Gregory Wegmann
email: gregory.wegmann@u-bourgogne.fr

Johei Oshita

A comparison between Japanese and French cost management -Contingency and institutional perspectives

Keywords: Japan, France, cost accounting, cost management, comparison

 and is culturally grounded in a few old-industrialized countries, in Europe: Germany, United-Kingdom, France, but also in Japan and in the United-Stated. Notwithstanding the cultural dominance of English-speaking countries on management, non-English speaking countries keep strong institutional and cultural roots that still influence their ways of managing companies. This is the case when looking at management accounting and more especially at cost accounting and cost management practices (now cost accounting/management). This paper, based on contingency and institutional frameworks, explores what are the similarities and differences of Japanese and French cost accounting/management. The first part describes the theoretical background, the second part presents the Japanese context, the third part the French one and the fourth part discusses the question. The historical, institutional and cultural backgrounds of the two countries are reviewed, focusing on cost accounting / management with a view on target costing. Through a comparative approach, specific French and Japanese ways of managing the costs are put in evidence. The research shows that the attitude towards target costing and the specific practices of cost accounting/management observed in France and Japan highlight the differences and similarities between the two countries. Both countries are also influenced by the Anglo-Saxon practices, and evidences of convergence become apparent.

INTRODUCTION

From an historical perspective, management accounting is a very recent phenomenon (see [START_REF] Johnson | Relevance Lost: The Rise and Fall of Management Accounting[END_REF] and is culturally grounded in a few old-industrialized countries, in Europe: Germany, United-Kingdom, France, but also in Japan and in the United-Stated. Notwithstanding the cultural dominance of English-speaking countries on management, non-English speaking countries keep strong institutional and cultural roots that still influence their ways of managing companies. This is the case when looking at management accounting and more especially at cost accounting and cost management practices (now cost accounting/management). This paper, based on contingency and institutional frameworks, explores what are the similarities and differences of Japanese and French cost accounting/management. It seems to be interesting to compare two non-English speaking countries, with strong cultural identities and a high level of internationalization of their companies. Because of these two strong cultural identities, Japanese and French companies have their own cost accounting/management practices, regardless of the influence of English-speaking countries, and particularly the United States. The first part describes the theoretical background, the second part presents the Japanese context, the third part the French one and the fourth part discusses the question. The historical, institutional and cultural backgrounds of the two countries are reviewed, focusing on cost accounting / management practices with a view on target costing. So the paper considers practices concerning cost accounting/management in Japan and France, more than theory.

Through a comparative approach, specific French and Japanese ways of managing the costs are put in evidence. Both countries are also influenced by the Anglo-Saxon practices and this could also be the cause of convergence.

THEORETICAL BACKGROUND

Contingency theory states that universally applicable cost accounting/management systems which equally satisfy the needs of different organizations under different circumstances do not exist. However, generalizations for classes of business settings can be made. It is assumed that managers adapt their organizations to the contingencies in order to enhance performance. A specific setting of factors (contingent variables) matches certain aspects of cost accounting/management. Using the contingency theory requires to identify correlations between contingent variables and cost accounting/management systems characteristics by defining a contingency framework. Contingent variables are considered to be outside of the organization's control. Variables that are controllable by the organization are arranged to adapt to the contingencies in an appropriate configuration in order to improve performance [START_REF] Otley | The contingency theory of management accounting: Achievement and prognosis[END_REF].

To be able to distinguish between practices in different countries, contingent variables varying on a national level have to be identified. Numerous scholars have proposed a selection of contingent variables for comparative research on a national basis [START_REF] Shields | Management accounting practices in Europe: a perspective from the States[END_REF]; Birkett (1998). Differences in culture are expected to impact cost accounting/management systems. Culture is the most frequently-stated influence factor in management accounting [START_REF] Chow | The effects of management controls and national culture on manufacturing performance: an experimental investigation[END_REF]1999). The cultural framework of Hoftede1 ( 2001) is commonly applied by management accounting researchers [START_REF] Carr | Context, culture and the role of the finance function in strategic decisions. A comparative analysis of Britain, Germany, the U.S.A and Japan[END_REF]. Because this research focuses on the characteristics and consequences of the differences between Japan and France, rather than on their cultural causes, these studies are only considered peripherally.

The institutional background and the economic situation of the particular country are also influence factors [START_REF] Pistoni | Comparative management accounting in Europe: an undergraduate education perspective[END_REF]Shields, 1998, p. 505-506). Institutional theory focuses on the influences of the systems surrounding organizations [START_REF] Scott | Institutions and Organizations[END_REF]. Institutions are social constructs directing actions while being sensitive to their influences [START_REF] Granovetter | Economic action and social structure: The problem of embeddedness[END_REF]. These institutions include both formal rules such as government policies, regulations and laws (economic orientation of institutional theory: [START_REF] North | Institutions, Institutional Change and Economic Performance[END_REF][START_REF] Coase | The New Institutional Economics[END_REF] and informal constraints such as norm of behavior, conventions and codes of conduct coming from cultural and social influences (sociological orientation of institutional theory: [START_REF] Dimaggio | The Iron Cage Revisited: Institutional Isomorphism and Collective Rationality in Organizational Fields[END_REF][START_REF] Schein | Organizational Culture and Leadership[END_REF][START_REF] Scott | Institutions and Organizations[END_REF]) and provide us with multiple insights (economic, social, and cultural) into economic phenomena [START_REF] Polanyi | The Economy as Instituted Process[END_REF]. According to [START_REF] North | Structure and Change in Economic History[END_REF][START_REF] North | Institutions, Institutional Change and Economic Performance[END_REF], allowance for institutions appears to be inescapable when evaluating corporate behavior. So firms are embedded [START_REF] Granovetter | Economic action and social structure: The problem of embeddedness[END_REF] in a social, political, institutional, and cultural context. Their behaviors have to be compatible with certain socially accepted values, norms, and rules. In accordance with the neoinstitutional approach, firms are mainly looking for legitimacy [START_REF] Dimaggio | The Iron Cage Revisited: Institutional Isomorphism and Collective Rationality in Organizational Fields[END_REF][START_REF] Scott | Institutions and Organizations[END_REF]. DiMaggio and Powell distinguish three kinds of isomorphism: coercive (regulations coming from the authorities), normative (from an economic domain, certain professional associations, etc.), and mimetic (trying to be like other firms).

Grounded on an institutional perspective, Shields (in Bhimani;[START_REF] Bhimani | Management Accounting: European Perspectives[END_REF] identifies seven factors that have shaped and currently influenced management accounting practices in European nations. These factors are: education, government, professional associations, consultants, technology, and the inter-nation transfer of information. With the same theoretical background, [START_REF] Ramirez | Understanding social closure in its cultural context: accounting practitioners in France (1920-1939[END_REF] puts in evidence four major determinants: -

The economical context, -

The professional context, -

The teaching contexts, -

The relationship with the authorities. This theoretical background seems relevant for this study, looking at different types of institutional influencestraditions, politics, administration, academic, etc.on Japanese and French cost accounting/management. The study is grounded on historical elements of French and Japanese traditions that enable to consider a tangle of institutional and economic factors affecting the characteristics of cost accounting/management systems.

