Hatem Younes

Sandrine Voros

Jocelyne Troccaz

AUTOMATIC NEEDLE LOCALIZATION IN 3D ULTRASOUND IMAGES FOR BRACHYTHERAPY

Keywords: Needle segmentation, 3D Ultrasound image, Bayesian classifier, Prostate brachytherapy

Needle segmentation in ultrasound images may be the indispensable step to solve other problems, such as the detection of radioactive seeds in ultrasound images for brachytherapy treatment. In this paper we propose a novel method to localize accurately curved paths of flexible needles in threedimensional (3D) ultrasound images. Our method is based on an automatic thresholding step where the bayesian classifier theory is applied to select a needle's voxels from the background. The next step consists in detecting the needle and representing its curvature. Here we propose to fit a parametric polynomial of low degree to representing the parabolic curvature. We describe a first evaluation on a data set of 9 patients.

Introduction

Prostate cancer is the second most frequently diagnosed cancer in men worldwide, with 1.1 million new cases in 2012 [START_REF] Torre | Global Cancer Statistics[END_REF]. In Europe, prostate cancer is the first leading type of cancer in terms of new cases and the third leading cause of cancer death among males [START_REF] Ferlay | Cancer incidence and mortality patterns in Europe: Estimates for 40 countries in 2012[END_REF]. In 2012 the highest incidence rates were estimated in Northern and Western European countries such as Norway (193 per 100,000) and France (187 per 100,000) [START_REF] Ferlay | Cancer incidence and mortality patterns in Europe: Estimates for 40 countries in 2012[END_REF]. Among the wide range of possible treatments [START_REF] Peschel | Surgery, brachytherapy, and external-beam radiotherapy for early prostate cancer[END_REF], brachytherapy with permanent seeds (also called low-dose brachytherapy) is considered an effective treatment modality for low risk prostate cancer with low urinary toxicity [START_REF] Raziee | Permanent seed brachytherapy for low risk prostate cancer, long term outcome, and urinary toxicity[END_REF].

Physicians use brachytherapy needles to implant seeds into the patient's prostate. Parallel needles are inserted through a grid named "template". Needles insertion and seeds placement are performed under the control of ultrasound images -most often a set of parallel axial images (figure1) resulting in a reconstructed 3D volume-. Because brachytherapy aims at delivering a specific and homogeneous radiation dose to the prostate whilst limiting the dose delivered to organ at risks (urethra, rectum and bladder), it is based on a planning phase determining the optimal placement of seeds. Due to the limited precision of seed placement, planning generally needs to be updated during the procedure. In the perspective of an automated process, needle position monitoring is a mandatory preliminary stage. Furthermore needles are frequently used in other interventional procedures such as biopsy and their automatic detection is of primary interest. In this paper we propose a novel method for automatic needle segmentation in 3D ultrasound images. This modality, widely used because of its non-invasiveness, its ability to visualize moving soft tissue in real-time and its low-cost, suffers from intrinsic limitations: noise, artifacts-due to the anatomy or to needles for instance-, signal attenuation with depth and high variability depending on the user and the patient. All these reasons make the needle detection challenging.

Several approaches have been proposed for segmentation and localization of needles in 3D ultrasound images. Novotny et al. [START_REF] Novotny | Tool localization in 3D ultrasound images[END_REF] proposed to apply the PCA algorithm method on the clusters of thresholded voxels and to identify the needle as the longest line on the brightest cluster. Hough Transform (HT) was also used to localized straight needles, Zhou et al. [START_REF] Zhou | Automatic needle segmentation in 3D ultrasound images using 3D Hough transform[END_REF] proposed two improvements of the classical HT by introducing 3D and randomizing aspects. It is important to underline that although needles are initially straight, they can bend during insertion in the living tissue. Ding et al. [START_REF] Ding | Automatic needle segmentation in three-dimensional ultrasound images using two orthogonal two-dimensional image projections[END_REF] proposed to segment the needle in two orthogonal planes. This method was developed by Aboofazeli et al. [START_REF] Aboofazeli | A new scheme for curved needle segmentation in threedimensional ultrasound images[END_REF] to detect a curved needle. They used a ray casting process to obtain projected 2D images. Ayvaci et al. [START_REF] Ayvaci | Biopsy needle detection in transrectal ultrasound[END_REF] first proposed machine learning techniques to detect a biopsy needle. They minimized an energy function from MRI/TRUS fusion guided biopsy videos. Zhao et al. [START_REF] Zhao | Evaluation and comparison of current biopsy needle localization and tracking methods using 3D ultrasound[END_REF] proposed a comparison study of biopsy needle localization methods in 3D ultrasound images.

