Francois Bonnet Pacte

Frank Bovenkerk

Rogers Brubaker

Naïma Makri

Ana Miškovska Kajevska

Mary Pattillo

Maria Reimann

Francesco Ragazzi

Eva Rosen

Clément Théry

Sudhir Venkatesh

Sharon Zukin

How to perform non-racism? Colorblind speech norms and race-conscious policies among French security personnel

How do individuals and organisations anticipate or deflect allegations of racism? This problem is especially sensitive in the context of crime control. There are two strategies to perform non-racism: colorblind and race-conscious. This article is about how French police officers and security guards perform 'not being racist', based on an analysis of the discourse and policies of 60 respondents in a shopping mall and a railway station. France promotes an ostensibly colorblind approach to being not racist, urging its citizens to avoid using racial categories. How do security people manage to perform non-racism when the majority of their clients are minority youth? The main finding is that while respondents display a strong command of colorblind speech norms (to perform non-racism), the security policy of the shopping mall is equally strongly race-conscious (also to perform non-racism).

To investigate this question, I interviewed 60 people in a shopping mall and a train station in France, a country that embodies the colorblind model on race relations. Malls and stations are places where people gather and move through fairly continuously, and are ostensibly filled with strangers. These are spaces where minority youth are perceived as posing all sorts of problems from shoplifting to vandalism and disorder. In the shopping mall, a riot involving mostly minority youth and happening on the last day of Ramadan had occurred a few years ago, heightening the concern over racism. I conducted fieldwork on law enforcement and private security, focusing on the everyday work of security guards and police officers and on the development of security policies by the management.

What is especially interesting with suspicions and accusations of racism is that they can affect individuals as well as organisations. In effect, this study is about the discursive strategies of non-racism-how people manage implicit or potential accusations of racism in the course of interviews; and about the policies of nonracism-how organisations perform non-racism through security policies they implement.

Unsurprisingly, security personnel are particularly concerned with performing non-racism: their legitimacy depends on it. The counterintuitive finding is that while respondents adopt the colorblind strategy to perform non-racism during interviews, in practice the mall's security team has to implement a race-conscious security policy. Indeed, they have little choice. Since the riot, this security team has been charged with implementing a security policy that would not incite further rioting. Store managers actively hired non-white security guards, typically Arab and Black, to deal with minority shoplifters and troublemakers, working on the assumption that racial familiarity would lead to more effective policing by deflecting the accusations of racism. My findings enable me to reflect here on the relevance of national models of race relations for making sense of local situations, and on what is tacitly defined as non-racist in a given situation.

Performing non-racism

Why would people have to perform non-racism?

In contemporary public space and social science, racism is under a strong and near-universal disapproval: very few people claim to be racist. For example, in the past twenty years, public opinion research in the United States has found fewer respondents willing to endorse racist views [START_REF] Blauner | Black Lives, White Lives: Three Decades of Race Relations in America[END_REF][START_REF] Dimaggio | Have Americans' Social Attitudes Become More Polarized?[END_REF][START_REF] Schuman | Racial Attitudes in America: Trends and Interpretations[END_REF]. In many Western democracies, overt racist speech is publicly reviled and there are legal statutes criminalising racist speech and discrimination, or aggravating punishment for hate crimes (see [START_REF] Bleich | The Freedom to be Racist? How the United States and Europe Struggle to Preserve Freedom and Combat Racism[END_REF] for a comparative analysis of the regulation of racist speech). Even extreme-right political parties, whose appeal largely rests on racial resentment, vehemently object to being labelled as racist (Bastenier 2004:14). In everyday life, this translates into a social world where people do not want to sound racist: "Many of us exhibit particular worries about being 'racist' with our very language" (Pollock 2004:1).

Because of the changing desirability of racism, people may want to avoid an accusation of racism; they may have to "perform non-racism". By "non-racism", I mean "the fact of not being racist", or "the contrary of being racist". If something or someone may be racist, then something else or someone else may "not be racist". For instance, most public figures are concerned with "not appearing racist"-with performing non-racism.

Note the difference between non-racism and anti-racism. Anti-racism is a form of activism for social and racial justice, while performing non-racism only refers to "the presentation of self in everyday life". This idea of performance refers to what people (or organisations) want to ostensibly communicate. Performing non-racism does not mean that one is not racist. One can publicly perform non-racism while privately harbouring racist thoughts or covertly engaging in racial discrimination and hateful acts. As Goffman (1959:23-25) argues, performances may or may not be sincere. What matters for this article are the norms and power relations that constrain people to perform non-racism, and not the sincerity of their performance, nor what is "inside people's head" (Eliasoph 1996: 265).

This conception is at odds with two distinct strands among the vast literature on racism.

Performances of non-racism: beyond what people "really" think

The first strand comes from structural conceptions in the sociology of racism [START_REF] Bonilla-Silva | Rethinking Racism: Toward a Structural Interpretation[END_REF], 2004, 2006;[START_REF] Desmond | What is Racial Domination?[END_REF]. Within this framework, performances of non-racism are distractions for racially naïve sociologists (see for instance Bonilla-Silva 2002:62's critique). To explain the paradox of declining overt racism and persisting racial inequality, these authors argue that front stage civil race talk (professed tolerance) is a cover for backstage discrimination and "tolerance" for racial inequality. Racism exists to maintain and perpetuate whites' privileged position, and the "new racisms" (symbolic [Kinder and[START_REF] Kinder | Divided by Color : Racial Politics and Democratic Ideals[END_REF][START_REF] Kinder | Prejudice and Politics: Symbolic Racism versus Racial Threats to the Good Life[END_REF], laissez-faire [START_REF] Bobo | Laissez-faire racism: the crystallization of a kinder, gentler, antiBlack ideology[END_REF], colorblind [START_REF] Bonilla-Silva | Les effets pervers du partage de la sécurité. Polices publiques et privées dans une gare et un centre commercial[END_REF], two-faced [START_REF] Picca | Two-Faced Racism: Whites in the Backstage and Frontstage[END_REF]) are the best functional way of maintaining racial domination. This literature translates into research that aims at uncovering backstage racism behind front stage tolerance [START_REF] Bell | Diversity in Everyday Discourse: The Cultural Ambiguities and Consequences of 'Happy Talk[END_REF][START_REF] Bonilla-Silva | The Linguistics of Color Blind Racism: How to Talk Nasty about Blacks without Sounding "Racist[END_REF][START_REF] Lewis | The Impact of 'Colorblind' Ideologies on Students of Color: Intergroup Relations at a Predominantly White University[END_REF][START_REF] Myers | Race Talk: The Perpetuation of Racism Through Private Discourse[END_REF][START_REF] Zamudio | From Traditional Racism to Liberal racism: Living Racism in the Everyday[END_REF]. From this perspective, performances of non-racism only make sense insofar as they reveal covert racism.

The second strand is the literature on discrimination [START_REF] Pager | The Republican Ideal? Ethnic Minorities and the Criminal Justice System in Contemporary France[END_REF]Shepherd 2008, Quillian 2006), which, remarkably, displays a lack of interest in the concept of racism, as if it was a less-than-scientific notion ii . By renouncing racism as a category of analysis because it is flawed, discrimination scholars prevent themselves from studying the everyday relevance of racism as a category of practice (for discussions of racism as a category of practice, see Wacquant 1997, Brubaker and[START_REF] Brubaker | Beyond Identity[END_REF]. Furthermore, like racism scholars, discrimination scholars share a concern with knowing what their subjects think "deep down"what would they say if it was not for the social desirability bias [START_REF] Pager | The Republican Ideal? Ethnic Minorities and the Criminal Justice System in Contemporary France[END_REF][START_REF] Pager | Bayesian Bigot? Statistical Discrimination, Stereotypes, and Employer Decision-Making[END_REF][START_REF] Pager | Walking the Talk? What Employers Say Versus What They Do[END_REF]; on social desirability, see Krysan and[START_REF] Krysan | Race in the Live and the Virtual Interview: Racial Deference, Social Desirability, and Activation Effects in Attitude Surveys[END_REF][START_REF] Phillips | Some Effects of "Social Desirability" in Survey Studies[END_REF][START_REF] Phillips | Some Effects of "Social Desirability" in Survey Studies[END_REF]. Discrimination scholars are thus looking for ways to "overcome" what they identify as a methodological obstacle. I, however, argue for studying social desirability in itself, as a relevant social phenomenon, because racism as a category of practice does matter in people's everyday lives.

What both perspectives overlook is how important it is for people and organisations to avoid an accusation of racism, i.e., to perform non-racism, and how this is performed.

Colorblind vs. race conscious strategies for non-racism

As mentioned previously, there are basically two strategies to perform nonracism: the colorblind (or universalist) strategy, and the race-conscious (differentialist) strategy [START_REF] Brubaker | The return of assimilation? Changing perspectives on immigration and its sequels in France, Germany and the United States[END_REF], D. Fassin 2006, E.Fassin 2006[START_REF] Lamont | Ordinary Cosmopolitanisms. Strategies for Bridging Racial Boundaries among Working-class Men[END_REF][START_REF] Lamont | Everyday Antiracism. Competence and Religion in the Cultural Repertoire of the African American Elite[END_REF]. The colorblind approach consists of denying race altogether, on the ground that all humans belong to one race, and the race-conscious approach consists of recognising racial difference with the intention of remedying racial inequality. Although each of these strategies is perfectly coherent and has been endorsed by moral leaders and antiracist activists, they are contradictory. From the colorblind point of view, raceconscious perspectives are inherently racist, since they acknowledge differences based on race. Conversely, from the race-conscious point of view, colorblindness is a pretext to prevent the implementation of policies such as affirmative action that can achieve racial equality. Either position (colorblind or race conscious) that is non-racist in one country may be deemed racist in the other.