Through a contingent, cultural and institutional framework to detect what are the most important and controversial problems in the field of cost accounting/management in France and Japan, the research focuses on the followings determinants: -The origins of cost accounting techniques with the economic background of the countries, -the management accounting profession and the teaching context, -The relationship with the authorities, -The inter-nations transfer of information: the influence of other countries on cost accounting/management, -Cultural influences on management styles with for instance decentralization/centralization and cooperative behaviors.

Mixing these determinants, the two next parts analyze respectively the Japanese and French contexts.

COST ACCOUNTING AND COST MANAGEMENT IN JAPAN

At first sight Japanese cost accounting/management practices seem to be relatively conventional and not dissimilar to Western practices. Looking back to traditions of cost accounting/management in Japan, nothing appears really specifically Japanese [START_REF] Okano | A History of Japanese Management Accounting, Handbook of Management Accounting Research[END_REF]. But looking in more detail, specificities appear. The first paragraph describes the Japanese cost accounting system and its specificities. The second paragraph analyses its determinants.

3.1

Description of the Japanese cost accounting system Historically, Japanese companies have used direct costing systems. As the overhead costs increased, a department costing system was invented (Shimizu,. In this system, overhead costs are assigned to operating and support departments and costs in support departments are allocated to operating and other support departments through four full costing techniques: direct, step-down, grouping step-down and reciprocal. Shimizu explains that few Japanese companies have experimented ABC (p. 24). Using a questionnaire survey, [START_REF] Shimizu | Cost Accounting Available to Practitioners[END_REF] shows that the rate of overhead costs in Japanese companies is lower than in US companies, because the Japanese companies directly reduce overhead costs. Lean accounting [START_REF] Maskell | Practical Lean Accounting. A Proven System for Measuring and Managing the Lean Enterprise[END_REF] clearly explains why with lean management practices overhead costs are reduced, with reductions of setup, batches and inspections costs, and thanks to inter-organizational cost management [START_REF] Cooper | Interorganizational cost management and relational context[END_REF].

In the same inquiry [START_REF] Shimizu | Cost Accounting Available to Practitioners[END_REF] shows that 55.5% of the companies use a direct costing method and around 42% a full costing one with 24.1% using the grouping step-down method. So, with all the techniques available, Japanese companies don't really need the ABC method and their allocation systems of overheads are quite effective. Some Japanese companies use a hybrid system combining step-down full costing method and Japanese Cost Accounting Standards (CAS) which is a good substitute for ABC systems (Shimizu,p. 30). The advantage of this system is that Japanese companies don't need to gather additional data, unlike ABC which is time and resource-consuming.

Institutional and cultural decrypting

When observing in depth the situation, due to the highly endemic cultural identity of Japan, the Japanese managers totally reinvented cost accounting/management. Japan is regarded as a collective and long-term oriented society [START_REF] Hofstede | Culture's consequences: Comparing Values, Behaviors, Institutions, and Organizations Across Nations[END_REF]. Institutional factors also show that permanent employment [START_REF] Cole | Work, Mobility and Participation. A Comparative Study of American and Japanese Industry[END_REF], flexibility and multi-skilling of the workforce [START_REF] Nishimura | Management accounting -Feed forward and Asian perspectives[END_REF] are widespread in Japan.

The economical context

After the period of isolation, and due to the Second World War, the Japanese industries were strongly influenced by the American way of doing management. For instance, Edward Deming came to Japan to advice leaders of Japanese industry, introduce the quality concept and influence the practices. [START_REF] Locke | The End of the Practical Man: Entrepreneurship and Higher Education in Germany, France, and Great Britain, 1880-1940[END_REF] explained that the adoption by Japanese firms before the Second World War of German cost accounting techniques were overlaid by American practices after the Second World War. More recently, the reverse flow of Japanese target costing to America and Germany has been observed. So a particular attention is paid to industrial and operational issues. In Japanese companies, what comes first are industrial and operational questions through the work of engineers setting up procedures and organizational innovations. In this context, cost accounting/management are primarily operational tools dedicated to engineers, middle managers and workers. Cost accounting/management do not exist alone, but are integrated in operational management systems such as just in time, quality control, and target costing.

-A market orientation of cost accounting/cost management (see next part about target costing) quite developed.

-A distinct preference to non-financial, physical, concrete and operational data, contrary to American managers who privilege financial and global measures. As explained [START_REF] Okano | A History of Japanese Management Accounting, Handbook of Management Accounting Research[END_REF], "…unlike the US where monetary terms were utilized in order for the State to rationally allocate resources, the Japanese seemed to have developed some distrust in the monetary valuations." -Cost accounting/management systems are highly decentralized to be as close as possible to the field. As Koike explains (1993, p.162), "NEC gradually separated its cost control activities from the core of the accounting division, and entrusted them as ''autonomous activities'' of each business division (…) NEC placed an emphasis on responsibility accounting, management by exception and Koto-mae (i.e., before-things-happen or causal) management." Another factor that favors decentralized cost accounting/management is the characteristics of profit centers in Japan which are, according to [START_REF] Cooper | When Lean Enterprises Collide: Competing through Confrontation[END_REF] "Microprofit Centers". Cooper observed in some Japanese companies the division of manufacturing, sales and support departments into small groups. Shimizu (2017, p. 136-147) describes the case of Kyocera, Corporation, a fine ceramics manufacturer. Products and sales departments of Kyocera are divided into small groups of five to ten people, called "amoebas", which are real microprofit centers. "Amoeba" leaders have to drive costs and make profits, using income statements (direct costing system) and a range of cost management indicators such as hourly efficiency, setting planned profits through an open book accounting philosophy.

-A collaborative way of using cost accounting/management inside and outside the firm (e.g., McMann and Nanni, 1995;[START_REF] Yoshikawa | Some aspects of the Japanese approach to management accounting[END_REF][START_REF] Yoshikawa | Cost accounting standard and cost accounting systems in Japan. Lessons from the past -recovering lost traditions[END_REF]. With the Japanese-style transformation of quality control after the second world war, both buyers and suppliers should grow together and cost accounting/management became a common language (see the next part).

The professional and educational contexts

Hiramatsu (1992) accounts for a job rotation for accountants to other functions every five to ten years. Consequently, the degree of specialization in cost accounting/management is lower in Japan than in France. When it comes to the practical education (such as cost accounting), employees are trained essentially in-house and by job rotation, neither in universities (where the programs are more theoretical), nor in professional high schools (where the programs are practical but more elementary). Another characteristic of Japan's collectivism and long-term orientation is the seniority system: wages and promotions rely on seniority instead of performance [START_REF] Kono | Strategy and structure of Japanese enterprises[END_REF].

Due to the institutional and cultural factors described, the main characteristics of Japanese cost accounting/management are as follows:

-Structures in Japanese companies are less specialized and management accounting is not professionalized. According to [START_REF] Hiramatsu | Accounting education in Japanese corporations[END_REF], accountants do not have any accounting major in 69.4% of Japanese organizations. Non-accounting departments have cost accounting/management responsibilities. More especially, design and production engineers are frequently involved in cost-reduction programs.

But recently, these features have gradually changed.