Most methods start with a binarization step which consists in classifying voxels into two classes given their intensity. Voxels with level grey higher than a threshold τ belong to the needle whilst the others correspond to the organs. Since all further steps process the binarized images, the choice of τ has a very high impact on the segmentation performance. As mentioned previously, this choice is made difficult due to high variability depending on the patient and ultrasound acquisition parameters tuning. τ might even vary in a single image due to signal attenuation. We thus propose to apply a bayesian classifier to obtain thresholded voxels (section 2.1) for improved accuracy and robustness to image variability. Then, the needle is detected among the thresholded voxels as a spatial parametric polynomial curve fitting (section 2.2). Results are described in section 3.

Methods

The proposed method processes each needle separately by considering a region of interest (ROI) constructed around the planned trajectory of the needle. The insertion direction in the template allows defining a parallelepipedic bounding box where the needle has to be searched for.

Thresholding

A set of voxels X, with intensity I(X) ⊆ R and their coordinates X ⊆ R 3 is divided into binary groups: needle voxels (X n ) and background voxels (X b ).

X n = {x ∈ X : I(x) ≥ τ } X b = {x ∈ X : I(x) < τ } (1)
To define τ automatically for various data sets, several authors proposed to use histogram value. For example Beigi et al. [START_REF] Beigi | Needle trajectory and tip localization in real-time 3-D ultrasound using a moving stylus[END_REF] chose the threshold value based on the cumulative sum of the number of pixels. Ultrasound image contains bright artifacts which intensities may be greater than needle intensities, resulting in wrong classification of these voxels. To solve this problem, authors used speckle reduction techniques, which are high complexity procedures. So we propose to apply instead a Bayesian classifier to accomplish thresholding without any preprocessing step.

Gaussian mixture model

Histogram value (figure 2) shows the existence of two separated peaks which describe the background and the needle. The histogram can be modeled using an additive Gaussian mixture model:

P (y) = M i=1 π Xi p i (y) (2)
Where y = I(x) is the intensity of x ∈ X, M is the number of mixed components, π Xi is the coefficient of i th component and p i (y) =

1 √ 2πσi e - (y-µ i ) 2 2σ 2 i
is the density that defines the i th component. To select the needle peak we use a Bayesian classifier where we assume that there are exactly two classes.

Naive Bayes classification

Bayesian classifiers assign the most likely class to a given observed feature [START_REF] Rish | workshop on empirical methods in artificial intelligence[END_REF]:

C Bayes (y) = argmax Xi P (Cl = X i | I(x) = y) (3)
Where Cl ={X n (Needle), X b (Background)}.

Naive Bayes classifier assumes that the intensities I(X) are independent within each class, so the posterior probability of each class is calculable as follows based on Bayes theorem: }.

P (Cl = X i |I(x) = y) = π Xi P (I(x) = y|Cl = X i ) P (I(x) = y) (4
So naive Bayesian classification is summarized by the followed inequalities :

π Xn P (I(x) = y|Cl = X n ) Xn ≷ X b π X b P (I(x) = y|Cl = X b ) (5) Voxel x ∈ X is classified in X n if the posterior probabil- ity of X n is larger than X b .
The application of this probabilistic model requires the knowledge of several parameters which vary from image to another, or even from one needle ROI to another in the same image. These parameters are optimized according to a criterion of maximum likelihood to approach as much as possible the sought distribution. This procedure is often done iteratively via the expectation-maximization algorithm (EM [START_REF] Dempster | Maximum Likelihood from Incomplete Data via the EM Algorithm[END_REF]) explained as follows.