Whether one strategy or the other is valid varies in time and space: [START_REF] Bonnett | The Americanisation of anti-racism? Global power and hegemony in ethnic equity[END_REF] has analyzed how anti-racism initiatives in Latin America were being "Americanized". The early Civil Rights Movement in the United States fought racism on a colorblind stance [START_REF] Sears | Presidential Address: reflections on the Politics of Multiculturalism in American Society[END_REF]. Today, colorblindness in the United States is often equated with neoconservative racism [START_REF] Bonilla-Silva | Les effets pervers du partage de la sécurité. Polices publiques et privées dans une gare et un centre commercial[END_REF][START_REF] Winant | Race and Race Theory[END_REF], 2007). In France, colorblindness is the preferred non-racist ideology [START_REF] Bird | Racist Speech or Free Speech? A Comparison of the Law in France and the United States[END_REF][START_REF] Bleich | The French Model: Color-Blind Integration[END_REF], see also [START_REF] Keaton | The politics of race-blindness. (Anti-)Blackness and Category-Blindness in Contemporary France[END_REF][START_REF] Ndiaye | Profiling Minorities: A Study of Stop-and-Search Practices in Paris, Open Society Justice Initiative[END_REF] for critical assessments). The variation in time and space of the types of non-racism has led to the conception that there are national models of race relations. It is, for instance, widely believed in the social sciences that France is exemplary of an idiosyncratic conception of citizenship, nationhood and immigrant integration. [START_REF] Brubaker | Citizenship and Nationhood in France and Germany[END_REF] book on French and German conceptions of citizenship and [START_REF] Favell | Philosophies of Integration: Immigration and the Idea of Citizenship in France and Britain[END_REF] on philosophies of integration in France and Britain have depicted quasi-ideal typical models of nationhood.

In many respects, France seems like an embodiment of colorblind non-racism. The French Constitution's first article states that "France (…) ensures the equality of all citizens before the law, without distinction of origin, race or religion". This makes it difficult for race-based affirmative action to be enforced, and for ethnic statistics to be collected: the French census is strictly colorblind. The colorblind discourse is especially meaningful in France in the context of the extermination of European Jews and post-war decolonisation [START_REF] Bird | Racist Speech or Free Speech? A Comparison of the Law in France and the United States[END_REF]. The denial of the Holocaust and race-based hate speech are criminalised under French law [START_REF] Bird | Racist Speech or Free Speech? A Comparison of the Law in France and the United States[END_REF]. Colorblindness is as much part of the French polity as secularism and many (not all) French anti-racists sincerely believe that ethnic statistics and affirmative action favour thinking in racialised terms, and therefore favour racism. France's emerging racial question is linked to postcolonial immigration. The two main immigrant groups are North Africans (also referred to as Arabs or Maghrebi), who mainly come from Algeria, Morocco and Tunisia, and Sub-Saharan Africans, mainly from francophone West Africa. Estimates (ethnic statistics being forbidden) suggest that there are around 3 million Arabs and 3 million Sub-Saharan Africans, roughly 10% of the nation's population [START_REF] Tribalat | Une estimation des populations d'origine étrangère en France en 1999[END_REF]. There is widespread suspicion that non-whites endure socio-economic inequality, residential segregation and discrimination, especially in the criminal justice system (Body-Gendrot and Wihtol de Wenden 2003, Open Society Institute 2009[START_REF] Pager | The Republican Ideal? Ethnic Minorities and the Criminal Justice System in Contemporary France[END_REF], suggesting a trajectory of downward assimilation [START_REF] Portes | The New Second Generation: Segmented Assimilation and its Variants[END_REF]. Chronic urban riots in the banlieues over the last 30 years reinforce the idea of a French racial question (Lagrange 2006, Waddington, Jobard andKing [ed.] 2009).

In short, France is a strategic case because its emerging racial question may challenge its colorblind ideals, and because national models need to be assessed from the bottom up. The research questions then are: How do security personnel perform non-racism when they concentrate their efforts on minority youth? What strategies do they use to avoid accusations of racism? Are these strategies predefined by established cultural models?

Research design

To explore these questions, this article draws on a qualitative study of law enforcement and private security in two urban spaces, a shopping mall and a railway station, in Lyon, France's second urban area (1.4 million inhabitants).

A qualitative study of security in train stations and malls

Railway stations and shopping malls are mass private properties where people come and go by the tens of thousands every day. Security and order are prime concerns for the management of these spaces, sharing similar concerns about the management of crowds.

The mall has a particular racial history that makes it a more sensitive space for the study of the performance of non-racism. During the 1990s, white middle classes perceived the shopping mall as declining in attractiveness, partly because it was old, and partly because of the increased presence of minority youth. The mall acquired a reputation for disorder and delinquency, which the mall's management responded to with a tougher security policy, based on the harassment of minority youth. The situation deteriorated until a full-scale riot erupted, on the last day of Ramadan (the month of prayer and fasting in Islam), resulting in nine people injured and fifteen arrests. The riot caused significant vandalism and looting, and further damaged the reputation of the mall. The station (which is near the mall) did not experience the riot with the same intensity, and its security actors are not as concerned with a riot as their colleagues from the mall (Bonnet 2009). In this article, therefore, I use the station as a counterpoint to the rather exceptional situation of the mall.

In the two spaces, I observed the interaction of security personnel and customers, including minority youth. As my familiarity with the spaces grew, I conducted 60 interviews with security guards, their various sponsors, and police officers:

-28 interviews in the station: the station manager, 6 railway company employees (including 2 labour representatives); 1 social worker; 3 shopkeepers and 2 employees (including one labour representative); 2 police managers, 3 police officers; 2 security managers for the railway company and 8 security guards. -29 interviews in the mall: 6 managers or shopkeepers, 9 employees (including 2 labour representatives), the security manager of the mall, 3 security guards (including 2 labour representatives), 3 security managers; 1 police manager and 6 police officers. -in addition, I interviewed the precinct commander, the alderman and a city official, both with responsibility for security.

Interviews ranged from 45 to 90 minutes and were transcribed and inductively coded [START_REF] Glaser | The discovery of grounded theory: strategies for qualitative research[END_REF]. The interviews questions were about the relations between private and public security, and interviews typically featured questions about the content of the respondents' daily work, their perception of security concerns, their perception of offenders, and the quality of their relations with other actors.

Speech norms and actual policies

Interviews revealed two types of performances of non-racism, both in discourse (how respondents talk about race-first empirical section) and policies (what security people are told to do about minority youth, and what they actually dosecond empirical section). Both sections are based on the same research material (interviews), but I analyze these interviews differently.

The first empirical section of the article is about discourse and speech norms. Like other scholars (e.g., [START_REF] Eliasoph | Political Culture and the Presentation of a Political Self. A Study in the Spirit of Erving Goffman[END_REF][START_REF] Eliasoph | Everyday Racism in a Culture of Political Avoidance: Civil Society, Speech, and Taboo[END_REF][START_REF] Young | The Minds of Marginalized Black Men: Making Sense of Mobility, Opportunity, and Future Life Chances[END_REF]), I use the tension that animates conversations on race as research material; it is the interaction between respondents and me which constitutes the data. Because I am a French white male, at that time a PhD candidate in sociology from an elite school, I unmistakably embodied dominant norms. I use threats to the interaction order [START_REF] Goffman | The Interaction Order[END_REF] during the interview as a tool to reveal speech norms. I am not making claims about the intentions of the speaker (whether she means to convey racial undertones or not). The problem for French respondents is that straightforward designations of minorities sound racist, regardless of the intentions of the speaker. In this article, I do not make claims about the racist meaning of racial undertones because it is not important to my argument. What matters is that respondents have to perform not being racist, and how they do it.

The second empirical section of the article is about the security policy designed by the management of the shopping mall (and implemented by its security guards). Studying actual policies is necessary, given the sometimes wide discrepancy between discourses and practices [START_REF] Pager | Walking the Talk? What Employers Say Versus What They Do[END_REF]. I focus on the shopping mall's security policy (and deliberately set the station aside) because of the consequences of the race riot and its strategic relevance for my argument about the performance of non-racism. I have researched the mall's security policy through the 29 interviews mentioned before, confronting multiple accounts of the many actors involved, in the tradition of policy case studies; reliability is enhanced by way of triangulation [START_REF] Gerring | What Is a Case Study and What Is It Good For?[END_REF][START_REF] Lin | Bridging Positivist and Interpretivist Approaches to Qualitative Methods[END_REF]. I have done non-participant observation of the interactions in the mall. As I will develop in the empirical section, the policy consists of ethnic hiring and mediation towards minority youth. Ethnic hiring has obvious consequences, and the reality of the policy of mediation is confirmed by its most bitter opponents-police officers and petty shopkeepers.

To summarize: in the first section about discourse, I study respondents performing non-racism to me, the then graduate student, during the interview interaction, both in the station and the mall. In the second section about policy, I study respondents in the mall performing non-racism to potentially rioting minority youth, as part of their security policy. I do so to show that in the mall, the same people use contradictory strategies to perform non-racism in discourse and in practice. In the second section, I use the station as a silent counterpoint to highlight the contingent aspect of the mall's peculiar security policy.