Relations with the authorities

They are regulations in the Cost Accounting Standards (CAS) in Japan since 1962. Even these standards are not compulsory, manufacturers still use them. CAS regulations are quite relative, according for instance to the classification of operating departments versus support ones and to the allocation of support departments. Apart from these standards, the influence of the Japanese authorities on the formalization of cost accounting models in companies seems quite low as a diversity of practices as been observed (see 3.1). But even now, Japan Cost Accounting Association is actually considering the revision of CAS regulations in order to adapt them to new business circumstances [START_REF] Takahashi | Toward the Relevance Regain of Cost Accounting Standards[END_REF]).

Besides CAS regulations, it is necessary to remember that the three decades after the Second World War was just a time of government leadership in Japanese cost accounting/management. [START_REF] Nishimura | Management accounting -Feed forward and Asian perspectives[END_REF] shows that Japanese government leadership established modern management accounting in Japan by introducing advanced accounting system from USA after the Second World War. Especially the Ministry of international Trade and Industry (MITI)'s Industrial Rationalization Council played a critical role in promoting the spread of modern management accounting techniques. From July 1951 to the 1970s, MITI published many reports on modernization (Nishimura (2003, p.24)):

- And finally, [START_REF] Nishimura | Management accounting -Feed forward and Asian perspectives[END_REF], concerning the characteristics of post-war Japanese accounting methods, notes first that Japanese management accounting made much more use of the total control system than it did of strategic decision-making. This is because of the demands of foreign loans and the 'triangle' system of governments, main banks and companies. Second remark made by Nishimura is that Japanese companies made better use of human factors related to productivity than the dependency on professional strategic managers (2003, p.25).

The inter-nation transfers of information

If Japanese cost accounting/management seem at first quite similar to the Anglo-Saxon model, specificities finally emerge and remain from the practices.

Cost accounting/management in Japan are culturally grounded, considering a non-sense the Western quest for true cost and rejecting for instance activity-based costing. Cost accounting/management are mainly influential tools. In other words, costing is more important for decision making than for control purposes. Japanese management accountants are aware of overhead cost allocation problems, but they prefer cost information for influencing decisions and for motivating employees [START_REF] Hiromoto | Another Hidden Edge -Japanese Management Accounting[END_REF]. For instance, Japanese managers sometimes use to allocate overheads on a labor-cost base despite the fact that labor costs are only a small proportion of total costs. This allocation is made to focus design engineers' attention on increasing productivity through automation and for influencing employees, but not for giving information about the cost situation [START_REF] Hiromoto | Another Hidden Edge -Japanese Management Accounting[END_REF]. Due to the collectivist culture of the country, Japanese companies focus more on behavioral control and on motivating employees to reduce costs than on cost control.

According to [START_REF] Hutchinson | Zen Accounting: How Japanese Management Accounting Practice Supports Lean Management[END_REF] who try to link the cost accounting/management practices to the Japanese philosophy, "Japanese management accountants have taken a Zen-like approach to developing cost accounting systems" (p.34). Considering that the perfect cost accounting system does not exist, the objective of management accountants is not so much to produce precise information. In this perspective, "Thinking about cost systems is more important than costing itself." (p.34) The objective of cost accounting is more to make employees act in accordance with the company's strategy. This philosophy is in accordance with the Strategic cost management stream (Shank and Govindarajan, 1989). In that sense, a kind of convergence between Japanese management accounting and a modernized approach of management accounting in Western countries can be detected.

The next part focuses on target costing and inter-organizational cost accounting to illustrate the specificities of cost accounting/management in Japan.

An illustration of the Japanese context through target costing, inter-organizational cost accounting and other management practices

As Feil et al. (2004, p.10) explain, early traces of target costing came from practices in US companies such as Ford. These practices were imported in Japanese companies after the Second World War. Then, the Japanese companies transformed these practices in what is called today target costing. In a sense, target costing is a "travel idea"2 .

Even if Japanese scholars don't agree exactly on the same meaning of target costing, there is a global convergence on the fact that it is a global management process. During the early experiences, target costing was a more focus instrument dedicated to control the cost of purchases. But today, according to Oshita (1996), target costing is neither accounting nor costing in traditional meanings, rather it is frequent and mutual communicative act and strategy that enables comprehensive profit planning and management. The most famous Japanese academic experts share the same view [START_REF] Kato | Target costing support systems: lessons from leading Japanese companies[END_REF][START_REF] Monden | Target costing and Kaizen costing in Japanese Automobile Companies[END_REF][START_REF] Sakurai | Target costing and how to use it[END_REF][START_REF] Tanaka | Target costing at Toyota[END_REF][START_REF] Tani | Target cost management in Japanese companies: current state of the art[END_REF].

The figure below (Oshita, 1996) illustrates this global view about the target costing process.

Note: From Japanese Accounting Association (1996, p.46) Figure 1: Target costing, a global managerial system

The Japanese expression ''genka-kikaku'' may be misleading. It literally means ''costplanning'' and that is how target costing is regarded in Japan, rather than a costing method. "Genka-kikaku" has much to do with planning, coordination and improving in advance to actual production and its calculation. Some Japanese scholars focus more on market orientation of target costing (Hiromoto), while some others privilege the engineering dimension (Sakurai) and others the product aspect (Tanaka). But they all agree on the fact that target costing has become a global managerial and philosophical process. According to [START_REF] Kato | Target costing support systems: lessons from leading Japanese companies[END_REF], target costing is not only a costing but a major element of a more global management system. Kato (p.43) also considers that target costing "…has its dysfunctional aspects..." and "should be socially and environmentally conscious".

So, an ideal form of target costing in Japanese companies is a comprehensive profit management system centering on planning, development and design of products. It is a global process with: the conceptual design (from product planning to target cost setting with various stages), the product development and design (core of the target costing), the transition to mass production and the follow-up. The process described comprises: methods of determining target costs, systems that support target costing activities and the organization for achieving target costs (with suppliers' relationships). An upstream management determination of the profit or cost, customer satisfaction and cross-functional cost determination in cooperation with suppliers complete this description. Considering a contingency approach, a target costing system is specified by the strategy and the form of the considered organization.

So, differences between a Western approach and the Japanese one mainly lies on the management philosophy concerning top leadership, team orientation, commitment to work and mutual work, education, the role of Keiretsu and information systems (Feil et al, 2004, p.17). Most Japanese employees have a cross-functional experience which facilitates to take part to target costing experiences.

Concerning the Keiretsu and the information systems, one important feature is the integration of cost management systems between suppliers and buyers. According to [START_REF] Cooper | Interorganizational cost management and relational context[END_REF], inter-organizational cost management systems (IOCM) constitute a specificity of cost accounting/management in Japan. They describe for instance the case of engineers of two companies, Komatsu and Toyo, a large heavy industrial manufacturer and an independent manufacturer of heat exchange equipment involved in a supply chain, who initiate an inter-organizational cost investigation to optimize the costs. Target costing lies at the heart of IOCM because it links customer demands through product design to the parts acquisition process. The increased scope of the design changes enables the identification and implementation of greater cost savings across the entire supply chain. Two ways to reduce costs appear: by redesigning the product and its components and by changing the location of activities.

The next part presents the specificities of cost accounting/management in France.