The aim of EM is to estimate the unknown parameters representing the mixing value between the Gaussians (the means, the variances and the prior probabilities) which maximize the posterior probability given an observed feature. EM algorithm is a dedicated iterative process, which computes the maximum-likelihood of an observed vector of features for each iteration. Jeff A. Bilmes [START_REF] Bilmes | A Gentle Tutorial of the EM Algorithm and its Application to Parameter Estimation for Gaussian Mixture and Hidden Markov Models[END_REF] developed in his tutorial the procedure of the EM parameter estimation.

π new Xi = 1 N N j=1 p(X i |x j , θ ) µ new Xi = N j=1 x j p(X i |x j , θ ) N j=1 p(X i |x j , θ ) σ new Xi = N j=1 x j p(X i |x j , θ )(x j -µ new Xi ) 2 N j=1 p(X i |x j , θ ) (6)
Where N is the number of voxels within the ROI and θ is the vector of parameters to be estimated at the th iteration.

The algorithm proceeds by using the newly derived parameters, calculated as equations ( 6), as the guess for the next iteration until convergence. EM typically converges to a local optimum estimated when parameters values stabilize: |θ -θ +1 | < . While EM converges to a local optimum, it requires a meaningful initialization. We want the means (µ Xn , µ X b ) to converge to the values of peaks corresponding to the needle and the background classes. Histogram of figure 2 shows us that the background class is the dominant one, so any initialization value of µ Xi between the peaks values will converge to the background peak. The best initialization value of µ Xn to reach the needle peak is 255. Any initialization of µ X b between peaks values [120, 210] converges to the background peak. Furthermore we choose the initialization values of the variances (σ Xn , σ X b ) to be sure that EM remains able to converge to the values of peaks of the two classes. We selected (σ Xn , σ X b ) = (7,60). These values are drawn from the variance values observed on histograms of bounding boxes of the first image and were robust when applied to the other images. Finally prior probability of the needle class π Xn is drawn from the number of voxels which correspond to the needle within a boundary box: (π Xn , π X b ) = (0.03,1-π Xn ). Using EM algorithm we are able to estimate parameters required to construct naive Bayesian classifier. Therefore, needle voxels are identified from needle volume (left of figure 3), and the point cloud corresponding to the thresholded needle is shown in the right of figure 3, when inequality (5) is satisfied.

Needle Localization

The second stage consists in localizing the needle within the thresholded voxels selected by the naive Bayesian classification. Most tool localization methods in thresholded 3D ultrasound images are based on Hough Transform(HT) for straight needle [START_REF] Zhou | Automatic needle segmentation in 3D ultrasound images using 3D Hough transform[END_REF], and on the model-fitting approach [START_REF] Uhercík | Model Fitting Using RANSAC for Surgical Tool Localization in 3-D Ultrasound Images[END_REF], [START_REF] Barva | Automatic Localization of Curvilinear Object in 3D Ultrasound Images[END_REF] for curved tool. Random sample consensus (RANSAC) algorithm [START_REF] Fischler | Random sample consensus: A paradigm for model fitting with applications to image analysis and automated cartography[END_REF] is used to compute the parameters of the model Fig. 3: A 3D needle volume (left) before and (right) after thresholding step fitting which makes the procedure costly. We chose to use model fitting to localize the curved needle among the thresholded voxels, and limit candidate crossing points for parameters calculation to still have a real time results.