Race muted: colorblind speech norms governing interviews

Whether true or false, there is a consensus in the mall and in the train station among respondents (shopkeepers, employees or security personnel) that the main troublemakers are minority youth from disadvantaged neighbourhoods-mostly (not all) Arabs. As a consequence of colorblind speech norms, French respondents cannot use straightforward words for minorities such as "Arabs" or "North-Africans". This is not specific to the people in my sample. There is a socialscientific literature about how French institutions manage to deal with race without naming it (Fassin and Fassin [ed.] 2006). Urban policy has, for instance, been analyzed as affirmative action in disguise, with space serving as a proxy for race [START_REF] Doytcheva | Une discrimination positive à la française? Ethnicité et territoire dans les politiques de la ville[END_REF][START_REF] Kirszbaum | La discrimination positive territoriale : de l'égalité des chances à la mixité urbaine[END_REF]. "Urban violence" in the French media really means "minority crime" or "race riot" [START_REF] Macé | Les violences dites 'urbaines' et la ville : du désordre public au conflit dans l'espace public[END_REF]. In this section, I document how respondents perform non-racism during interviews by remaining "colormute" [START_REF] Pollock | Colormute. Race Talk Dilemmas in an American School[END_REF] or by carefully managing the stigma that comes with breaking colorblind speech norms.

"Colormute" respondents and the art of suggesting race

The main route to colormuteness is the use of words that are semantically void of racial meaning. To refer to minority youth, many respondents use "youth", a short form of "urban youth". In this exchange, neither party is saying anything explicit about race. The security manager is concerned by "gangs of youth", and her boss, the general manager, is merely concerned about "comfort" in the store. But the manager takes pains to tell me that she is not racist, indicating her awareness that any comment about gangs of youth might be taken as such. In this context, suggesting race while performing non-racism requires interactional skills. Skilled respondents use coded innuendoes to suggest race in the most allusive ways. These respondents may for instance allude to the clothing style of minority youth. Expensive track pants, such as Lacoste's, become a metonymy for Arab.

If you want to see problems, go to Lacoste. (Saleswoman 1, luxury superstore, white female, mall)

The multiple mentions of Lacoste are not random. In France, the Lacoste brand is widely associated with minority youth, who often dress in designer brand track pants [START_REF] Pringle | Lacoste victime de sa popularité[END_REF]. Such expensive outfits seem unaffordable to poor immigrants, and many French people believe that minority youth acquire them illegally.

Researcher: What kinds of goods are stolen?

Respondent: There is a targeted demand for certain products, men products, brands, perfumes.

Researcher: Which brands?

Respondent: Lacoste [she stares at me in silence] (Assistant manager, luxury superstore, white female, mall)

By referring to Lacoste, respondents are able to suggest race without saying race, and to refer to both race and crime without threatening colorblind speech norms. The ambiguity about racial undertones allows for plausible deniability. To reduce the ambiguity and abide more safely by colorblind speech norms, some respondents use replacement words that are less directly allusive to race, such as "fools", "tough guys with an urban style", "kids", "little morons". Again, the point is not that, in fact (inside their head), respondents are racist (or not). I document how respondents perform non-racism in a context of colorblind speech norms. Paradoxically, in the interview setting, the most neutral words ineluctably acquire a racial undertone.

Managing the stigma of a potential accusation of racism

Because of colorblind speech norms, it is difficult in France to mean race (to carry racial undertones) without sounding racist. The context of crime control means that racial undertones-already inappropriate by colorblind norms-are even more likely to be suspected of racism. Security guards and police officers in France are fully aware that their focus on minority youth fuels accusations of racism and discriminatory practices.

In private security, we have two reputations which are negative: we are bullies and even better, racists (Security manager, leisure store, mall)

Yet, some respondents, pressed with questions, cannot (or do not want to) remain colormute. How do police officers and security guards perform non-racism when they have to explicitly designate their main target?

The first solution to perform non-racism while engaging in explicit race talk is contrition, the expression of sorrow and repentance, which is a way for respondents to signal that they respect, understand, and usually abide by, the dominant norms of non-racism (since racial categorization is already a form of racism in France). In numerous instances, respondents become demonstrably apologetic in their explanations of talking directly about race, anxious that they are violating speech norms. The point is not whether they are truthful or sincere, but how they speak about race.

Researcher: Who are the offenders?

Respondent It is remarkable that even minority respondents use oratory precautions to violate colorblind speech norms. Assertive respondents act as if they want to convince that they are non-racist, instead of apologizing for saying something that might offend speech norms, as contrite respondents do. In short, the interviews force respondents into tricky performances of non-racism who reveal colorblind speech norms.

Deviance to speech norms

Not all respondents abide by colorblind speech norms. One main reason for this is the evolution of the relationship between the sociologist and the respondent. Most of the explicit quotes in this article are the result of a progression in the interview. But some respondents are truly deviant. The following excerpt is from an interview with a white police officer in his fifties who speaks about the criminal activity in the mall. After discovering by chance that he and I are distant relatives, the respondent spoke as if speech norms do not apply any more. It is worth noting that this quote is unique in my sample (his colleagues had warned me that he was a fringe character). What is nonetheless meaningful is that the deviant respondent makes speech norms explicit ("I won't say who, I'd be called a racist", "we are not allowed to say it"). Most of his argument belongs to the register of defensive assertiveness. He implicitly presents himself as an extreme-right voter whose electoral behaviour is explained by the over-criminality of minority youth and the speech norms preventing discourse about minority crime (and not by an irrational hatred of minority): in effect, he communicates "I violate speech norms, but not out of racism". It is striking that even he attempts to perform non-racism. In the next section, I show how non-racism also has to be performed in policy-not just interviews.

Race enacted: strategically race-conscious security policies

To study policy, the mall's case is of special relevance because of its racial history. The mall's management and its security team have to police the premises because crime and disorder threaten business' profitability [START_REF] Lee | From Civil Relations to Racial Conflict: Merchant-Customer. Interactions in Urban America[END_REF]. As a consequence of commercial decline and racial tensions that culminated in a race riot, the management was changed and supervised a $30-million dollars renovation of the mall. The new management has implemented a security policy which does not derive from a change in law [START_REF] Dobbin | The Strength of a Weak State: The Rights Revolution and the Rise of Human Resources Management Divisions[END_REF] nor is it due to the rise of human resources professionals (Dobbin 2001(Dobbin -2002)), rather it stems from the realization that performing non-racism is key to avoiding another riot.

Security personnel have to handle two types of criminal problems in the mall: shoplifting and disorder [START_REF] François | Un crime sans déviance : le vol en interne comme activité routinière[END_REF]2008b). In my sample, most respondents perceive Arab youth as mainly responsible for most of the petty shoplifting and for disorderly conduct (loud arguments, exuberant behaviours, rude comments towards teenage girls who object to being sexually insulted). Security guards need to control shoplifting and to limit disorderly behaviours, so that customers do not perceive the mall as unwelcoming and/or an "Arab territory". But they must do so in ways that appear non-racist, to avoid another riot.

To protect themselves against potential accusations of racism, the mall's management and the stores have hired security guards with minority backgrounds.

In the communal parts of the mall, which are run by the mall's management, most of the security guards are Arab. In the stores, which pay rent to the mall and are subjected to problems of shoplifting, guards typically are Black. The rationale of the policy is the following: since minority teenagers are involved in a significant amount of shoplifting and in the vast majority of disorders, security guards need to focus their attention on minority teenagers, and, if security guards were white, minority teenagers could accuse them of being racist. However, since guards also are minority, the accusation becomes irrelevant.

For one thing, you can't say they [minority guards] are racist. (Security manager, white male, mall)

Again my point is not to assess the truth of respondents' folk theories, but to examine its logic and effects. Why are guards Arab in communal parts and Black in the stores? In communal parts, the shopping mall's management wants to reduce tensions between the security team and teenagers, and to prevent further disturbances. Arab security guards are hired from the same housing projects where teenagers are supposedly from. Cultural proximity between teenagers and guards allegedly allows for better communication.

And then, they know the kids, they have the same language. It's better than if someone else was taking care of the problems. (Security manager, white male, mall)

The explicit mission of Arab guards is to ensure that relations between storekeepers and teenagers do not escalate. They must prevent conflicts, keep situations under control and make sure that both side's feelings are spared. Their role is similar to the Black employees of Jewish-owned stores in African American neighbourhoods: they act as "cultural brokers" Lee (2002:89). To avoid public arguments and volatile scenes, security guards are instructed to bring turbulent youth out, into the service corridors, outside public view. There, the guards call the mediator-an older (Arab) guard dressed up in a suit as opposed to an uniform-who specializes in talking youth into calming down and even apologizing to the people they wronged. This occasionally creates tensions with shopkeepers who cannot hire their own guards, and have to rely on the mall's.

There are these hoodlums iii These tensions demonstrate the reality of the policy of appeasement towards minority youth: small shopkeepers expect a kind of support from the mall's guards that is very different from the support they actually get. Shopkeepers want aggressive policing from the guards, but guards have been instructed by the mall's management to avoid the type of aggressive policing that eventually leads to riots. In fact, managers cite the lack of rioting by Arab youth in the past decade as direct evidence of their non-racism.

Within the bigger stores, the purpose of security is to prevent thefts and expel troublemakers. Security guards in the stores have a more coercive role than in the communal parts: it is not so much about negotiating with troublesome teenagers than about deterring arrested thieves from making scenes. Most guards in the stores are Black. Experience seems to have taught private security companies' managers and store managers that white guards do not intimidate young Arab, while Blacks have a reputation that allows them to fulfil a dissuasive role, so they are mainly hired for the heavier policing (corroborated by From an ethnomethodological point of view, these practices operate under the tacit assumption that minority guards are ontologically immune to an accusation of racism, that only whites can be racists and/or engage in racial discriminations.