COST ACCOUNTING AND COST MANAGEMENT IN FRANCE

Several contingency, cultural and institutional factors explain the specificities of the conventional French cost accounting system: a quite sophisticated system, mainly dedicated to industrial processes, with low managerial and predictive potentials.

The first paragraph describes this French conventional cost accounting system. The second paragraph analyses the determinants of this system.

Description of the French conventional full cost accounting system

Historically, the French cost accounting/management are highly influenced by full costing, privileging the multiplication of cost centers, drivers and the assignment of indirect costs to cost objects. The French cost accounting/management are historically separated from the budgeting system.

The emergence of the French full costing system: an historical overview

Compared to the Anglo-Saxon accounting system, the French accounting system is characterized by a net separation between cost accounting and financial accounting. Cost accounting is therefore in France explicitly developed for internal decision taking. France made an early contribution to the cost accounting literature [START_REF] Nikitin | Setting up an industrial accounting system at Saint-Gobain (1820-1880)[END_REF].

Even if the French cost accounting system comprehends methods based on full-as well as on partial-costing, historical developments mainly focus on full costing. Research into cost accounting/management practices of French firms during the nineteenth century has focused on the implementation of industrial accounting around 1820 with the accounting for glass production of MGF (Manufacture des Glaces de France) and the works of Pierre-Antoine Godard-Desmarest, who was the owner of Baccarat Crystalworks and ran the firm from 1822 until 1839 (see [START_REF] Nikitin | Setting up an industrial accounting system at Saint-Gobain (1820-1880)[END_REF]. MGF company distinguished between the three stages of the manufacturing process: pouring, abrasion, and polishing which led to an overall example of full process costing with a clear distinction between direct and indirect cost. The Baccarat industrial accounting system consisted of a double-entry bookkeeping system with cost calculations and production flow tracking at each stage of the manufacturing process. At the same period, in the field of mining, iron-making and sugar refining, Guilbault proposed the use of the average-cost method of pricing raw material into production and a process costing system with a distinction between fixed and variable overhead costs [START_REF] Garner | Evolution of Cost Accounting to 1925[END_REF]. The famous work of Lieutenant-Colonel Rimailho, with the Homogeneous sections method in military plants, confirmed this preponderance for full costing approaches. Early twentiethcentury French firms, like Renault, deployed such a full costing system.

French cost accounting/management have strong industrial and engineering backgrounds, due to the context of their emergence. The French cost accounting techniques were developed amongst others from industrial practices. In this institutional context, the homogeneous sections-method is the main concept of the French cost accounting system in the first part of the twentieth century. A homogeneous section is a cost center organized around a main activity to enable the allocation of its costs to products or to other sections. This notion was applied to the French military industries during the 1920's thanks to Lieutenant-Colonel Rimailho [START_REF] Lemarchand | The Military Origins of the French Management Accounting Model: A Return to the Mechanisms of Accounting Change[END_REF], who was head of the "Commission Générale d'Organisation Scientifique du Travail" (CGOST). Drawing on his military experience, Rimailho developed the homogeneous sections mechanism in such a way that, according to Lemarchand (Rimailho, 1928: 66;quoted in Lemarchand 2002) "…different specialized activities which make up the section are, in principle, employed in the same proportions for each task undertaken in the section". Renault, a major French car manufacturer developed sophisticated cost systems [START_REF] Bhimani | Indeterminacy and the Specificity of Accounting Change: Renault 1898-1938[END_REF][START_REF] Bhimani | Management Accounting: European Perspectives[END_REF] during the 1920's based on Taylor's shop floor management system. Other French firms followed the example of Renault. The implementation of budgetary control systems started separately in the 1930s [START_REF] Berland | La naissance du contrôle budgétaire en France [The birth of budgetary control in France[END_REF][START_REF] Berland | The Availability of Information and the Accumulation of Experience as Motors for the Diffusion of Budgetary Control: The French Experience from the 1920s to the 1960s[END_REF] and was based on a different process. Cost accounting and budgetary systems in the French tradition are two different cycles and the conventional cost process makes it difficult to draw a link between these two cycles. In fact, the cost analytical structure separates direct and indirect costs regarded cost objects. These cost objects are mainly products, which does not enable to put in evidence various kinds of margins based on the cost behaviors and the planning. To do that, a complex correspondence system that was difficult to implement at the time is required.

The French full costing method

The conventional French full costing method is a two-stage process. It is called today "Méthode des centres d'analyse" (Analysis centers method). This method is based on a hierarchical vision as described by Fayol's who defined the firm as a set of functions (Cauvin, Neumann, 2007). It assigns indirect costs to jobs or products in two stages. In the first stage, the method identifies indirect costs with various production and service departments. Service department costs are then allocated to production departments. The system assigns the accumulated indirect costs for the production departments to individual jobs or products based on predetermined departmental cost driver rates. The analysis centers could be viewed as cost pools.

Figure 2: French full costing approach: The two-stage assignment process These centers modelled on the production process are called the main sections when they contribute directly to the manufacture of a product. They are complemented by auxiliary sections that contribute indirectly to the manufacture of a product. The fundamental assumption of this two-stage allocation method is the absence of a strong direct link between the support activities and the products manufactured. For this reason, service department costs are first allocated to production departments using one of the conventional two-stage allocation methods previously described. This stage is called "primary repartition".

Once all charges have been either attached to a product or to a section, a physical quantity dedicated to the production of an output is defined for each center. Therefore, the users of the output (a product for instance) are charged for the output's quantity they consume. The cost per unit is the unit cost charged to the user for each unit physically consumed. It is most of the time a cost driver expressed in terms of quantities (number of direct labor hours, number of machine hours, etc.) During the process described, one important step is to establish the "indirect cost allocation table". This table gathers important information: the indirect costs of the centers, the primary and secondary repartitions and the costs of the units.

The next part analyses the institutional and cultural determinants of the system described.

Institutional and cultural decrypting 3.2.1 The economical context

France is an old industrialized country, with a centralized conception of the development of its industries. Since its early stages, French cost accounting/management are attached to the French industrial environment. The expression "industrial accounting" was used at that time and several features could be stated: -

The increasing competition experienced by French organizations after the 1820s, which facilitated the emergence of cost accounting/management to enhance productivity, -

The influence of the Scientific management movement (Taylorism), in relation to the strong influence of engineers in France,

-

In 1880's, the role of these French engineers who developed cost accounting techniques looking for measures for the industrial processes. These measures also aim to help reducing the transformation costs and supporting the mass production and the development of innovations. The focus was therefore more on production matters than on markets ones, compared to the USA.

-

The French engineers had also at that time a social view of their role. They see cost accounting/management as instruments to regulate competition, to justify selling prices, to inform unions and to promote career advancement for employees.

-

In addition to the role played by French engineers and the military industry [START_REF] Lemarchand | The Military Origins of the French Management Accounting Model: A Return to the Mechanisms of Accounting Change[END_REF], the French centralized administration has been very influential. After the Second World War, France had a State pricing policy. This context reinforced the role of the French administration and cost accounting remained influenced by government authorities through a full costing system. This context remained unchanged until the mid-1960s when demand exceeded supply and production capacity was saturated. The late 1960s saw the beginning of an era in which production capacity regularly exceeded demand. In this context, French full costing was challenged by the introduction of direct or variable costing, but managerial mentality was still strongly in favor of full costing. In the 1980's, Activity-based costing has been an opportunity for reopening the debate.