Parametric curve

The needle axis is represented by a spatial parametric polynomial curve c(t; H) : R -→ R 3 of low order n -1:

c(t; H) =    h 11 , ..., h 1n h 21 , ..., h 2n h 31 , ..., h 3n         1 t : t n-1      , t ∈ R (7)
where H = h i,j is a matrix of the curve's coefficients and t is a curve parameter. We use n = 3 to model needles which exhibit a parabolic curvature. So the matrix H is found by solving the system of linear equations as follows:

c(t i ; H) = p i , 1 ≤ i ≤ 3 (8)
Where p i ∈ R 3 are the 3 control points through which the curve is required to pass and t i = (pi-p1)k0 ||k0|| an estimate of the projection of p i onto the curve as the principal direction k 0 of point cloud. For each possible triplet of control points, both the matrix of coefficients H and its cost C are calculated. C corresponds to the distance between the point cloud and the curve.

C(H) = x∈Xn d(x; H) 2 (9)
where d(x; H) is the distance between point x and the curve approximated as ||x -c(t; H)||, where t is the projection of x onto the curve. The curve c(t; H * ) that minimizes C(H) is selected as the model that best approximates the needle.

H * = argmin H C(H) (10)
We propose to generate all the triplets of points (p 1 , p 2 , p 3 ) from three regions (X 1n , X 2n , X 3n ) ⊂ X n . These regions are drawn from the cloud of voxels X n where X 1n , X 2n , X 3n correspond to the beginning, middle and end of the detected voxels in the needle direction (figure 5).

Results

The algorithm was implemented in CamiTK which is an open source, cross platform generic tool, written in C++ [START_REF] Fouard | CamiTK: A Modular Framework Integrating Visualization, Image Processing and Biomechanical Modeling[END_REF]. It was tested on anonymized 3D ultrasound images of 9 real patients. The image size is about 765x575x65. The bounding box size is about 50x50x65. The size of a voxel is 0.26x0.26x1. The number of needles per image is 14.

We evaluated the results of the implemented algorithm using a manual needle segmentation. Thus, we have firstly measured the objectivity of our manual segmentation by repeatedly segmenting two arbitrary needles in two different images, at distant time. The standard deviations obtained were {tip < 0.5 mm, axis < 1 • }.

Images of real patients may contain some needles not fully detectable, for instance, when crossing a shadowed region (figure 4). Table 1

Conclusion

We have proposed a novel technique to detect curved needles without any pre-processing. However, information from the planning of needle position could allow to optimize the size of the region of the bounding box. We used naive Bayesian classifier to select needle voxels among volume voxels. Localization of the needle within selected voxels is done by spatial parametric polynomial curve of low order. The proposed algorithm was tested on realistic images of patients. Needle path would be excellent prior information to localize seeds, it will be our next objective.

Fig. 1 :

 1 Fig. 1: 2D axial image, the needles perpendicular to the slice are figured as the points radiating linear artifacts

Fig. 2 :

 2 Fig. 2: Typical histogram in a bounding box with needle and background peaks

π

  ) Therefore, in the case of binary classification, naive Bayesian classifier assigns to each voxel the class which maximizes argmax Xi { Xn P (I(x)=y | Cl=Xn) P (I(x)=y) , π X b P (I(x)=y | Cl=X b ) P (I(x)=y)

Fig. 4 :Fig. 5 :

 45 Fig. 4: Results: (left) two needles completely visible, (right) needle containing an occluded region

  provides the number of needles per category (fully/partially visible) in each image of the data set. Error in the tip localization for fully visible needles is in [0.6;2.7]mm with a mean value of 1.4mm, and error in the needle axis is in [0.5 • ;3.5 • ] with a mean value of 1.5 • . Error in the tip localization for partially visible needles is in [2;8]mm with a mean value of 4.2mm, and error in the needle axis is in [2 • ;7 • ] with a mean value of 6 • . The average needle length being 45 mm; the error in the tip localization is about 3% of the needle length for fully visible needles and about 9.3% for partially visible needles. Each needle detection

Table 1 :

 1 Number of fully/partially visible needle per imagePatient 25 s (un-optimized program implemented on a system with a 3.50-GHz processor and 32 GB of RAM). The EM algorithm was able to converge on all the 126 needles localized showing the robustness of the needle peak detection.