A focused study such as this cannot tell whether the mall is the norm or the exception in its strategic ethnic hiring practices. In the case of the railway station nearby, however, there is no evidence of such policies, suggesting that the choice and implementation of a policy depends on local context and contingent factors. Race-conscious security policies certainly exist elsewhere in colorblind France: one of the respondents, a movie theater manager who has worked in Paris in the past, recalled similar matching strategies.

It depends on who the customers are, here we don't have too many Black people. In the 13 th district of Paris, there is a lot of Asians, so they have Asian guards, it's simpler. They communicate more easily, they understand each other, they speak in their language, it's in their own culture. In Cergy, at Les Halles, black guards, they didn't do the trick. There was a bad feeling with the customers, it was a problem. At Les Halles, it was the Eastern Europeans [guards] who did well. It depends on the place, there is a category that works better. (Manager, movie theater, white male, mall)

Such practices are interesting for they illustrate the discrepancy between raceconscious policies and colorblind speech norms. While interviewees have to speak colorblind to perform 'not being racist' in their interaction with me, they also have to implement a race-conscious security policy, precisely also to perform 'not being racist' in the everyday interactions that occur in the mall between the security team and the minority teenagers. This race-conscious security policy stems directly from the mall's management determination to avoid further rioting, and is consonant with similar efforts of ethnic hiring in law enforcement undertaken in the United States after race riots (Sabbagh 2011, p. 111-113;[START_REF] Skrentny | The Ironies of Affirmative Action: Politics, Culture, and Justice in America[END_REF]. Following Ward (2006:69)'s conceptual framework, the absence of riots in the mall suggests that ethnic hiring among the private security workforce affects both the substance of policing and its symbolism. This is a significant result (albeit restricted to this case), since research on the effects of a diverse workforce in the criminal justice system tends to find little tangible consequence to diversity [START_REF] Farrell | Race Effects of Representation among Federal Court Workers: Does Black Workforce Representation Reduce Sentencing Disparities?[END_REF]Bourreau 2009, Ward 2006).

Conclusion

How to perform not being racist? I have argued that the exact same people resort to symmetrically contradictory strategies: the security guards of the shopping mall have to speak colorblind to perform non-racism in their interaction with me, but they have to implement a race-conscious security policy, precisely also to perform non-racism in the everyday interactions with minority youth, and all this in a colorblind country. This finding raises several concluding remarks.

An analysis which aimed to assess how "racist" the mall's security personnel are would miss the point on the local dynamics of race relations. The concern over performing non-racism is more than just a way of covering up for inner racism: it may shape policies. Racism is a powerful negative label, for those who design the policy and those who implement it. Respondents work hard, both in speech and in practice, to avoid that label. This result challenges racism research that assumes racist backstage attitudes or behaviour behind front stage tolerance. Even respondents who harbour private racist attitudes have to implement a policy that performs non-racism.

This result does not apply for the case of the railway station, where respondents "only" have to perform non-racism in discourse. Clearly, the historical context of the mall-the race riot-is a key factor for the race-conscious security policy. This highlights that local context matters. The discrepancies between the station and the mall on the one hand, and between colorblind speech norms and raceconscious policies in the mall on the other hand, suggest that a national colorblind ideology does not translate into a world where racial cognitions have been erased from people's mind. Race is definitely a category of practice, a category most French people use in their making sense of the social world. A national colorblind model does not prevent people from engaging into race-conscious practices. The local definition of non-racism does not derive from national models. This has implications for the generalizability of the findings. Whether or not people and organizations have to perform non-racism, and how they do it, should not be inferred from national models, but assessed in local settings.

Regarding the matter of how non-racism is defined, it seems important to point out its interactional component. We have seen that speech norms are not about what can be said (in the abstract), but about to whom it is said. When an interview begins as a conversation between strangers, slurs are offensive. A few minutes later, when the interview interaction is redefined as a casual chat among acquaintances, the slurs may lose their offensiveness for the participants. Speech norms should not be analyzed in terms of political correctness (as [START_REF] Pager | The Republican Ideal? Ethnic Minorities and the Criminal Justice System in Contemporary France[END_REF][START_REF] Pager | Bayesian Bigot? Statistical Discrimination, Stereotypes, and Employer Decision-Making[END_REF][START_REF] Pager | The Republican Ideal? Ethnic Minorities and the Criminal Justice System in Contemporary France[END_REF]Quillian 2005 do) because nothing is intrinsically "politically incorrect": the violation of speech norms is interactional in nature. "Political correctness" essentially is a category of practice; it is a loose, constantly redefined label; to use it as a category of scientific analysis is to be blind to the "situatedness of race talk" [START_REF] Pollock | Colormute. Race Talk Dilemmas in an American School[END_REF]).

More importantly, non-racism is also defined in the daily interactions between law enforcement and private security and minority. To avoid damaging conflict, security personnel have to successfully perform non-racism to minority teenagers.

In the absence of institutional channels, youth ultimately decide by rioting or not whether the performance is successful or not. In practice, security guards have to take into account the teenagers' tacit definition of what is a non-racist policing.

The implementation of a race-conscious policy informs us on the tacit drawing of racism's boundaries in the shopping centre: white guards arresting minority teenagers falls inside these boundaries (it is "racist"), while minority guards managing minority teenagers remains outside (it is "non-racist"). The policy's race-conscious dimension shows that rather than deriving from the national model of non-racism, it derives from an empirical, bottom-up, practical understanding of the youth's tacit definition of non-racism. Of course, there can be many factors other than the mall's policy that explain the absence of rioting; but as long as the mall's management perceives its security policy as one of the relevant factors, the absence of rioting confirms the success of the policy's performance of nonracism.

  Although these euphemisms can denote other social attributes, like class and space (see Kirschenmann and Neckerman 1991), a greater sensitivity arises around race.Security manager: Gangs of youth. My top priority is to deter those youth that come in groups. So, when they come, we know them. I have made a priority of fighting against those youth. (…) This is a private area, open to the public. We intend on staying in charge of our own territory.

	Researcher: What is your main security problem?

Store manager: Gatherings are not forbidden, if they are chatting together, it's no big deal, but provocation is a problem. Sometimes we have gangs running across the store laughing. We don't want to run any anti-youth or racist policy. I don't want this interview to go that way but we are working for the comfort of our customers. (Security manager, white male, and store manager, white female, department store, mall)

  second solution for interviewees is to deflect a potential accusation of racism by engaging in defensive assertiveness. In this speech act, respondents violate colorblind norms in ways that communicate an awareness of said norms and a rationale for the violation. It is as if they were saying to me, "I know that you suspect me of racism, but I'll try to convince you that I am just doing what my job asks of me". Assertive interviewees act as if displaying contrition would be an implicit admission of their guilt, and they take on a more direct approach by trying to reframe the terms of the debate.The rationale of defensive assertiveness is that the over-criminality of minority youth is a "sad truth" that cannot be denied and that security people have a functional requirement to engage in what may appear as racist behaviour. Interestingly, a minority respondent-therefore less in need of performing nonracism-also used defensive assertiveness to speak about disproportionate minority delinquency. This Arab security guard walked me out of the mall after an interview. He put his hand on my shoulder and lowered his voice:

	You want me to be honest with you? You really want me to be honest
	with you? Objectively speaking? 90% of my clients are North-Africans
	or African youth. It won't please everybody but sadly, it's the truth.
	90% of my clients. The 10 remaining percent... It's a fact, my clients, I
	am not going for them, they are coming to me [awkward laughs].
	(Security guard 1, white male, station)
	: [Hesitant] They are young…yes young, but in [the train
	station] since I've been here, I haven't had much to do with
	delinquents, there are more problems like infringements of SNCF
	rules; when they don't have their train ticket, identity papers, stuff like
	that, yes, they are mostly young people… [short pause] er…[lowers his
	voice] North-Africans. So I am telling you the truth as it is, it's
	not…er… (Security guard 2, white male, station)
	Contrition ostensibly communicates that respondents are aware of colorblind
	speech norms and of the gravity of their offense. Contrite respondents perform
	non-racism by showing deference to norms: they seem to check that I am aware of
	their acceptance of conventional speech norms. Contrition thus actually reveals
	that colorblind speech norms are norms.

A I'm going to speak frankly, I am a Maghrebi. The youth, I tell them: "who's screwing around, it's us, who spits on the sidewalk and can't behave in the street, it's us.

Yet, at home, in Algeria, in Morocco, you don't act like that! (Security guard 5, Arab guard, mall)

  [foreign-sounding names] you arrest 50 of them. Delinquency, it's 95% from Maghreb [North Africa]. Ah, that's France, you can't say anything… They are promoting Jean-Marie [extreme-right leader Jean Marie Le Pen]. The énarques [the liberal elite], these dickheads, they tell you that everything's fine, but people are not dumb, when they

	steal your bike, you know it's them, they spit at you, they call you "son
	of a bitch", so obviously, people arrive at the voting booth, they can
	vote right, they can vote left, and they vote Jean-Marie. (Police officer
	3, white male, mall)
	The problem, it's always the same youth, the same gangs we get, I
	won't say who, I'd be called a racist, they come to steal, to drink, to
	fuck with the population. Well, back there in Italy, they can hire as
	many security guards as they please, they do have [racial slur for

Arabs], they do have Romanians, and also because Italians are thieves. It depends on race! We are not allowed to say it, but it depends on race. Durand, Dupont, Bonnet [French-sounding names], we get one maybe twice a week, the Benhamou, the Karim, the Zoulou

  [START_REF] Hug | Les agents de sécurité noirs. Un exemple de discrimination dans le monde de la sécurité[END_REF]; see Jackson 2006 about the exoticization of racialised masculinity). The guards' Blackness is simultaneously a solution to the problem of projecting a credible threat of coercion towards Arab youth and to the problem of performing a non-racist security policy.