As a consequence, the main weakness of the French full costing system is the quite static approach of the costs analyzed that not permits contribution margins studies. Moreover, many studies have put in evidence homogeneity problems in a lot of centers that lead to crosssubsidization phenomena. Another weakness of the French full costing method is that it does not take into account capacity problems. For that reason, it needs to be completed with rational imputation. Another point is that it does not distinguish fixed from variable costs. So to summarize, the French full costing method needs to be completed by other systems: for planning, capacity problems, decision-making, etc.

The professional and educational contexts

In France, the accounting profession has a low level of recognition. The management accounting profession did not really exist until recently and the development of management accounting was mainly supported by the engineering profession. [START_REF] Levant | Contemporary evolutions in costing methods: Understanding these trends through the use of equivalence methods in France[END_REF] explain that France was characterized by a dominant engineering culture during the 19th and 20th centuries and that the influence of Taylorism was high in France. So, during the 19th century, several French engineers work on cost accounting and develop high level of rationalization with sophisticated full cost systems. Burlaud and Simon (1981, p.22) explain that "the French manner of product costing, embracing the idea of a 'complete cost' is an attempt to link scientific precision with practical policies. Their opinion is that while the procedure is rooted in the law, it also has gained popular acceptance due to the Cartesian element of the French psyche". It was also a way for engineers to strengthen their overall influence. French famous entrepreneurs like Renault had also an influence on the promotion of full costing.

As a consequence, a quite sophisticated, analytical and rational method became quite popular, the French homogeneous sections method. This method, dealing with indirect charges through cost centers resembles the German full costing method (see [START_REF] Lebas | Managerial accounting in France Overview of past tradition and current practice[END_REF][START_REF] Cauvin | French cost accounting methods: ABC and other structural similarities[END_REF]. As explain Boyns et al. (1997 a, p.418), "… the slower development of the French accounting profession provided a window of opportunity for new ideas to be advanced…" with a major role played by the engineering profession.

But, due to the professional situation described, the cost accounting and budgetary systems in the French tradition remained two different cycles and the traditional cost process makes it difficult to draw a link between these two cycles. It is still the case today. The cost analytical structure separates direct and indirect costs with reference to cost objects and a complex correspondence system, difficult to implement, tries to articulate the two cycles.

Concerning the teaching context in France, the development of management accounting was supported by practitioners and the Chambers of Commerce. Cost accounting and management training did not really exist in the French universities. So the French education system was characterized by a practical and technician orientation of the courses on cost/management accounting, without any academic background. Unlike Germany for instance, this situation generated in France a brake on innovation and a conservative view of accountants and management accountants.

Relations with the authorities

France has a long history of strong governmental influence with a powerful technocracy. It was the case since the origins of capitalism with mercantilism and state intervention in the economy (1630-1720). Bhimani (1994) described this situation with the "manufacture des glaces de France" (the Royal Glass Work) case, Boyns et al. (1997 b) through the industrial accounting during the nineteenth century and [START_REF] Nikitin | Setting up an industrial accounting system at Saint-Gobain (1820-1880)[END_REF] with the costing system of Saint-Gobain about 1820.

The French full costing system was institutionalized in the "Plan Comptable" (chart of accounts) with formal rules and fiscal matters which influenced the costing systems. Influenced by the German occupation of France, the chart of 1942 was very similar to the German plan of 1937 and was preserved after World War II. The French state continued to control cost accounting after World War II with the introduction of centralized planning and national accounts. As [START_REF] Levant | Can cost & financial accounting be fully reintegrated?[END_REF] explain, the French state played a major role in the separation of accounting systems and the failed attempt of the "système croisé" (dual system) to re-integrate them. The move towards standardization was so strong that the separation of accounting systems became inevitable.

As a consequence, the French state was a conservative factor. The influence of legal obligations on accounting policy was so strong that French firms were reluctant to divulge their costs and keep a strong separation between financial and cost accounting. French companies tended not change their cost accounting systems, allocating real costs to the products via the homogenous sections method.

But external influences must also be mentioned.

The inter-nation transfers of information

Lock (1984) describes the mutual influencing movements between countries, for example between Japan and the United-States, and between the United-States, France and Germany. Shield describes the American costing techniques in France and Germany in the 1920s, echoing Fordism and Taylorism.

Another major U.S. influence on French cost accounting/management dates back to the early years of the Marshall Plan (early 1950's). As a consequence, a number of leading French managers studied management accounting in the United States. Thus, an increased emphasis on budgeting and management control was observed in the following years. More recently, the adoption of activity-based costing, balanced scorecard and target costing are good examples of the inter-nation transfers of information.

The next section analyzes the adoption of target costing in the French industry.

About target costing in a French perspective

For Western countries companies, target costing is more a technique than a global managerial philosophy [START_REF] Feil | Japanese Target Costing: A Historical Perspective[END_REF]. For the Japanese accounting association (Oshita, 1996), Western countries use a simplified picture of target costing.

In a French perspective (Oshita, 1996), target costing is first a market-oriented pricing system than a global system integrated in other systems. Due to the French analytical perspective, target costing is seen as a cost management system with a product life cycle approach. The starting point is the recognition of the discrepancy in timing between cost determination and accrual. For [START_REF] Lorino | Target Costing ou gestion par coût-cible [Target Costing or management by target cost[END_REF], at the end of the target costing process, value and cost meet. So, among French scholars and practitioners today, there is still a debate to understand target costing as a true global management system or just as one tool among others.

DISCUSSION AND CONCLUSION

This paper compares Japanese and French cost accounting/management. It shows that there are specific French and Japanese ways of recognizing the concepts of cost accounting/management, different from the Anglo-Saxon approach. This first observation has justified the research objective.

Notwithstanding the homogenization of management practices around the world, Japanese and French cost accounting/management are keeping their traditions. French accounting looks like old-fashioned; but it is a wrong understanding. French cost accounting/management are just grounded on a strong analytical and control tradition. Differently, Japanese cost accounting/management are more synthetic and globally integrated to the whole management system of companies.

Through institutional, cultural and contingency approaches, several compared determinants between French and Japanese cost accounting/management traditions have been discussed. The research shows that the attitude towards target costing and the specific practices of cost accounting/management observed in France and Japan highlight the differences and similarities between the two countries.

The points raised are: -

The two countries traditions of cost accounting/management are rooted in an industrial history dominated by the role of the engineers, promoting rational, technical and manufactureoriented systems.

-

The weakness of the French and Japanese management accounting professions (compared to the Chartered institute of management accounting in United-Kingdom for example) backs up the industrial orientation of cost accounting/management in both countries.

