are shown in Table 1. DMOS is the difference of MOS from CRT and LCD :

DMOS=MOS CRT -MOS LCD (1)
Thus, subjective quality measured on LCD is lower than the one measured on CRT. It can be observed that DMOS is varying strongly with the sequences, as a result CRT MOS and LCD MOS are not well correlated. As an approximation of SD 576i sequences, SDTV sequences (definition 960´540) are computed from HD sequences through a half band filtering followed by subsampling by a factor of 2 (both along horizontal and vertical directions). Both fields are decimated separately before reconstitution in progressive format. This technique is motivated by the fact that this definition is very close to "real 16/9" SDTV (1024´576) and no interpolation is required. Furthermore, this results in a half-height video (QHD in figure 2, for Quarter HD), which can satisfy both recommended distances for SD (D=6H) and HD (D=3H), H being the video's height [START_REF] Ichiro | Fundamental requirements for high-definition television systems[END_REF][START_REF]Methodology for the subjective assessment of the quality of television pictures[END_REF][START_REF]Subjective assessment methods for image quality in high-definition television[END_REF]. As with HD sequences, SD videos have been encoded using the H.264 reference software, with the same parameters but for two bitrates corresponding to two common SD broadcast quality. They have been chosen to be representative of an excellent (Q80) and of a rather good (Q60) subjective visual quality, respectively. It means to get scores of around 80 and 60 on a continuous subjective quality scale of 1 to 100.

Protocol

The protocol is derived from the comparative method with adjectival categorical judgment described in BT.500-11 ITU Recommendation. Series of comparison tests have been performed through several presentations. A presentation consisted of one or several visualizations of two video sequences labeled "video A" and "video B". HD and SD videos were assigned letter A or B randomly. A visualization was the viewing of both videos A and B. During each visualization, the observer compared A and B. After an initial visualization, he/she could replay any video as much as he/she wanted before voting. This random access increases reliability, as shown in the EBU's SAMVIQ methodology. After each presentation, the observer had to report the existence on a preference category scale (7 categories, see Table 3). To avoid screen flickering and screen's manual switching between HD and SD, SD videos have been inserted at the center of an HD video with gray background.

Caption to choose

Value stored I prefer much more A than B +3 I prefer more A than B +2 I prefer a little more A than B +1 I have no preference 0 I prefer a little less A than B -1 I prefer less A than B -2 I prefer much less A than B -3 Table 3: Comparison scale for comparison test protocol.

Tests have been performed in the same environment described in the previous section. The HDTV screen used was a CRT JVC DT-V 1910CG which can display 1080i sequences. Uncompressed 1080i video sequences were played with a V1-UHD player.

Observers were mostly male students in their mid twenties. All are familiar with SDTV and cinema but not with HDTV. They were checked and screened according to the method described in section 2.

Results

3.2.1.Observation Distance

Prior to the preference test, observers were asked to determine their set their own observation distance. All went farther than 3H for HDTV. Average viewing distance measured about 8H with a standard deviation of 1.5 H. This result is due to the habit of SDTV usual observation distance. People have a certain television viewing culture, and without informing them that HDTV is supposed to be watched closer, they don't adapt their habits to the new media. It is also interesting to notice that when positioned at 3H, most observers expressed the feeling that there was too much information to process, therefore observers often needed to stand back in order to consider so much information. The screen was so close that motion in the peripheral area of the retina was disturbing for many.

3.2.1.Resolution versus distortion trade off

PSNRs of the sequences have been computed to measure the distortion importance. For each sequence and each SDTV bitrate, DPSNR is defined as DPSNR=PSNR HD -PSNR SD . It measures the artifact difference between HD and SD. Figure 1 presents the preference, e.g. mean opinion score (MOS), as a function of DPSNR with 95% confidence intervals for the Stockholm Pan sequence. Since results from both configurations (Q80 and Q60) are plotted, this figure indicates that the user preference of HD over SD depends on the distortion importance difference. Results with other sequences are similar. The arrow indicates the DPSNR 0 value, which equals the DPSNR value for isopreference (MOS=0) between HD and SD. If DPSNR 0 is negative, it means that when users have no preference between HD and SD, the HD sequence contains more artifacts. In Table 4, the DPSNR 0 and PSNR HD values are given for each sequence. These values have been obtained by linear interpolation. Some conclusions can be drawn from Figure 1 and Table 4. First, with a decreasing DPSNR, SD is preferred to HD, which is the expected behavior. Then, when HD is compared to SD of a good visual quality (Q80), DPSNR is always negative. This means that HD may contain more distortion and still obtain the same preference level.