	In the stores they are Black; I was told "I send you Blacks; they scare
	Arabs". Security guards-the people they calm down, they are often
	Arabs. My boss told me "Arabs fear Black people". (…) The fact is
	also that Blacks are well-built, they are impressive, they are calm, and
	they are not French [white]. (Assistant general manager, sports
	superstore A, white male, mall)

It has been financially supported by the SPACECONTROL project (ANR n o ANR-10-ESVS-001-01), at Maison européenne des sciences de l'homme et de la société (MESHS -USR 3158) and by the Dynamics of Citizenship and Culture program group at the University of Amsterdam.


C Richardt

J Lopez-Moreno

A Bousseau

M Agrawala

G Drettakis

Vectorising Bitmaps into Semi-Transparent Gradient Layers

Editing result (b) Multi-layer vector representation (a) Input photograph

Figure 1: Our interactive vectorisation technique lets users vectorise an input bitmap (a) into a stack of opaque and semi-transparent vector layers composed of linear or radial colour gradients (b).

Users can manipulate the resulting layers using standard tools to quickly produce new looks (c). We outline semi-transparent layers for visualisation; these edges are not part of our result. We rasterised figures to avoid problems with transparency in some PDF viewers. See supplemental material for vector graphics.

Introduction

Vector graphics enjoy great popularity in graphic design for their compactness, editability and scalability. Skilled vector artists commonly blend multiple layers, each composed of simple colour and transparency gradients, to represent the appearance of an object. Each layer typically corresponds to a single aspect of the shading, such as diffuse shading, specular highlights, shadows or Fresnel reflections. The challenge is to fully capture complex shading effects while also maintaining a small number of layers and gradient parameters so that the vector representation remains compact and easy to edit. † Inria authors are at REVES/Inria Sophia-Antipolis Méditerranée.

Automatic image vectorisation techniques convert bitmaps into a set of editable vector paths and gradients. (We use the term gradient to refer to the smooth colour variations encountered in vector graphics; this should not be confused with the gradient of a function, i.e. the vector of its partial derivatives.) We believe that the use of semi-transparent layers has largely been overlooked by existing algorithms which focus on the generation of opaque vector graphics [LL06,SLWS07,LHM09,XLY09,LHFY12]. The resulting vector representation is often very complex and difficult to edit because it combines all aspects of the shading into a single opaque layer.

We present the first interactive decompositing tool that can generate a simple, editable set of opaque and semi-transparent vector layers directly from a bitmap. Users progressively select smooth colour regions in the image, which our system separates into a foreground region filled with a vector gradient and a background bitmap layer. Users can repeat the process to further separate the background bitmap into additional gradient layers. The final step is to vectorise the remaining background layer. Figure 1 shows how the resulting vector representation separates the transparent highlights, shadows and liquid for a photograph of a wine glass. Users can edit each layer independently and then re-composite them to quickly generate a variety of looks and appearances.

Semi-transparent layers are especially challenging to extract as they require us to invert the compositing equation [PD84], a non-linear mixture of foreground and background colours. While this problem has been considered in the context of alpha matting [SB96, WC07] and reflection separation [LZW04], existing methods estimate bitmap layers rather than vector gradients and therefore do not provide a small set of parameters suitable for editing the matted layers. Our main technical contribution is a decompositing algorithm that exploits the parametric nature of vector gradients to jointly separate and vectorise semi-transparent layers. In particular, we constrain the foreground colours to vary according to linear or radial gradients, allowing us to solve for a small set of parameters per layer instead of the thousands of unknowns over all pixels in a region. The few degrees of freedom of linear and radial gradients also offer a good trade-off between goodness of fit and editability, while simplifying colour variations in the image to reproduce the clean and sharp look of hand-made vector art. We have integrated our decompositing algorithm in Adobe Photoshop and we export our gradients as Illustrator layers, allowing vector artists to create and edit semi-transparent layers with the tools they are familiar with.

Our approach is limited to foreground layers composed of linear or radial gradients, and therefore cannot capture complex semi-transparent textures or reflections. However, vector images are often not meant to have the complexity of natural images. Instead, vector artists create images that represent objects with stylised lighting. We facilitate the creation of similar vector art from studio photographs, with lighting designed to be simple and effective. We demonstrate that our approach can successfully vectorise a diverse set of studio photographs and drawings into a layered representation (Figures 1 and 6). We also show how the resulting representation supports a variety of editing operations such as modifying the shape of highlights, altering the diffuse colour or texture of a region, or changing the gradient parameters to significantly alter the surface appearance of an object.

Related Work

Image vectorisation The goal of vectorisation algorithms is to facilitate or automate the creation of vector graphics from bitmap images. Most methods segment the input image into smooth colour regions that are then represented by vector gradients. Lecot and Lévy [START_REF] Lecot | Ardeco: automatic region detection and conversion[END_REF] fit linear and radial gradients to generate vector images in Art Deco style. Gradient meshes represent complex gradients by interpolating colours over the faces of a quad mesh, making them a powerful primitive to capture the smooth colour variations of natural images [START_REF] Sun | Image vectorization using optimized gradient meshes[END_REF]. However, this smooth interpolation is less effective in representing sharp colour discontinuities, which require the introduction of holes [START_REF] Hu S.-M | Automatic and topology-preserving gradient mesh generation for image vectorization[END_REF] or closely-spaced mesh lines on either side of image edges [START_REF] Sun | Image vectorization using optimized gradient meshes[END_REF]. This limitation motivates Xia et al. [START_REF] Xia | Patch-based image vectorization with automatic curvilinear feature alignment[END_REF] and Liao et al. [START_REF] Liao | A subdivision-based representation for vector image editing[END_REF] to represent the image as a piecewise-smooth surface whose control mesh aligns with strong image edges, using a multiresolution representation for the latter. These approaches also share similarities with diffusion curves [OBW * 08], which vectorise images by storing colours on each side of strong edges while computing smooth colour variations in-between the edges using a diffusion process. All of these algorithms are designed to vectorise opaque objects into a single layer.

Our approach complements these techniques by decompositing the image into multiple transparent and opaque layers. We represent transparent layers with linear or radial gradients because artists often use such simple gradients to represent reflections and shadows in vector art. We also found that linear and radial gradients are sufficient to represent the highlights, shadows and transparency encountered in stock photographs, which are typically shot using large area light sources. In contrast, opaque layers often contain more complex shading variations, which we express with any suitable representation, such as gradient meshes [START_REF] Sun | Image vectorization using optimized gradient meshes[END_REF]. The main advantage of our layered representation is that each layer is simpler than the composed image.

A few vectorisation algorithms consider the creation of layered representations for opaque objects, for example relying on motion detection to separate dynamic foreground objects from a static background when vectorising cartoon animations [ZCZ * 09]. Price and Barrett [START_REF] Price | Object-based vectorization for interactive image editing[END_REF] decompose objects into several gradient meshes using a hierarchical segmentation algorithm. In contrast, we focus on semi-transparent layers, which require inversion of the compositing equation.

The versatility of vector layers motivates Eisemann and colleagues [EWHS08,EPD09] to convert 3D models into vector images, separating cartoon shading, highlights, shadows and occlusions. Lopez-Moreno et al. [LMPB * 13] generate editable shading effects as vector layers by first building normal fields from line drawings. We share a similar goal of generating and editing vector layers, but our input consists of bitmap images. Thus, we must estimate layers by inverting the compositing equation rather than rendering them from a 3D model or a normal field.

Image decomposition

The key observation that guides our approach is that complex images often can be explained with just a few simple layers. In fact, researchers have shown A user selects a region in an image (a), which our algorithm decomposites into a vector foreground layer and a background bitmap (b). This process is repeated and opaque layers are handled with existing tools to create a complete set of vector layers (c). Layers can be edited easily (shown as blue lines in d) and added (green lines), and re-composited to enable powerful editing applications (e).

that the human visual system infers transparent layers if they result in a simpler explanation of the visual stimuli [START_REF] Adelson | Perceptual organization and the judgment of brightness[END_REF][START_REF] Zomet | Learning to perceive transparency from the statistics of natural scenes[END_REF]. Vector graphics represent transparency using alpha compositing [PD84], which composites the foreground colour F over the background B using the foreground's opacity channel α:

I = F • B = α • F + (1 -α) • B.
(1)

Matting algorithms aim to invert this equation in order to recover the RGB colours F and B and the scalar α at each pixel of an input image I. Existing algorithms solve this problem using assumptions on the colour distributions of the foreground and background layers [SB96, CCSS01, WC07, LLW08]. While we adopt a similar strategy, we adapt it to our context by constraining smooth regions to be filled with linear or radial gradients, allowing us to jointly separate and vectorise the foreground layer into commonly used vector gradients.

Reflection-separation methods handle semi-transparent layers by estimating colour channels pre-multiplied by the alpha channel, reducing the image formation model to a sum of two layers I = F + B [LZW04, LB14]. While this approach linearises the problem and facilitates its resolution, it prevents the subsequent independent manipulation of colour and transparency as is common in vector graphics applications. Yeung et al. [START_REF] Yeung S.-K | Matting and compositing of transparent and refractive objects[END_REF] describe a user-assisted method to extract semi-transparent objects from photographs and re-composite them. They introduce a three-layer bitmap representation to capture bright highlights, smooth colour transmission and refractive deformations. While we do not support refractive deformations, we unify highlights and colour transmission by expressing both effects as parametric gradients.