-

The relationship with national authorities is quite different between France and Japan concerning cost/management accounting. France is a highly centralized country and the influence of state administration explains the usage of quite standardized and separated cost accounting systems, from a conservative perspective. French controllers focus more on the checking behind the control, while Japanese controllers could favor the steering. One reason is that French cost accounting systems focus on historical analyses and Japanese ones on planning. So, French managers need other information systems such as "Tableaux de bord" (dashboards) for decision making. French management accounting is rooted in two contradictory and complementary traditions [START_REF] Lebas | Managerial accounting in France Overview of past tradition and current practice[END_REF]: a tradition of a standardized cost accounting system as a universally accepted method, the homogeneous section method (French full costing method), and a tradition of engineers, developing autonomous methods (GP/UVA method3 and French dashboards) and a strong interest for foreign methods such as activity-based costing and target costing. The French dashboard ("Tableau de Bord") focuses on monitoring physical and financial indicators for operational and strategic control (such as the balanced scorecard). Differently, cost accounting/management systems in Japan are adapted to the companies' operational needs. They are contextualized. Target costing is a global managerial method that needs to be fit to the specificities of each situation. Target costing is not a specific technique as the French full costing method, but a set of principles sustained by a managerial philosophy.

-A "non-cooperative" culture of cost accounting/management in France, inside the companies and between them. As centralization is historically grounded in French companies, cost accounting/management can be perceived as instruments of dominance, controlled by experts and high level managers. French companies tend to favor centralized decision making systems. This is not the case in Japan and target costing is a good illustration of this difference. Target costing involves employees at the middle management and operational levels with selfcontrol mechanisms.

Cost accounting/management in Japan are more external-oriented than in France with a focus on customers, suppliers and partners. Target costing is a good illustration of this orientation. Concerning the cooperative dimension of Japanese cost accounting/management, companies in close business relations share common information through value chains. This is the case for instance in the automotive industry between main suppliers and constructors. Target costing is also a good illustration of this cultural trend with inter-organizational costing and open-book accounting approaches. In France, there is a reluctance to share costing information, even if target costing is now quite popular in French industries.

Both countries are also influenced by the Anglo-Saxon practices, and evidences of convergence seems to be emerging: the popularity of methods and trends such as balanced scorecard or the influence of IFRS (International Financial Reporting Standards). But these influences would only be on the surface and would not have a strong impact on managerial behaviors. But literature also indicates that Japan converges towards Western countries managerial behaviors. For instance, the seniority and permanent employment principles decrease in Japan [START_REF] Pudelko | The End of Japanese-Style Management?[END_REF]. According to [START_REF] Hopper | Cost accounting in small and medium sized Japanese companies: an exploratory study[END_REF], several Japanese companies start generating more detailed cost information and adopting individualized payment systems. In France, the influence of English-speaking countries with the strategic cost management and strategic control trends shakes up traditions to move towards more external and strategic-oriented cost accounting/management systems. The Japanese influence with the lean management movement is also transforming the French habits.

Two proposals emerge from the research: Proposal 1: Japan and French keep specific cost accounting practices despite the phenomena of globalization observed Proposal 2: Due to cultural and institutional factors, the differences with which Japanese and French companies manage costs are even greater.

For a future research, it would be interesting to analyze in depth these subtle changes through case studies. A study about a Japanese subsidiary in France and a French subsidiary in Japan is planned.

[START_REF] Hofstede | Culture's consequences: Comparing Values, Behaviors, Institutions, and Organizations Across Nations[END_REF] develops the famous five dimensions of power distance, uncertainty avoidance, individualism vs. collectivism, masculinity vs. femininity and long versus short-term orientation.

[START_REF] Madsen | The Balanced Scorecard: Fashion or Virus?[END_REF] describe the use of Balanced Scorecard in Scandinavian countries with the concept of "travel idea".

GP-UVA is a French equivalent method first designed by Georges Perrin (GP) in the 50's; after, it was renamed UVA (value added units for "unités de valeur ajoutée") in the 80's. GP-UVA simplifies the calculations by fictively reducing multi-product firms to firms that only produce one product (see[START_REF] Levant | Georges Perrin and the GP cost calculation method: the story of a failure[END_REF].


Enric Celaya
email: celaya@iri.upc.edu

Jose-Luis Albarral
email: albarral@iri.upc.edu

Pablo Jiménez
email: jimenez@iri.upc.edu

Carme Torras
email: torras@iri.upc.edu

Visually-Guided Robot Navigation: From Artificial To Natural Landmarks

Landmark-based navigation in unknown unstructured environments is far from solved. The bottleneck nowadays seems to be the fast detection of reliable visual references in the image stream as the robot moves. In our research, we have decoupled the navigation issues from this visual bottleneck, by first using artificial landmarks that could be easily detected and identified. Once we had a navigation system working, we developed a strategy to detect and track salient regions along image streams by just performing on-line pixel sampling. This strategy continuously updates the mean and covariances of the salient regions, as well as creates, deletes and merges regions according to the sample flow. Regions detected as salient can be considered as potential landmarks to be used in the navigation task.

Introduction

The robotics community has devoted many efforts to specific indoor navigation schemes for wheeled robots, often strongly dependent on structured visual features [START_REF] Desouza | Vision for mobile robot navigation: a survey[END_REF]. Nowadays, the demand for outdoor applications is increasing, and the research attention is shifting towards more general, though perhaps less accurate, navigation strategies that rely on unstructured visual cues.

Not only unstructured environments pose new challenges, but also the mobility capabilities of the robots themselves -outdoor robots usually use legs, tracks, or deformable frames-imply that concepts like what is actually an obstacle have to be reconsidered. Even the very goal of outdoor navigation is usually different from that of indoors. The typical goal in indoor navigation applications is to reach a desired place in a previously known environment, usually using a map, whereas in outdoor navigation, e.g., in a rescue task, the environment is often unknown and the goal must be specified on-line and redefined during the navigation task, based on the observations made from the positions already reached. Full teleoperation is not always possible, as shown by NASA's Mars Exploration Rovers project [START_REF] Leger | Remote Image Analysis for Mars Exploration Rover Mobility and Manipulation Operations[END_REF][START_REF] Maxwell | The Best of Both Worlds: Integrating Textual and Visual Command Interfaces for Mars Rover Operations[END_REF] operating Spirit and Opportunity rovers in the surface of Mars, due to the time delay introduced by the travel of the control signal between the Earth and Mars. The possibility of fully autonomous navigation may also be severely limited by the processing capabilities available to the rover's CPU.

Thus, in applications such as planetary exploration or rescue in catastrophic areas, partly teleoperated robots, able to reach autonomously a destination marked by a human operator on the image as seen by the robot, are agreed to be very handy. Updating the target as the robot advances, permits long-range journeys even with low-bandwidth and long-delay communications. Additionally, it would be desirable that these robots were able to find their way back autonomously without relying on any a priori knowledge of the environment. This so-called homing problem has been addressed in some works [START_REF] Kirigin | Bearings based robot homing with robust landmark matching and limited horizon view[END_REF], where visual homing with bearings-based control is experimented in natural environments by using correlation of SIFT features [START_REF] Lowe | Distinctive Image Features from Scale-Invariant Keypoints[END_REF] in images taken from different robot poses.

In our visually-guided navigation approach we have addressed these issues by relying on landmarks. Only few previous works have tackled a similar visually-guided landmark-based approach. In [START_REF] Murrieta-Cid | Visual navigation in natural environments: from range and color data to a landmark-based model[END_REF] a landmark-based environmental model is incrementally built using color and stereo range information. Then, the landmarks in the model are used for the robot localization and for long-range robot navigation, through the selection of different landmarks that form a sequence of sub-goals that the robot must successively reach. The system differs from ours in that, instead of being user-guided, it performs an automatic goal selection by searching for the landmark with the highest elevation peak among all detected landmarks in the scene.