Acknowledgements

We thank LATIM and Brest University hospital for giving access to patient images. This work was partly supported by the French ANR within the Investissements dAvenir program (Labex CAMI ANR-11-LABX) and through the FO-CUS project (ANR-16-CE19-0011).


Samuel Gantier
email: samuel.gantier@univ-valenciennes.fr

Michel Labour
email: michel.labour@univ-valenciennes.fr

User Empowerment and the I-Doc Model User

HAL is

Since 2008, several hundred "interactive documentaries" (i-docs) have been published on the English-and French-speaking Internet. We define an i-doc in line with Gaudenzi (2013: 73), for whom an i-doc is not the mere extension of linear documentary into digital media, but "something else". The digital nature of an idoc implies modularity, in that it is created by independent objects linked to each other where each file is accessible and independent from the others (Manovich, 2001:31). An i-doc also implies that it is "not something fixed once for all, but something that can exist in different, potentially infinite versions" (Manovich, 2001:36). This variability means that an i-doc can change and evolve, allowing collaborative creations that were not possible with film and video. In this context, an i-doc seeks to combine digital media to the long heritage of non-fiction films. With Web 2.0 and the changes of the broadcast industries, connected interactive interactions, such as the non-stabilised i-docs, come with ideological discourses focused on a "New Writings 3 " approach. This rhetoric of newness legitimates a change of creative paradigms concerning the traditional roles of the author and spectator as "users". This includes in examining how an i-doc opens up the possibilities for a user-led "documentary voice" (Nash, 2014 4 ).

With the New Writings paradigm, users are afforded a central place with an active, if not empowered, role in the designing of an i-doc. In this light, broadcasters suggest that i-docs be "gamified" as a way of "giving voice (to users) by providing the means, or tools that will induce others to speak for themselves, and the context in which they may be heard" [START_REF] Daniel | On Politics and Aesthetics: A Case Study of 'Public Secrets' and 'Blood Sugar[END_REF]: 217, in Nash, 2014: 3), and even to create their own content. This is what Nash (2014: 6) calls voice-as-authorship.

The overall aim, however, was to engage users actively beyond being passively involved in the i-doc process (see a broadcaster's view on this issue in Table 2.2, below). The implication for the editorial strategy was to facilitate an interactional communication approach by formulating ways of creating solid links with a community of empowered Internet users within social networks 5 (voice-as-social participation, Nash, 2014: 6).

If the emerging i-doc format appeals to media professionals, the empirical grassroots reality of an active, if not empowered, user raises numerous issues for 3 The term "nouvelles écritures" (New Writings) was coined in 2011 by the Nouvelles Ecritures department of the French state-funded national television group, France Télévisions, to spearhead innovative digital contents, see http://nouvelles-ecritures.francetv.fr (viewed 9 March 2014) 4 For Nash (2014: 6) the two-dimensioned concept of a use-generated documentary voice allows the possibility "for audiences to challenge the documentary's point of view". The first concept is voice-as-authorship, where users are involved in decisions taken about what the documentary publishes. The second concept is voice-as-social participation. This refers to the ability of users to "connect and engage" with each other through the documentary. 5 One can speak of an interactive documentary ecosystem that regroups public funding in France and Canada for the "New Writings" format via a growing network of festivals and professional training schemes.

designers. In effect, the injunctions of upstream deciders to place the user in the center of editorial, artistic and ergonomic consideration comes up against numerous problems in the different phases of the life cycle of an i-doc.

In this chapter, we examine the various ways in which designers, as social actors, formally portray, or "modelize", an i-doc user. In this light, the chapter outlines the research methodology, followed by an overview of the research findings. We then present in detail the three dimensions of the Model user. First, the Exogenous (importing) User is described in his or her penchant for the heterogeneous way in which the projected user blurs the identity of the overall Model User. Then we explain the characteristics of the Interfacial (navigation design) User and the issues it brings up in designing an i-doc. Finally, the Interactional (empowered) User is analyzed in its various facets.