Therefore, the impact of the large screen effect is predominant and rules the observer's preference in this case. On the contrary, when HD is compared to SD of lower but still rather good quality (Q60), this effect decreases and is even reversed in some cases with DPSNR0>0. Distortions in HD become more prevalent and disturbing. Here observers tend to prefer SD implying distortions on a large image is more disturbing than on a small one. In this case, the effect of distortions is predominant over the large image effect. This means that image size is a factor of visual comfort when images are only slightly distorted. But larger image size becomes a drawback when the level of distortions increases. Observers then prefer standard definition, as this reduces the visual impact of the distortions. Therefore HDTV has to reach a high level of quality to be successfully adopted by consumers.

Sequence

IMPACT OF CONTENT ON H264 CODED VIDEO SUBJECTIVE QUALITY

The impact of H.264 artifacts on subjective quality is still under investigation [11]. Many studies exist concerning subjective quality assessment of coding artifacts, most of them consider the influence of several coding artifacts on subjective quality. Farias [START_REF] Farias | No-reference and reduced reference video quality metrics: new contributions[END_REF] synthesizes such artifacts in order to apply them independently or combined on isolated regions of the sequence. This is a content-independent approach. Wolff [11] uses sequences distorted through the use of the H.264 coding scheme. Two tasks are then asked of observers. The first is to assess the global annoyance caused by all visible impairments on the entire sequence. The second is to rate the strength of each type of artifact. We propose an alternative methodology: instead of considering different artifacts, H.264 is considered to produce, due to quantization, different perceived annoyance depending on the local spatio-temporal content. Actually, the perception of the distortions strongly depends on the local content of each distorted spatio-temporal region. For example, applying the same quantization error gives particularly visible distortions in smooth areas, whereas these distortions can be fully masked in highly textured areas. In the same way, quantization produces different results on edges. Therefore, the proposed approach is to distort only selected coherent spatio-temporal regions in terms of type of content with real coding artefacts in order to reflect common broadcasting usage.

Content classification

Since we need to study the perception of impairments depending on the local spatio-temporal content, several categories of content have to be defined. Each category should correspond to a type of content with a certain spatial activity, so to a certain impact of the H.264 coding artefacts on the perceived quality. We consider five categories defined as follows: smooth areas with low luminance (C1), smooth areas with high luminance (C2), fine textured areas (C3), edges (C4), strong textured areas (C5). In order to obtain sequence per categories, we first apply a segmentation process that creates elementary spatio-temporal volumes, then a classification of each spatio-temporal volume is assigned to its right category.

The concept of 2D+t tubes has been introduced by Wolf and Pinson [START_REF] Wolf | Spatial-temporal distortion metric for in-service quality monitoring of any digital video system[END_REF] for an objective quality video metric. In Wolf's approach, tubes are fixed in time while in the proposed approach, they are oriented along the local motion. Consequently, the temporal tubes are coherent in terms of motion and spatial activity. Therefore, the segmentation process is a block-based motion estimation which enables the evolution of spatial blocks to be followed over time. This is performed per group of five consecutive fields of the same parity (odd and even fields). For each group of five fileds, the center field i is divided into blocks and a motion estimation of each block is computed simultaneously using the two previous fields and the two next fields as shown in Figure 2. The second part is the classification itself of the tubes based on a spatial processing. This step uses four spatial gradients (DH, DV, DD45° and DD135° ) computed on every pixel of each tube. Means (DH, DV, DD45° and DD135°) over the tube of the absolute gradients are used in two spatial activity spaces P and P' in order to label the tube. Both spaces have the same geometric properties as shown in Figure 3. Plane geometry determines the global sequence block classification. Polar coordinates are used for content in order to get most relevant classification for each sequence. P' space is used only for data labelled as C5 in P. Depending on these features, a tube may be labeled as corresponding to a smooth area (C1 or C2 in both planes), a fine textured area (C3 in both planes), a strong textured area (C5 only in P') or an edge (C4 in both planes).