The paper is structured as follows: Section 2 describes our approach to visually-guided navigation. In Section 3, we show the viability of the approach by testing it for indoor navigation using artificial landmarks. Section 4 introduces our approach to natural landmark detection, and some results are shown for real image sequences taken by a mobile robot in an outdoor setting. Finally, some conclusions and future work are pointed out.

Our approach to visually-guided navigation

A problem we had to face while designing our visually-guided navigation system was how to specify the navigation target. Map-based approaches, and those based on absolute coordinate systems like GPS, were discarded since we required our system to be useful also in completely unknown environments. Ideally, the user should be able to select any object in sight and mark it as navigation target. As the navigation proceeds, the user could select new targets, thus guiding the robot towards the desired location in a way similar to what a human explorer would do himself.

The problem with this approach is that, what can be a clearly identifiable object for the user, may be difficult to discern for an artificial vision system, so that the robot can be lost very soon. Our strategy to solve this problem consists in limiting the selection of possible targets to those that the robot is able to identify with its vision system. Thus, it is the system that first shows to the user the available landmarks, and the user may select one of them as the target. To overcome the limitations that this approach could have in environments with sparse identifiable landmarks, it is possible for the user to directly drive the robot in the desired direction.

The Navigation Interface

Fig. 1 shows the two main windows of the navigation interface: The Camera Window and the Robot Control Interface. In the first, the user can see the images taken from the camera of the robot and control its gaze direction. Superimposed on the image, landmarks detected by the vision system are marked in red. The user may select one of these landmarks as the navigation target with a mouse click. Alternatively, the target can be selected using the Robot Control Interface, where landmarks are displayed in a simulated environment. In developing our platform for visually-guided navigation, our aim was to achieve a robot-independent control, so that the user could use it without being concerned about specific features of the robot that is performing the navigation task. The only assumptions we made about the robot are that it carries a camera, it is endowed with appropriate sensors to detect obstacles, and it is able of a short-term estimation of its own displacements (e.g. by odometry). For the tests of the system, two robots as different as a four wheeled Pioneer II and a six-legged Lauron III (Fig. 2) have been used.

The interface allows two control modes: Manual Drive and Go to Target. In the Manual Drive mode, the user directly controls the robot speed and direction with a slider and a driving wheel. Once a landmark has been selected as navigation target, the Go to Target mode can be triggered to let the robot reach the target in a completely autonomous way. Both modes can use the option collision reaction, which implements a high priority behavior that performs a pre-defined evading maneuver when an obstacle is detected. Obstacle detection is specific of each robot. In the case of the Pioneer, an activation of the bumpers or a null advance of the robot in response of a movement command are interpreted as obstacles. In the legged robot, the activation above a threshold of the front infrared sensor, the collision with one leg at a given height, or the impossibility to find a stable support, are all considered as obstacles. Both robots also consider very close landmarks as obstacles.

In the Go to Target mode, by default, the robot simply heads directly towards the direction of the target. This simple reactive form of goal attraction, together with the collision reaction behavior, is enough to succeed in simple environments where path planning is not strictly necessary. For more complex environments, the Navigator option can be used to invoke a landmarkbased navigation algorithm with map building and path planning capabilities. Our current implementation features the algorithm described in [START_REF] Busquets | A multi-agent approach to qualitative landmark-based navigation[END_REF], but other landmark-based navigation algorithms could be used.

Navigating with artificial landmarks

A major problem in landmark-based navigation outdoors is a reliable identification of landmarks. In order to validate our visually-guided navigation approach, we isolated the landmark recognition problem and provisionally solved it by using artificial landmarks. For this, we designed a set of landmarks inspired by the work of Scharstein and Briggs in [START_REF] Scharstein | Real-time recognition of self-similar landmarks[END_REF], who proposed the use of self-similar patterns, invariant to scaling, rotation, and viewing angle, whose detection indicates the presence of a landmark. A second pattern attached to the landmark composing a binary code served as identifier. This design has the drawback that most of the landmark area is used to indicate the presence of a landmark, but only a small area is devoted to the landmark code. This may prevent a correct identification of landmarks that are far away.

To improve on this, we modified the landmark design so that the code pattern is at the same time part of the landmark-presence indicator. The result is a set of landmarks like those marked in red in fig. 1, consisting in a header rectangle and five half-sized ones, whose relative positions, at the right or the The navigation interface has been tested indoors using artificial landmarks with robots as different as a four wheeled Pioneer II and a six-legged Lauron III (Fig. 2). Users not familiar with the interface could easily drive the robot using the manual mode, no matter what robot was at work. On the other hand, the simple Go to Target mode, with the Navigator inactive, shows itself to be very intuitive. The collision reaction capability allows for a successful task completion even when the target is behind some obstacle. An unanticipated way to drive a robot was soon discovered: taking the selected landmark away causes the robot to track it, despite the fact that it is continuously moving or suddenly changes its position. With respect to the Navigator mode, it could be seen that the interaction between the navigation algorithm and the collision reaction behavior, naturally adapts the resulting trajectory to the actual capabilities of the robot performing the task, a feature that could be hard to implement in a more traditional map-based approach. To show this, we put the two robots in the same environment, and launched the navigator with the same target (Fig. 3). Since the legged robot has a much greater capability for obstacle crossing, what for the wheeled robot was an obstacle that caused the navigator to plan an alternative path, was considered just as an irregular patch of terrain by the legged robot, which followed its way walking over it with no further planning. Note that, for a map-based navigator to do the same, it should be provided with robot-specific maps, with different distributions of obstacles and free space.

Natural landmarks

Having shown that our visually-guided navigation approach was satisfactory, our next focus was on solving the problem of natural landmarks detection. For this, we relied on the assumption that useful landmarks must be salient, i.e., they must constitute distinctive regions in the image, so that its repeated detection and identification is facilitated. The saliency of a region is not determined by the absolute value of any intrinsic magnitude, but rather, by the contrast or difference of this value with respect to the value of the same magnitude in the surroundings [START_REF] Nothdurft | Saliency from Feature Contrast: Additivity Across Dimensions[END_REF]. This is also known as opponency [START_REF] Todt | Detection of natural landmarks through multiscale opponent features[END_REF]. Many variables can be used to define the saliency of a region, like color components, intensity, or feature orientation.Works like [START_REF] Itti | A Model of Saliency-based visual Attention for Rapid Scene Analysis[END_REF] compute saliency as a combination of the opponency values of these three variables. In what follows, we present a very simple approach that just considers RGB color values, but the same idea could be used with more informative features.

Detecting Salient Regions: a random exploration approach

Instead of processing each incoming individual frame captured by the camera, the sequence of snapshots is treated as a continuously varying image. To this end, individual pixels are picked up at random from the current image and processed independently of whether the frame has already changed or not. In this way, a frame-rate and resolution-independent processing system is obtained, and the power of the computational resources only affects the number of pixels that can be processed per time step.