Research Methodology

A grassroots participant-observation approach was used to study the design process of an i-doc from an ethnographic perspective. One of the researchers of this chapter was a film-editor of the design team involved in designing an i-doc for the French national television network. From this angle, each member of the design team was considered as "informers" engaged in complex social practices. The dual role of the researcher as an embedded social actor as a former freelance filmeditor and the present role of as a researcher constitutes one of the characteristics of the participant-observation approach with its potential strengths and weaknesses (for further discussion of this point, see the Conclusion, below).

Our research postulate was drawn from the theory of textual cooperation of Umberto Eco (1989), notably his well-known concept of the "Model Reader", which we call, here more broadly, "Model User". We thus posited that the design team would, more or less, consciously create a de facto "Model User" via design strategies as an attempt to guide user's sense-making processes. Thus, one of the objectives of our participant observation was to identify how the Model User was constructed and what the end result was.

Following the sociological Actor-Network Theory (Akkrich, [START_REF] Akrich | Sociologie de la traduction: textes fondateurs[END_REF], our study retraced the sociotechnical negotiations of the authorfilmmaker with the different members of the design team. This involved identifying negotiations via innovative technical injunctions between "actants" (symbiotic relations between human and non-human actors of an operational network) that prefigured the Actor-Network Theory inspired "prescribed usage scenarios" (Utilisateur-Projet) of [START_REF] Akrich | Sociologie de la traduction: textes fondateurs[END_REF] of the technical framework of an i-doc. In our study, the challenges tackled by designers were seen as a way to understand how actants could creatively use an i-doc.

In this context, a case study method (cf. Labour, 2014) was put in place to define the conceptual and operational parameters of our study into a real-life i-doc design process "in action". The ensuing field data were analyzed by using the precepts of "grounded theory" (Glaser and Strauss, 1967). In a nutshell, a "grounded theory" approach involves a bottom-up approach to theory creation based on inductively creating conceptual categories taken from empirical data. The more recent works of [START_REF] Paillé | L'analyse qualitative en sciences humaines et sociales[END_REF] was used to establish the protocol for conceptual category building (see Table 1, below). Data collection was conducted by cross-matching information taken from technical documentation on i-doc production, email conversations between design team members, semi-guided interviews, team meetings, statistical data, notes taken from observation of team members, and "debriefings" with research colleagues not involved in the B4 project. The triangulation of these different sources was conducted within the framework of what Elias (1983Elias ( /1993: 25-26, 35) : 25-26, 35) calls the two levels of knowledge in a toand-from dynamic between the "engagement" of the employee-participant in an activity and the critical "distance" of the researcher from the activity observed.

The underlying question that guided our study was to understand, for example, how the preconceptions and the reasoning of the different members of the design team impacted on the different pathways proposed for the designing of the i-doc. This led to questions about how expected media use by future Internet users was formally portrayed in the i-doc design? What did the upstream members of the design team of the i-doc put in place to facilitate the active participation of its projected Internet users? How did the final i-doc design formally portray "participative interactivity" with Internet users? What were the different action modes made available to the user wanting to make sense of an i-doc.

Overview of the Findings

Our analysis is based on a published French i-doc, called B4, fenêtres sur tour (B4, Windows of a block of apartments)6 . This i-doc is a literary adaptation inspired from George Perec's prize-winning "novel", La vie mode d'emploi (1978). The i-doc is composed of 96 video interactive modules placed in a graphical interface portraying a 12-storey, inner-city block of apartments. The different video sequences lasts between one to three minutes. They show in a fragmented and singular manner the way in which a dozen inhabitants of this type of mass-habitation appropriate their everyday living space.