Overview

A layered image consists of multiple layers L i that are composited from back to front onto the bottom-most layer L 1 . Thus, each pixel x of an image In composed of n layers is formed recursively as To decomposite an image into its constituent layers, we must invert this equation. However, the inverse problem is underdetermined as n layers result in n unknown RGB colours and n-1 unknown alphas to explain one RGB observationper pixel. We adopt a greedy approach, removing one layer at a time in their reverse compositing order, from front to back. That is, at each step, we separate the image I k into a foreground vector layer L k with its alpha channel and the remaining background bitmap I k-1 , which we further decompose in subsequent steps.

In(x) = Ln(x) • I n-1 (x) n > 1 L 1 (x) n = 1. ( 2
We additionally constrain each semi-transparent foreground layer by modelling it as a parametric gradient function f = c • g = c(g(x, θ), θ), composed of a gradient function g(x, θ) that maps a pixel coordinate x ∈ N 2 to a parametric value β ∈ R, and a colour function c(β, θ) that maps this parametric value to an (r, g, b, α) colour, all based on a set of parameters θ. As shown in Figure 3, these parameters encode the colours of a gradient as well as their transition, through the colour and gradient functions, respectively. The gradient function g defines linear gradients by a direction vector θv and signed offset distance θo from the origin, and radial gradients are defined by centre θp and radius θr:

g linear (x, θ) = x • θv θv 2 + θo and
(3)

g radial (x, θ) = x -θp θr . ( 4
)
The colour function c maps the parametric gradient to colours using piecewise-linear interpolation between gradient stops with colours θ ci (i = 1, . . . , k) at offsets θ si ∈ [0, 1], such that θ s1 := 0, θ sk := 1. We express gradients with 2 and 3 stops as

c 2 (β, θ) = mix(θ c1 , θ c2 , β) and
(5)

c 3 (β, θ) =    mix θ c1 , θ c2 , β θs2 β ≤ θ s2 mix θ c2 , θ c3 , β-θs2 1-θs2 β > θ s2 , (6)
with mix(a, b,t) = (1t) • a + t • b for linear interpolation.

The colour functions can easily be extended to more stops.

We represent opaque layers with arbitrary colour fills, which include constant colour, linear and radial gradients of multiple colours, and gradient meshes.

Our interactive method proceeds as follows. The user first selects a smooth image region for decompositing using Adobe Photoshop's selection tools. The user additionally indicates if the region is opaque or semi-transparent, if it is a linear or radial gradient, and how many stops it has (we have presets for two and three). The user can optionally indicate that a region is a highlight or a shadow, which further constrains the decomposition. Our algorithm then separates the selected region from its background and vectorises it. It first converts the boundary to a Bézier spline using the open-source vectorisation tool Potrace [START_REF] Selinger | Potrace: a polygon-based tracing algorithm[END_REF]. It then separates the region into a foreground gradient layer, by fitting a gradient to the colours inside the region (Section 4.1), and a background bitmap layer (Section 4.2). The user can choose to further decomposite the background bitmap into additional gradient layers by repeating this procedure, or vectorise the background as a one or more opaque layers.

Decompositing

Extracting a semi-transparent gradient layer requires us to estimate the parameters θ of the foreground gradient f as well as the bitmap background B that together best explain the input image. We cast this estimation as a least-squares minimisation over the selected region R:

arg min θ,B ∑ x∈R I(x) -f (x, θ) • B(x) 2 , ( 7
)
where the compositing implicitly involves the foreground alpha:

f (x, θ) • B(x) = f α (x, θ) • f rgb (x, θ) + (1-f α (x, θ)) • B(x), (8)
and subscripts select (r, g, b, α) channels: f α (x, θ) is the alpha component of the foreground gradient, while f rgb (x, θ) is its

RGB colour vector f r (x, θ), f g (x, θ), f b (x, θ) .
Finding the foreground gradient parameters θ and background image B that minimise Equation 7 requires solving an under-determined system. Each input pixel in I provides one colour and we must recover one colour for each background pixel as well as the set of foreground gradient parameters for the region. Our algorithm tackles this problem in two steps, by first estimating the foreground gradient (Section 4.1) and then the background bitmap (Section 4.2). We can separate the foreground 2-stop gradient if at least two iso-contours (dashed lines) are observed over two different background colours. This condition extends to each linear piece of multi-stop gradients. We assume the background is continuous across the region boundary, so blue locations approximate the background at the green locations.

Solving for a semi-transparent foreground

Equation 7 corresponds to the matting problem, which Smith and Blinn [SB96] proved to have a unique solution in the context of bitmap images if each pixel of the foreground is observed over at least two different known background colours. In our context, the foreground pixel values are the same along each iso-contour of the gradient, which implies that we can solve for the matting problem along an iso-contour if we observe at least two of its pixels over different background colours. Furthermore, two different iso-contours are sufficient to constrain the location and colours of two-stop gradients (Figure 4). Our problem is hence well-posed if we observe at least two different iso-contours, each with at least two pixels on different backgrounds, for each linear segment of a multi-stop gradient. In practice, we erode and dilate the hard selection by a few pixels to account for anti-aliasing or blurry edges in the image, sampling x along the boundary ∂R of the eroded region (shown in green in the inset) and b(x) outside the dilated boundary (shown in blue). We then solve for the optimal parameters Θ of f solely based on the boundary pixels as

Θ = arg min θ ∑ x∈∂R I(x) -f (x, θ) • I(b(x)) 2 . ( 9
)
We minimise Equation 9 using a standard non-linear leastsquares solver. We found that the IPOPT solver [START_REF] Wächter | On the implementation of a primal-dual interior point filter line search algorithm for largescale nonlinear programming[END_REF], wrapped by the CoMISo package [BZK12] with automatically differentiated Jacobians and Hessians, achieves good convergence. We constrain variables expressing colours, opacities and stop offsets (i.e. θ ci and θ si ) to the unit range to produce valid vector gradients. When the background is uniformly coloured, we cannot obtain two observations of the foreground on different background colours and therefore cannot uniquely solve Equation 9. In this case, we offer users two additional constraints. Users may choose to fix the foreground to have a known constant colour so that only the alpha channel is unknown. We provide this constraint to users in the form of two presets: a white colour for highlights and a black colour for shadows. Users can alternatively choose to favour a semi-transparent solution, which we express as an additional weak regularisation term ε| f α (x, θ) -0.5| 2 in Equation 9.

Limitation Our assumption of a continuous background across boundaries may not hold when the background contains strong edges that are coincident with the region boundary. We later account for incorrect background pixel estimates in a subsequent step (Section 4.3).

Recovering the background

Given the foreground gradient f (x, Θ), we estimate the background bitmap B from the image I by inverting the composition equation (1) at each pixel of the selected region as the background estimate

B(x) = I(x) -f α (x, Θ) • f rgb (x, Θ) 1 -f α (x, Θ) + ε , ( 10
)
with ε = 10 -20 for robustness to opaque pixels.

However, the resulting background layer can be polluted with three types of artefacts (Figure 5). First, while the user specifies a hard selection boundary, the region often has a blurry boundary in the original bitmap, which produces halo artefacts after foreground removal (Figure 5b). We address this problem by computing the alpha matte M(x) of the soft boundary in the image, and then replacing f α (x, Θ) with M(x) f α (x, Θ) in Equation 10. We use the Laplacian matting algorithm to compute the matte [LLW08], dilating and eroding the user-specified region boundary to generate a suitable trimap (see Figure 5c).

The two other artefacts we observe are noise from the input bitmap, which is amplified by the inversion of the compositing equation, and a low-frequency residual due to smooth variations in the foreground that are not modelled by a linear or radial gradient, producing a colour mismatch on the border of the region. We treat both artefacts using smoothing (Figure 5d) and Poisson blending [START_REF] Pérez | Poisson image editing[END_REF] (Figure 5e), solving for the background that minimises arg min

B λc • ∑ x∈∂R B(x) -I(b(x)) 2 + λp • ∑ x∈R 1 -f α (x, Θ) • ∇B(x) -∇ B(x) 2 +λv • ∑ x∈R ∇B(x) 2 , ( 11
)
where the first term favours continuous colours across the region boundary, the second term preserves variations of the initial estimate B within the region, and the third term penalises variation overall. We additionally weight the second term by (1f α ) because the division by the same factor in Equation 10 amplifies noise in the image, such as sensor noise or quantisation, particularly for more opaque foregrounds. The optimisation can be solved using a standard linear leastsquares solver, such as the backslash operator in MATLAB. We use weights λc = 10, λp = 1 and λv = 0.1 for all results.

We also experimented with total-variation (TV) denoising |∇B| that better preserves sharp edges but is non-linear [ROF92]. We use ∇B 2 by default and allow users to apply total variation if they are not satisfied by the diffused result. We solve the optimisation with total-variation denoising using iteratively reweighed least-squares [RW09].

Iterative gradient refitting

Image pixels outside a region are not always similar to the background colour behind the region, for example if the region boundary aligns with an edge in the background. These incorrect background samples are outliers that bias the estimation process and therefore need to be excluded. We iteratively exclude these outliers and refit the gradient until convergence. Specifically, we compare the sampled image colour I(b(x)) to the initial background estimate obtained from Equation 10 and remove any samples that differ significantly. The complete inlier criterion is

(1 -f α (x, Θ)) • I(b(x)) -B(x) < τ, (12)
where we weight the Euclidean colour difference with a (1f α ) term as the background estimate is less accurate for more opaque pixels. We use ten iterations and a default value of τ = 0.2. Users can alternatively refine this outlier rejection by interactively erasing samples along boundaries that should not contribute to the estimation. We also provide controls for users to directly constrain the background to a specific constant colour where necessary.