Subjective quality assessment

Distorted sequences are generated from the original HDTV sequence, the H.264-distorted sequences at several bitrates and the classification of the original sequence. H.264 coding is performed with the H.264 reference software at High Profile. Several bitrates of H.264-distorted sequences are selected in order to cover a significant range of quality. Parts of the distorted sequence corresponding to a category are inserted in the original sequence. This process creates one sequence per category with one spatio-temporal homogeneous content part distorted.

Subjective quality assessment tests are designed to individually measure the impact of each category on the perceived quality. According to ITU recommandations for the test environment, assessments are performed using the SAMVIQ protocol. The monitor used is a 1920´1080 HDTV 37PF9731D/10 Philips LCD display. Uncompressed 1080i HDTV sequences are played with a Doremi V1-UHD player. The test session for one content at bitrate B is composed of the following sequences: explicit reference (high anchor) ; C1-only distorted sequence at B ; C2-only distorted sequence at B ; C3-only distorted sequence at B ; C4-only distorted sequence at B ; C5-only distorted sequence at B ; entirely distorted sequence at B ; entirely distorted sequence at a low bitrate (low anchor) ; entirely distorted sequence at a third bitrate, corresponding to a quality of 40 or 60 (on a 100 scale), it is defined depending on B and on the low anchor bitrate ; hidden reference (high anchor).

Height original uncompressed sequences from the Swedish television broadcaster SVT have been used for these tests. The tested relation uses the sum of the DMOS of some or all the classes, without any weights. Table 5 presents the combinations and the associated Correlation Coefficient (CC). For CC < 0.9, only combinations with one category have been kept. These combination results reveal the relative importance of each class in the merging process made by the mean observer. Both combinations with a CC over 0.94 use almost exclusively the categories C2 and C5. Therefore, those two are particularly important in the merging process. Despite its low proportions and single combination correlation (0.5472), the category C4 is present in five of the six first combinations. The distortions on these three categories (one with edges, one with smooth areas and one with textures) are closely related to the global quality of the sequence. At the bottom of the table, single category combinations provide the lowest correlations and the highest errors, revealing that using only one category is not sufficient to explain the global behaviour. Furthermore, these values confirm the high importance of the category C2 with the highest CC for a single combination (0.7664) and the low importance of C1 and C4 alone. The latter tend to be of importance only combined with other categories. Despite its simplicity, such an approach provides high correlations with very few strategic categories. Therefore, it is possible to envisage a pooling of the partial qualities of the categories into a global one for the sequence. These combination results reveal the relative importance of each class in the merging process made by the mean observer.

Results

A Mean Opinion

Combination

VISUAL ATTENTION AND SUBJECTIVE QUALITY ASSESSMENT

Visual attention is a main feature of the Human Visual System (HVS). Knowing and using the mechanisms of the visual attention could help to the improvement of image quality assessment methods. For example, an artifact that appears on a region of interest is much more annoying than a degradation appearing on inconspicuous area. This is all the more important as the viewing angle is wider in order to identify which part of the video is exciting fovea and paravovea.

Visual attention involve two processes : the topdown and the bottom-up processes. The bottom-up process is guided by the low-level features of the viewed stimuli, and the top-down process is guided by high level cognitive factors [START_REF] James | The Principles of Psychology[END_REF]. Which kind of saliency should be taken into account ? A free-task visual selective attention or a quality oriented visual selective attention. In order to deal with this issue, eye tracking experiments have been conducted both in free-task and in quality-task. We recorded and evaluated the discrepancy between these two types of oculomotor behavior in order to show the impact of the viewing task on visual strategy.