The research findings brought to the fore three complementary sub-dimensions of the Model User. These are the exogenous, interfacial and interactional dimensions. The table, below, summarizes the findings based on an analysis of the data using a grounded theory approach (see above). The category building approach of [START_REF] Paillé | L'analyse qualitative en sciences humaines et sociales[END_REF] was used to create the "conceptualized categories" of Table 1.

Portrayals of the i-doc format

A set of sociotechnical dimensions that pre-constructs an i-doc format (spectator of non-fiction films, Web 2.0 Internet users, reader of youth literature, etc.).

Exogenous User -borrows and hybridizers from pre-existing genres.

Negotiation of interfacial interaction modalities

Debate between "guided" navigation and "intuitive" navigation.

Interfacial User -seen by designers as having a certain level of media literacy.

Negotiation of participative modalities

Debate between "open" works and "closes" works.

Interactional User -able to feels empowered to act on an i-doc.

The Exogenous Dimension of the Model User

In this we examine the facets of the exogenous dimension for the deciders of the design team (filmmakers, producers, web developers, graphic artists, television broadcasters, etc.) having to cope with a wide diversity of i-doc designs. This diversity can be explained by the still emerging editorial formats of i-docs that bring together designers from different professional perspectives, for example, about "what the user can do and wants to do". These different perspectives can lead to a clash of design methodologies and a weakened workflow process.

The Heterogeneous Portrayals of an i-doc Format

We argue that different professional modes of operation, norms and values condition the aesthetic and ergonomic framework of an Interactive Documentary. Following the work of the Symbolic Interactionism movement, we assumed that the human actors (see Actor-Network Theory, in section "Research Methodology", above) of the i-doc design team were members of different "social worlds 7 ". The notion of social world is a particularly useful heuristic to "analyse conflicts and negotiations in the construction of technical artefacts" (Flichy, 2003: 119), notably, in terms of the professional, aesthetic and technical culture of each member of the design team that conditions expectations and implicit preconceptions about the hybrid and non-stabilised i-doc format. This means that the identification of the specificities of an i-doc can differ radically depending on the designers' professional culture and mode of operation. The final i-doc invariably ends up as a negotiation between the implicit logic of the televisual documentary industry and the demands of putting online a hypermedia artefact. It is in this context that Web 2.0 broadcasters, with a purported "innovative editorial" logic (we discuss the point in section "The Shared Unity of Choice", below), can associate an i-doc to a strong discourse on user participation. An i-doc thus can become a place of empowerment (see section "The Heterogeneous Portrayals of an i-doc format", below) in its enablement logic (see below) for designers in allowing them greater artistic freedom compared to the pre-established formats of television channels for authors-filmmakers and to the monetization logic of web sites for developers. We thus advance the hypothesis that i-docs, in the domain of transmedia production (see section "The Rival Editorial Models", below), are the result of a complex relation based on a series of pre-existing exogenous (external) references (film documentary, journalism, video-games, contemporary art, broadcast culture, Web 2.0 ideology, youth literature, etc.). If this hypothesis holds, it would then seem reasonable to assume that in the case of the designing of the i-doc B4, fenêtres sur 7 The notion of "social worlds" refers to group activities that have no clear boundaries, nor a stable formal "organisation". It is in this sense that the notion of social world differs from a classical concept of "organisation" having various internal social worlds. In our case, however, a social world is a phenomenon that can be common to different organisations. This implies that various loosely defined social worlds -common to different well-defined organisation -can rival each other, for example, in defining a professional mode of operation or a problem to resolve (Flichy, 2003: 118).

tour (B4, Windows of a block of apartments), the "social worlds" of its different members of the design team brought in a series of pre-conceived expectations and implicit preconceptions about the nature of the future hypermedia artefact. Tables 1.1 and1.2, below, present key aspects of B4 designers' social worlds regarding idoc formats.

Graphic designer

A sober and decorative graphic framework to facilitate film content.