Separating opaque layers

Opaque layers can be captured by any existing vectorisation algorithm. In our implementation, we provide users with the ability to vectorise an opaque region as either a linear or radial gradient, or a gradient mesh [START_REF] Sun | Image vectorization using optimized gradient meshes[END_REF]. Users can also use the image tracing tools in Illustrator or Inkscape for this task. As opaque regions fully obscure the background, they leave a hole that we fill with a colour diffusion [OBW * 08].

Implementation

We use Adobe's publicly available ExtendScript Toolkit to communicate with Photoshop. Using a script, we export the current image and selection (as a binary mask), and pass them to our program that implements the decompositing steps. The resulting foreground vector and background bitmap layers are loaded back into Photoshop, and control is returned to the user. We use Scalable Vector Graphics (SVG) as an interchange format for all vector layers, which we finally load into Adobe Illustrator to create the final vectorised result.

Results

Figures 1, 2, 6 and 7 show layers extracted from input bitmaps with our decompositing algorithm. All results have been produced with 2-stop gradients, except the camera lens and traffic cone that contain multiple 3-stop gradients. Most layers are linear gradients; some highlights in the glass (Figure 1) and the shadow of the vacuum cleaner (Figure 6) are radial gradients. While shadows and highlights are mainly black and white, the liquid in the glass and reflections in the lens, among others, are colour gradients. Our layers capture effects like Fresnel reflections on the side of the wine glass, but we do not recover and store the relationship between the colours of the reflection and the background. However, users can manually adjust the colours of gradient stops in the reflection layers to match the appearance of a new background.

Decompositing a single semi-transparent layer takes a few seconds to a minute, depending on the size of region and background estimation approach, which makes our interactive approach practical. The examples we show were created from scratch in a few minutes to an hour, depending on the number of layers and complexity of regions and images. Typical times for each region are: <10 s for selection in Photoshop, <1 s outline vectorisation using Potrace, 1-10 s foreground fitting (Equation 9), 1-5 s Laplacian matting and 5-30 s background estimation (Equation 11). The current bottleneck is the background estimation (Poisson and denoising) which could be accelerated with a multi-grid GPU solver [START_REF] Mccann | Real-time gradientdomain painting[END_REF].

Our current implementation vectorises each region independently. As a result, boundaries of regions do not always match up perfectly. We manually adjusted the boundaries between regions of the shoe, and the shadow and body of the vacuum cleaner in Figure 6. Automatic snapping could automate this task. To decomposite a hand-drawn bitmap, we first applied morphological closing to remove the contour lines (vacuum cleaner and purifier in Figure 6). The corresponding layers are provided in the supplemental material. Evaluation For ground-truth evaluation, we process a bitmap generated from vector art. We show two examples in Figure 7: a linear gradient on a textured button and two radial highlight on a round button. The results of decompositing are very close to the original layers used to create the illustration.

Limitations To make the decompositing problem tractable, our approach makes some assumptions (Section 4.1). While these assumptions are often satisfied for two-stop linear gradients, failure cases are more frequent for radial and multi-stop gradients as more iso-contours are susceptible to be missed. If the assumptions are violated, our approach may produce undesirable decompositions that nonetheless are often still valid, as shown in Figures 8 and9. Adding additional constraints, such as a white gradient colour or medium opacity, often resolves existing ambiguities.

Vector editing applications The layered vector art created using our approach can be edited with conventional vector graphics tools such as Illustrator or Inkscape. In Figure 6, we change the shape of the large highlight on the left of the bottle, and the texture of the shoe. We change the colour of the liquid layer in Figure 1 and introduce a layer for the straw. Figure 8: Examples of failure cases, for which our approach produces valid but undesirable decompositions. If gradient iso-contours (dashed lines) are not observed over two distinct backgrounds (a), the gradient colours and opacities are underconstrained. If only one iso-contour is observed (b), we obtain a constant-opacity gradient. Quadratic gradients (c) can be approximated as piecewise linear, but leave a residual in the background.

Our decomposition Recomposited Input photo Similarly, a layer is added in Figure 2 for the moustache. All of these edits are difficult or impossible to achieve starting from just a bitmap, but using our layered image vectorisation approach, they become much easier.

Potential automation Users of our tool currently need to manually select the gradient type (linear or radial), number of stops (two or three) and any other constraints (black, white or constant colour) of each region. Our framework can in principle allow for an automatic selection of these parameters, for example by selecting the gradient that minimises Equation 9. However, we found the choice of gradient primitive fairly easy; it also leaves artistic freedom to the user, for instance to constrain a highlight to be white even if it is slightly coloured in the input.

Conclusion

Vector artists create complex artworks by stacking simple layers. We have demonstrated the benefit of this strategy for image vectorisation, both to capture and edit transparency effects in drawings and photographs. Our approach facilitates the creation of such layered vector graphics from bitmaps, and we thus see our method as a valuable tool for professional artists and novice users alike.

While we focused on alpha compositing, our framework should easily extend to other popular blending modes, such as screen and overlay. Another extension of our work would consist in casting the decompositing problem as a global optimisation across multiple layers to better capture residual colour variations, potentially using our greedy solution as an initial solution. Finally, our layered representation could facilitate the creation of vector shade trees [LMPB * 13] from existing drawings and photographs, which would represent a first step towards the capture and transfer of material appearance in illustrations.

  (e) Re-composited result (d) Edited vector layers (c) Complete vector layers (b) Vector layer and background bitmap (a) Input image

Figure 2 :

 2 Figure 2: Interactive workflow.A user selects a region in an image (a), which our algorithm decomposites into a vector foreground layer and a background bitmap (b). This process is repeated and opaque layers are handled with existing tools to create a complete set of vector layers (c). Layers can be edited easily (shown as blue lines in d) and added (green lines), and re-composited to enable powerful editing applications (e).

Figure 3 :

 3 Figure 3: We define parametric gradients by composing a gradient function g (left) with a colour function c (right), that encodes colour and transparency as an (r, g, b, α) tuple.

Figure 4 :

 4 Figure4: We can separate the foreground 2-stop gradient if at least two iso-contours (dashed lines) are observed over two different background colours. This condition extends to each linear piece of multi-stop gradients. We assume the background is continuous across the region boundary, so blue locations approximate the background at the green locations.

  While in theory the background colour is unknown over the entire region, we make the additional assumption that it is continuous across the boundary of the region, which allows us to approximate the background colour of each pixel x along the boundary with the image colour at the closest location b(x) outside the region boundary (see inset).

Figure 5 :

 5 Figure 5: Background estimation using Equation 10 for image (a): a hard region boundary causes halo artefacts (b) that are removed with a matted boundary (c). A smoothness term suppresses noise (d) and combined with Poisson blending produces the final result (e). To clearly see the differences, please view this figure on a monitor.

Figure 7 :

 7 Figure 7: Ground-truth comparison on vector art: our decompositing results are close to the ground-truth decompositions as indicated by the low root-mean-squared errors (RMSE) of the background bitmaps. The vector gradients are correctly estimated despite the textured background and the subtle colour gradients in the two radial gradient examples.

Figure 9 :

 9 Figure 9: Example of a real-world failure case. If an image layer cannot be represented well by a colour gradient, our method tends to leave residual foreground colours in the background layer. However, we observed that these residuals are often smoothed away by subsequent vectorisation steps.

© 2014 The Author(s) Computer Graphics Forum © 2014 The Eurographics Association and John Wiley & Sons Ltd.

Acknowledgements We thank Julia Chatain for her work on an early version of this project, Robert Carroll and David Bommes for help with optimisation, and Pascal Barla and Frédo Durand for feedback on an early draft. This work was supported by the Inria CRISP associate team, ANR-12-JS02-003-01 DRAO, and a research donation from Adobe.

Image credits Images from Shutterstock: the wine glass (Givaga) and bottle (Dmitri Gristsenko), the red cup (George Dolgikh) and shoe (Picsfive), the magnifying glass (Ruslan Grumble) and the glossy round button (Roman Sotola). Lens image by Flickr user squinza (CC BY-SA 2.0). Vacuum cleaner and purifier drawn by Spencer Nugent on sketch-a-day.com. Striped green button by openclipart.org user inky2010.

For each result, we show the input photo or drawing on the left, and on the right an exploded view of all layers as well as editing results for some of the images. We added outlines to semi-transparent layers for visualisation; these lines are not part of our results. We use blue lines to show edited layers, green lines for added layers, and the re-composited result in the centre. Most gradients have two stops, except for the camera lens where three-stops gradients capture saturated and coloured highlights.


Marc Hässig
email: hassig@geoazur.unice.fr

Rolland Yann

Marc Sosson

Ghazar Galoyan

New P-T-t data on the metamorphic sole of the Amasia ophiolites and implications for the geodynamical process, NW of the Sevan-Akera suture zone, Lesser Caucasus (Armenia)

published or not. The documents may come

The ophiolites of the Lesser Caucasus belong to the Tethyan Ophiolitic Belt. In the northwestern part of the Sevan Akera suture zone (Lesser Caucasus, Armenia) ophiolites correspond to a major obduction of oceanic crust over the South Armenian continental block. Near the locality of Amasia (NW Armenia), our mapping evidenced a series of (1) unmetamorphosed gabbroic oceanic crust, resting on (2) serpentinites and a greenschist grade tectonic mélange composed of deformed pillow-basalts, radiolarites and cherts, and (3) a basal slice of garnet amphibolites bearing similar compositional features as the ophiolite.