Eyetracking experiments

In order to track and record real observers eye movements, experiments have been performed with a dual-Purkinje eye tracker from Cambridge Research Corporation. Experiments were conducted in normalized conditions (ITU-R BT 500-10). Image resolution was 512´512. They are displayed at viewing distance of four times the height of the picture. Twenty unpaid subjects participated to the experiments. Two protocols have been used : one for the free viewing task and another one for the picture quality assessment task.

Free viewing task

The free viewing condition is mandatory to lessen the top-down effects. Twenty pictures of various contents have been selected. Ten pictures present numerous artifacts. Each picture was presented to subjects in a free-viewing task during 8s. A gray picture is displayed during 3s between two test pictures. Subjects were instructed to "look around the image".

quality assessment task

In this experiment, participants have to assess the quality of a picture. The fact that a particular task is assigned will likely alter the oculomotor behavior.

To perform the picture quality evaluation, the standardized method DSIS (Double Stimulus Impairment Scale) is used. In DSIS, each observer views an unimpaired reference picture followed by its impaired version. Observer then rates the impaired pictures using a scale containing 5 scores related to impairments perception between reference and distorted image (from imperceptible to very annoying). The observers were able to give their quality scores with their eyes thanks to an interactive protocol between the display and the eyetracker. Distorted pictures are impaired by a JPEG, JPEG2000 compression or by a blurring filter. 120 impaired pictures are obtained.

Analysis

Average fixation duration

From the collected data, an average fixation duration is computed for each observer and for each picture. Two fixation phases are temporally separated by a saccade phase. To obtain the average fixation duration for a picture, we work out the average of the average fixation durations per observer for this picture. The average fixation duration can be compared when the following cases are considered:

the original picture is viewed by observers in a free-task configuration, -the original picture is viewed by observers in a quality-task configuration. This picture is just displayed before the impaired picture, -the impaired picture is viewed by observers in a quality-task configuration.

This analysis indicates that the average fixation durations are similar when considering the free-task and the quality-task configuration (with the impaired picture). In this case, the oculomotor behavior is not disturb by the task. It is important to stress that this result does not mean that observers pay attention to the same locations.

Considering the quality-task configuration with the original picture, the duration fixations are significantly longer than the previous ones. In this case, the oculomotor behavior is clearly modified. A possible explanation lies in the fact observers endeavor to accurately memorize some parts of the picture. Spatial memory seems here important to achieve the proposed task.

Saliency map

From the collected data, a fixation map is computed for each observer and for each picture. It encodes the saliency degree of each spatial location of the picture. This kind of map is often compared to a landscape map [START_REF] Wooding | Eye movements of large population : II. Deriving regions of interest, coverage, and similarity using fixation maps[END_REF] consisting of peaks and valleys. A peak, indicating the number of fixations, represents the observer's regions of interest. To determine the most visually important regions, all the fixation maps are merged yielding to an average fixation map SM. The average saliency map encodes the most attractive part of a picture when a large panel of observers is considered. Finally, the average saliency map is smoothed with a 2D Gaussian filter, given a density saliency sequence DM It is possible to test the correspondence between the different saliency density maps, one metric commonly used is the Kullback-Leibler, noted KL, whch assess the degree of dissimilarity that potentially exists between two probability density functions.

Correspondence between the different saliency maps

Figure 5 illustrates the four measures we have done: -test (A), Reference in quality-task versus reference in free-task: in this first test, we focus on the influence of the task on the oculomotor behavior [START_REF] Yarbus | Eye movements and vision[END_REF]. Do the observers look at the same area? -test (B), Reference in quality-task versus first reference in quality-task: the objective here is to show (or not) that observers adapt theirs visual strategies to inspect the original picture in a quality-task. Do they learn something in order to refine their quality judgment? -test (C), Degraded quality task versus reference free task: it is well known that the task acts on the allocation of attention. But we do not know to what extent a task modify the visual attention. This issue is here tackled by comparing saliency maps coming from a freetask and from a quality-task. Moreover, do the artifacts modify the saliency maps? -test (D), Degraded quality task versus its associated reference quality task: in a DSIS method, is the visual strategy the same for the reference and the impaired pictures? Results of the two first analysis are displayed on figure 6 whereas the two last analysis are given by figure 7. Average Kullback-Leibler divergence computed for each original picture. As shown in figure 3, the KL value is computed on one hand between the density map of the original picture in a quality-task and the density map of the original picture in a free-task, and on the other hand between the density map of original picture in a quality-task and the first density map coming from the first original picture viewed in quality-task.