In the process, a number of so-called units are competing for the most relevant parts of the image, which we call Regions of Interest: each time a pixel is examined, the unit that best responds to it is selected and updated. By responding we mean to have a similar color while being in its proximity, and by updating, the process of adjusting the location and dimensions of the unit so as to improve its response to this pixel. In the case that no unit responds to a pixel, it may be considered as the seed of a new unit.

Units have elliptic receptive fields in the image plane, which adapt through successive updates to cover regions whose pixels have similar colors. These ellipses arise naturally from considering normal distributions for pixels taken at random from each region: the center of the ellipse corresponds to the mean value of the pixels' positions, whereas its dimensions (the major and the minor axes) and orientation are given by the covariances. Each unit also has a spherical receptive field in the RGB color space centered at the mean color value and fixed radius, which is a parameter of the system. Units are implemented as data structures with the following fields:

• Center vector U xy and covariance matrix Σ xy in the image plane, • Center vector U rgb in the color space, • Contrast C U , a scalar that measures saliency, as explained below,

• Creation date, to record the time since which the unit exists, • Counter U pdates U for the number of times a unit has been updated, • Counter Inside U for the number of times an input pixel has fallen inside the receptive field of the unit in the image plane, and • Strength S U , a scalar that estimates the current proportion between the number of pixels to which the unit responds and those lying into the unit's receptive field in the image plane.

In the main loop of the algorithm, a random input pixel I is selected and, for each unit U , its Mahalanobis distance in the image coordinates and Euclidean distance in color space are computed as:

M dist xy (U, I) = (I xy -U xy ) ⊤ Σ -1 xy (I xy -U xy ), (1)
where Σ xy is the covariance matrix of unit U , and

Edist 2 RGB (U, I) = i (I i -U i ) 2 , with i ∈ {r, g, b}. (2)
The Mahalanobis distance is used in the image space in order to take into account the shape of the spatial distribution. If both distances are below given thresholds (MAXDISTXY and MAXDISTRGB, respectively) then the unit is said to respond to the input pixel. Among the units that respond, the one which is closest in the image space, is considered as the winner.

Updating the winner unit

The center of the unit is approached to the coordinates of the pixel in the image plane as well as in color space:

U i ← U i + γd i , (3)
where d i = I i -U i , whith i ∈ {x, y, r, g, b} and 0 < γ < 1

Covariances in image space determine the dimensions and orientation of the elliptic receptive field of the unit, and are updated according to:

σ ij ← σ ij + γ(d i d j -σ ij ),
where i, j ∈ {x, y}.

Finally, the counter of updates is incremented by 1:

U pdates U ← U pdates U + 1 (5)

Updating other units

If the Mahalanobis distance from the input pixel to a non-winner unit is below three times MAXDISTXY, the pixel is considered to lay in the unit's neighborhood, and the pixel color is used to update the unit contrast:

C U ← αC U + (1 -α)
i∈{r,g,b}

(I i -U i ) 2 , 0 < α < 1 (6)
i.e., increasing or decreasing according to the Euclidean distance in the color space between the input pixel and the unit. The updating of the strength S U and Inside U is done for all units for which the Mahalanobis distance of the input pixel is below MAXDISTXY, i.e., the pixel is in the receptive field of the unit. While Inside U is simply incremented by one, the strength is increased when the unit responds to the input (i.e., also chromatically) according to:

S U ← βS U + (1 -β), with 0 < β < 1. ( 7
)
If the unit does not respond to the input color, S U is decreased according to:

S U ← βS U (8)

Removing irrelevant units

To avoid an unlimited proliferation of units, their number is limited by a parameter of the system. To allow the creation of new units when the maximum number has been reached, the less useful ones must be removed. This is done in the following way: First, the unit with the lowest strength is sought. If its strength is below a given value, it is assumed that the region it represents is no longer there, and the unit is removed. Otherwise, a unit will only be replaced provided a contrast estimation of the unit to be created is above that of the lowest-contrast unit. The contrast estimation is given by the distance in color space of the input pixel to its spatially closest unit. The new unit is initialized with a circular receptive field with center at the position of the input pixel and radius equal to its distance to the closest unit.

Merging of units

If two units respond to a given pixel, they are probably representing different parts of the same region and can be merged. The center and covariance values of the merged unit are computed as a weighted sum of those of the merging ones, with weights proportional to the respective area and strength, roughly corresponding to the "mass", or number of pixels each unit responds to:

W eight(U ) = Area(U )S U (9)
The area is computed from the covariances as

Area(U ) = ((σ xx + σ yy + ∆) • (σ xx + σ yy -∆)) (10)
∆ = (σ xx -σ yy ) 2 + 4σ 2 xy
The contrast of the merged unit is set to the highest contrast value of the two original ones, and for the strength, the weighted sum of the strengths is computed, this time using the respective values of U pdates U as weights.

Output of the system

As for the output of the system, only the units that correspond to the most salient regions are returned as possible landmarks. To this end, units covering too large regions of the image are discarded, as they usually capture the background and are not useful for navigation. Regions that are too small are also discarded to remove isolated pixels or noise. Finally, units that have not been updated a minimum number of times are not considered, since they are still not reliable enough. From the remaining units, those with contrast values above a given threshold are selected and output as landmarks.

Navigation using natural landmarks

Fig. 4 shows some snapshots of a preliminary test of the use of the salience detector for the guidance of a robot outdoors. The legged robot Lauron III is facing the situation shown in frame 1, in which the blue bag is chosen as initial navigation target. After some progress towards it (frame 2), two bright objects enter the field of view and are identified as salient landmarks by the system (frame 3). One of these objects is selected as a new target and the robot proceeds towards it letting the old target back (frames 4-6).

Conclusions

We have presented a working system for visually-guided navigation, which differs from others in the following features: (1) The interface is robotindependent even if the notion of obstacle varies from one robot to another, depending on their locomotion capabilities. Experiments have shown that the same navigation algorithm generates different trajectories for a legged and a wheeled robot. (2) A partial teleoperation scheme has been adopted, in which the goal is selected by the user among a set of landmarks previously detected by the robot. (3) Navigation issues have been decoupled from vision ones by designing artificial landmarks that could be easily detected and identified. (4) Once we had a navigation system working, artificial landmarks were replaced by natural ones. A color-contrast saliency detection algorithm has been developed which relies on randomly sampling pixels from the continuous video stream acquired by an on-board camera. This algorithm has been tested on a legged robot in an outdoor scenario with promising results.

Fig. 1 .

 1 Fig. 1. Camera Window and Robot Control Interface.

Fig. 2 .

 2 Fig. 2. The six-legged robot Lauron III used in the experiments.

Fig. 3 .

 3 Fig. 3. Navigation experiment

Fig. 4 .

 4 Fig. 4. Six snapshots of the navigation experiment with the robot Lauron III.

Acknowledgments: This work has been supported by the Spanish Ministerio de Ciencia y Tecnología and FEDER under the project SIRVENT (DPI2003-05193-C02-01), as well as the European Union under the project PACO-PLUS (FP6-2004-IST-4-27657). We thank the IIIA for making available the environment for the navigation experiments and performing the tests with the Pioneer 2, and also for providing the implementation of the navigator.