They are sliced by post Eocene thrusts related to the shortening of the suture after the collision of the South Armenian Block [START_REF] Sosson | Subduction, obduction and collision in the Lesser Caucasus (Armenia, Azernaijan, Georgia), new insights[END_REF]. Structural observations allow us to consider these metamorphic rocks (garnet amphibolites and quartz-garnet-oligoclase-amphibole-chlorite schistes) to be the sole of the obducted ophiolite.

The metamorphic rocks allow the evaluation of pressure-temperature-time conditions endured by the base of the oceanic crust during the obduction of the Armenian ophiolite series. Pressure and temperature estimations were obtained by calculating equilibrium conditions between different mineral phases which coexist in the sample coupled with whole rock geochemistry using the software PERPLEX. The P-T data show a counterclockwise path with (1) a HT-LP peak of P = 6-7 kbar and T > 630 • C, followed by MP-MT peak at P = 8-10 kbar and T = 600 • C. In order to further constrain the evolution of the amphibolites and build a P-T-t path using 40Ar/39Ar dating on amphiboles and white micas (phengite). The ages obtained on both HT and MT peaks are similar within error (90.8 Ma ± 3.0 (2σ), and 90.8 Ma ± 1.2 (2σ), respectively), which suggests a very rapid cooling of the metamorphic sole during its stacking in the subduction zone.

The counterclockwise path determined confirms the observations made below the Oman ophiolite by [START_REF] Goffé | A case of obduction-related high-pressure, lowtemperature metamorphism in upper crustal nappes, Arabian continental margin, Oman: P-T paths and kinematic interpretation[END_REF]. This path is also in agreement with a scaling process of oceanic crust during the obduction. In this scenario slices of oceanic crust would have been dragged under the obduction front below a relatively hot oceanic crust. Afterwards, these slices would then have been amphibolitized along a HT-MP geothermal gradient. Rapidly, a thickening of the system at c. 91-90 Ma, and underthrusting of amphibolites would have caused the conditions to change to MP-MT. This infers that the South Armenian Block enters the subduction zone at this date.

A recent study has shown metamorphic rocks in the same structural setting were affected by a HP metamorphism in the Blue Schist facies [START_REF] Rolland | Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran[END_REF]. The age of this blue schist of Stepanavan, Armenia is slightly older (95-91 Ma) [START_REF] Galoyan | Geology, geochemistry and 40Ar/39Ar dating of the Sevan ophiolites (Lesser Caucasus, Armenia): Evidence for Jurassic Back-arc opening and hot spot event between the South Armenian Block and Eurasia[END_REF] than the amphibolites of Amasia (91-90 Ma). This assumes the occurrence of an active subduction just before the development of the HT-MP metamorphism of Amasia.

  New P-T-t data on the metamorphic sole of the Amasia ophiolites and implications for the geodynamical process, NW of the Sevan-Akera suture zone, Lesser Caucasus (Armenia) Hässig Marc, Yann Rolland, Marc Sosson, Ghazar Galoyan To cite this version: Hässig Marc, Yann Rolland, Marc Sosson, Ghazar Galoyan. New P-T-t data on the metamorphic sole of the Amasia ophiolites and implications for the geodynamical process, NW of the Sevan-Akera suture zone, Lesser Caucasus (Armenia). EGU General Assembly 2011, Apr 2011, Vienne, Austria. pp.EGU2011-7462-1. hal-01060852


Julie Ghirardi

Jérémy Jacob

Arnaud Huguet

Hugues Bauer

Claude Le Milbeau

Jean-Jacques Châteauneuf

Christian Di Giovanni

Florence Quesnel

A unique high

Keywords: biomarkers, climate, Eocene-Oligocene transition, vegetation

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The transition between Eocene and Oligocene (EOT) is known as one of the major climatic change of the Paleogene. This climatic transition is marked by the change from a greenhouse to an icehouse climatic mode. Although the effect of climate change on terrestrial setting correspond to major changes in plant and animal communities (the well-known "Grande Coupure", [START_REF] Stehlin | Remarques sur les faunules de Mammifères des couches éocènes et oligocènes du Bassin de Paris[END_REF], most of studies are realized on marine successions in which several side-effects were identified (stepwise cooling, faunistic turnover, deepening of the CCD, eg. [START_REF] Coxall | Rapid stepwise onset of Antarctic glaciation and deeper calcite compensation in the Pacific Ocean[END_REF]. Although more sensitive to climate changes, terrestrial settings suffer from the lack of continuous and well calibrated sections. Recently, the BRGM (French Geological Survey) has conducted a fully-cored deep drilling (CDB1) in the Cenozoic Rennes Basin. A continuous organic matter and clay rich lacustrine record encompasses the EOT as shown by palynology (Bauer et al., in prep.) and confirmed by magnetostratigraphy [START_REF] Dupont-Nivet | La transition Eocène-Oligocène identifiée par magnetostratigraphie dans le bassin continental de Rennes. 14 ème congrès de sédimentologie[END_REF].

In order to decipher the climatic evolution during the EOT and its impacts on ecosystems, we have performed a full palynological study accompanied by the analysis of molecular biomarkers and of their isotopic composition. After extraction and separation of lipids, quantification of branched Glycerol Dialkyl Glycerol Tetraethers (GDGTs) was achieved by liquid chromatography coupled to mass spectrometry, whereas other lipids were identified and quantified by gas chromatography coupled to mass spectrometry. Additionally, the D of vascular plants leaf-wax n-alkanes (with 27, 29 and 31 carbon atoms) was determined by gas chromatography coupled to isotope ratio mass spectrometry. Indices calculated from branched GDGT concentrations in 50 samples allowed us estimating the evolution of mean annual air temperature based on two calibrations developed in recent soils [START_REF] Peterse | Revised calibration of the MBT-CBT paleotemperature proxy based on branched tetraether membrane lipids in surface soils[END_REF][START_REF] Weijers | Environmental controls on bacterial tetraether membrane lipid distribution in soils[END_REF].

The paleohydrological reconstruction relies on the D of n-alkanes determined for 50 samples and on their Average Chain Length (ACL, [START_REF] Eglinton | Leaf Epicuticular Waxes[END_REF]. D of n-alkanes depend on hydrological parameters such as temperature at precipitation site precipitation amount and evapotranspiration [START_REF] Gleixner | Compound specific hydrogen isotope ratios of biomarkers: Tracing climatic changes in the past[END_REF][START_REF] Sachse | δD values of individual n-alkanes from terrestrial plants along a climatic gradient -Implications for the sedimentary biomarker record[END_REF]. ACL is used to decipher vascular plant adaptation to relative humidity: the longer the chain, the better vascular plants are adapted to dry conditions.

The two temperature records obtained from GDGTs show the same evolution, marked by two cooling trends: one during the late Early Priabonian and a second during the Early Rupelian. Conversely, two warming intervals are observed during the Early Priabonian and the Late Priabonian (EOT).

The ACL values vary between 26 and 28. From the Late Priabonian and the Early Rupelian, recurrent ACL variations attest to changes in relative humidity. One of these variations occurs during the EOT and shows the transition from humid to dry conditions. This interpretation is supported by the most negative n-alkanes D values recorded during the Late Priabonian to the Early Rupelian that either indicate high precipitation rates, lower evaporation rates. Then, and as for ACL, D values increase after the EOT, indicating the sudden settlement of dryer conditions. How did these climatic changes impact the environment, and especially the vegetation? The evolution of vegetation is unraveled through specific biomarkers of ferns (fernenes), gymnosperms (diterpenes) and angiosperms (triterpenes ketones) and compared to spores and pollen data. From the Late Eocene to the Early Oligocene, spores and pollen counts delineate an increase in mesothermal species compared to megathermal species.

In detail, we observe an increase in coniferous species from the Late Priabonian to the Rupelian that is classically interpreted as the result of the installation of a cool and dry climate. These evolutions are in agreement with temperature evolutions deduced from branched GDGTs and also with the global cooling that is characteristic for this transition. However, diterpenes evolution is different from that of gymnosperm pollen since they are as abundant during the Eocene as during as in the Oligocene, except for a peak of concentrations recorded the early Late Priabonian.

As for gymnosperms, fern and angiosperm biomarkers do not show any global trend in concentration but high frequency changes from the Late Eocene to the Early Oligocene. Remarkably, these high frequency changes in vascular plants are consistent with the rhythm of ACL values. Thus, the evolution of continental ecosystems during the Priabonian and Rupelian appears paced by changes in relative humidity.

A

  unique high resolution lacustrine record of climate and vegetation changes during the EOT. The CDB1 core, Rennes Basin, France Julie Ghirardi ( a,b ), Jérémy Jacob ( a ), Arnaud Huguet ( c ), Hugues Bauer ( b ), Claude Le Milbeau ( a ), Jean-Jacques Chateauneuf ( d ), Christian Di Giovanni( a ) and Florence Quesnel ( a,b ) _____________________________________________________________________________________________________________________________________________________ ( a ) Institut des Sciences de la Terre d'Orléans, ISTO, UMR 7327 du CNRS/INSU, Université d'Orléans, BRGM, 1A rue de la Férollerie, 45071 Orléans, France. ( b ) BRGM (French Geological Survey), Georessources Division, 45060 Orléans Cedex 2, France. ( c ) CNRS/UPMC UMR 7619 METIS, 75005 Paris, France ( d ) 8 quai du Châtelet, 45000-Orléans, France _____________________________________________________________________________________________________________________________________________________