As expected, the degree of dissimilarity between two saliency maps is important when two different tasks are considered (see figure 6). Second result of figure 6 concerns the adaptation of the visual strategy for a quality task. As observers saw several times the same unimpaired picture, the short term memory and the observers capacities to learn how assessed the picture quality (for example, to assess the picture quality, it is preferable to scan flat areas rather than textured areas) can likely modified the visual strategy. Although that it was reasonable to think that observers become more and more competitive, the results indicate that this hypothesis is wrong. Both the degree of dissimilarity and the confidence interval are weak.

Figure 7 allows to tackle two points: what are the differences between the free-task and the quality-task when the impaired picture is considered. The second points refers to the similarity of the visual strategy when an unimpaired and impaired pictures are considered. In others words, does an artifact have the capacity to attract or to significantly modify the visual attention? Concerning the first point, results indicates that there exists a significant difference between the visual strategy that is deployed for a free-task and a quality-task (the results of figure 6 are retrieved). Moreover, the confidence intervals are important compared to those of figure 6. It means that the type of degradation (Blur, JPEG, JPEG2000) has a significant influence. Concerning the visual strategy deployed on the unimpaired and the impaired pictures. Results suggest that there is few differences between the two saliency map (stemming from both the unimpaired and the impaired pictures). Average Kullback-Leibler divergence computed for each original picture, whatever the degradation is. The KL value is computed in on hand between the density map of the degraded picture in a quality-task and the density map of the original picture in a free-task, and in the other hand between the density map of degraded picture in a quality-task and the density map coming from its associated reference picture viewed in quality-task.

CONCLUSION

Subjective assessment quality tests have highlighted different behaviour regarding factors that impact on the perceived quality.

Concerning display, loss of quality between perceived quality on CRT and perceived quality on LCD for HD sequences with significant movements has been highlighted. One possible explanation is the motion blur. Since quantity of perceived blur depends on LCD temporal aperture, display manufacturers try to deal better with this parameter. Nevertheless, their effort will be grateful with respect of an assessment of the perceived quality improvement.

Concerning resolution factor, two important effects have been identified as critical in switching from SDTV to HDTV. Comparison tests highlight that distortions and a larger image both influence the observer's preference. Results tend to show that distortions are the predominant factor when HDTV sequences are compared with low quality SDTV. However, in the case of high quality SDTV, the image size effect becomes more important.

Concerning content factor, this paper proposed a new methodology to estimate the impact of H.264 artefacts on subjective video quality. Contrary to usual approaches that are artifacts based, the proposed methodology focus on the content. Therefore, to predict the global quality of a sequence, the presented methodology separates it into several content-based categories. It has been shown that it is possible to relate the impact of each of these categories on visual quality with the global quality of the distorted sequence.

Concerning visual attention factor, as we expected, a quality task has a significant effect on eye movements. The first result shows that the fixation duration increased on the unimpaired picture used in a qualitytask. It means that observers attempt to memorize some parts of the picture. The second important result concerns the variation of the visual strategy throughout the quality test. We show that observers are not more competitive at the end of test than at the beginning. In other word, there is no visual adaptation or task learning. Finally, it is interesting to note the type of degradation modifies the visual strategy.

Finally, all these factors, and specially resolution and visual attention, point out the gap that is remaining between needs in quality assessment and how far is our knowledge in the process involved in the quality judgment.

Figure 1 :

 1 Figure 1: ∆PSNR values in function of MOS for Stockholm Pan sequence.

Figure 2 :

 2 Figure 2: Tube creation process over five fields.

Figure 3 :

 3 Figure 3: P is the plane allowing block classification.

  Score, denoted MOS i,j,k , is obtained for each partly distorted sequence Sj and for each class Ci at each bitrate Bk. Examples of MOS i,j,k are presented on figure4. The difference between this partial MOS and the MOS of the original sequence (hidden reference) is called DMOS i,j,k . It indicates the quality loss induced by the distortions in class Ci. Each distorted class induces a quality loss which is part of the global quality loss of the entirely distorted version compared to the original one. As an attempt to determine a relation between the local DMOS of the classes and the global DMOS, an additive model has been tested. Such a relation would be very useful in order to design an objective quality metric using the presented methodology. Such a metric would evaluate the global quality from classes quality.

Figure 4 :

 4 Figure 4: MOS i,j,k for sequence Ulriksdals coded at 1 Mbps

Figure 5 :

 5 Figure 5: This schema summarizes the quantitative analysis that have been performed.

Figure 6 :

 6 Figure 6:Average Kullback-Leibler divergence computed for each original picture. As shown in figure3, the KL value is computed on one hand between the density map of the original picture in a quality-task and the density map of the original picture in a free-task, and on the other hand between the density map of original picture in a quality-task and the first density map coming from the first original picture viewed in quality-task.

Figure 7 :

 7 Figure 7:Average Kullback-Leibler divergence computed for each original picture, whatever the degradation is. The KL value is computed in on hand between the density map of the degraded picture in a quality-task and the density map of the original picture in a free-task, and in the other hand between the density map of degraded picture in a quality-task and the density map coming from its associated reference picture viewed in quality-task.

Table 2 .

 2 The selection of these bitrates has been done in order to cover the useful quality range from a poor to an excellent visual quality.

	Correlation coefficient and root mean square error
	between the two MOS sets are given below :
	CC(MOS CRT ,MOS LCD )=0.751 (2)
	RMSE(MOS CRT ,MOS LCD )=8.58 (3)
	Sequence	MOS CRT MOS LCD ∆MOS
	PARKRUN	86.28	81.32	4.96
	SHIELDS	84.68	77.95	6.73
	STOCKHOLM 83.56	81.74	1.82
	CONCERT	80.33	72.05	8.29
	FOOT	83.56	73.05	10.51
	VOILE	83.83	73.09	10.74
	SHOW	81.15	69.28	11.87
	CREDITS	82.7	73.76	8.94
	Table 1: Mean opinion scores by sequences and
		displays.

Table 2 :

 2 Chosen bitrates (in Mbps) per video sequence.

Table 4 :

 4 PSNR HD

	Q 80	∆PSNR 0	PSNR HD
	New Mobile & Calendar	-0.65	37.60
	Parkrun	-2.65	34.36
	Knightshields	-0.46	37.81
	Stockholm Pan	-0.97	37.81
	Sequence Q 60	∆PSNR 0	PSNR HD
	New Mobile & Calendar	1.27	37.39
	Parkrun	-0.87	33.23
	Knightshields	1.5	37.00
	Stockholm Pan	-0.43	36.78
		and DPSNR 0	for both SD
	sequences.

Table 5 :

 5 Combinations of classes ∆MOS and their respective correlation coefficients and RMSE withDMOS.

							CC
		C 2	+C 4	+C 5	0.9485
			C 2	+C 5	0.9440
		C 2	+C 3	+C 4	0.9094
	C 1	+C 2	+C 3	+C 4	+C 5	0.9058
	C 1	+C 2	+C 4	+C 5	0.9052
	C 2	+C 3	+C 4	+C 5	0.9041
					⋅⋅⋅	⋅⋅⋅
					C 2	0.7664
					C 3	0.7094
					C 5	0.6400
					C 4	0.5472
					C 1	0.5349

Authors would like to thank SVT for the freely available HDTV sequences, Arnaud Tirel, Fabien Beugne and Guillaume Courtin for their assistance in performing the subjective tests, Mathieu Carnec for its assistance concerning the content classification software and Olivier Le Meur for its assistance in the visual attention experiments analysis.