"One must not forget the main thing about the web documentary is that they are films to be seen. Behind the (graphic) scenery there is a technical framework. The aim of the game, for me, is to highlight as best I can the films that will be in the framework".

Developer

A cultural web site without the limits of monetizing web traffic.

"In the case of B4, I like the idea of publishing content; with the site being just an access tool to these videos and not a window to generate traffic and money".

The key point of the Model User is its ability to sustain a certain undetermined aspects of the innovation process. It is this element that qualifies an i-doc as a polysemic catch-all term. This type of all-embracing term participates in the hybridization of professional concepts and skills in the renewal of the non-fiction cinematographic process with the rise of computerized media. It is at this precise point that the portmanteau-word "empowerment" comes to the fore. Working between different technical, design and editorial boundaries, calls for discretionary choices based on the designer's creative "imaginary" skills. The challenge of this boundary shifting for the i-doc design team includes finding a way to encourage a user's empowering process. Such an empowerment enables users to voice a personal viewpoint in novel, or counter-current, actions regarding a shared problem situation. Empowerment entails individuals authorizing themselves to author discursive practices that nurtures the "freedom to have one's voice heard, (the) freedom to develop a voice worth hearing" (Hymes, 1996: 64). Empowerment is thus affirmatively communicational. It enables an individual to affirm an apparently personal repertoire of exchanges and topics in terms of social positions and gatekeepers, involved in the distribution of discursive resources and practices, within a "communicational situation" (cf. [START_REF] Mucchielli | Un cadre de référence pour "l'approche communicationnelle des TIC[END_REF]. At first glance, this definition of user empowerment is closely associated to what Nash (2014: 6) names "voice-as-social participation" highlighting the ability of users "to connect and engage with others" in active participation through a media. That having been said, our view of empowerment also includes "voice as authorship" relating to users ability to contribute to the documentary text as participation within a media (Nash 2014: 5).

Design team Portrayals of i-doc formats Verbatim

Video technician

A web program that adapts to feedback from the Internet users.

"The interactive documentary is an open scenario that can become a subject for discussion forums as a way to add on modules more or less on line with what the Internet users want".

Producer

A transmedia production that articulates the author's documented viewpoint in a participative to the Web 2.0 dimension.

"While the unusual, the violence and the futile are found side to side on the Net, we wish, on our part, to put the incredible possibilities of this new mode of creation to the benefit a documentary approach". Broadcaster A formal and technical experiment to strongly engage an audience via a participative dimension.

"The radical break of the digital universe is audience engagement where the public, the citizens now have a preponderant place. The creative view point implies integrating their commentaries and especially involving them at different content levels including offering them the possibility of participating and, even, creating content".

The term "apparently personal" is used as we posit that individuals are not isolated beings, but cultural co-participants of an ongoing reconstruction of the social fabric (whether individuals are aware, or not, of the socio-cultural dimension of their "personal" choices is beyond the ambit of our study). What our study does clarify, however, is how the phenomenon of "empowerment", as a form of social, personal and technical enablement, can be, in part, explained by what we dub the Exogenous User.

Our findings identified, within the Model User dimension, a user that typically borrows from pre-existing genres when exchanging with other members of the design team who have their own implicit and non-stated design formats (these designers, as actors of their respective social worlds, tend to consider their implicit preconceptions as "natural" or "common sense" knowledge needed to avoid the cognitive burden of reinventing everything at every moment). The Exogenous User is thus often heavily involved in designers' discourse and operation modes, notably in the way they participate in the design process, without being made aware of how they effectively operate during the life cycle of the project. As implicit knowledge, gained over the years, these preconceptions can be either a professional asset or liability when engaged in an innovative process that goes beyond "common sense" as received wisdom. This is where the enabling empowerment process steps in. The Exogenous User needs to be enabled to take the (calculated) risk of borrowing (relatively) unknown phenomenon from pre-existing genres, gleamed from other members of the design team, in order create a novel hybrid solution to a given problem situation. This "inter-disciplinary" approach carries with it the danger of misinterpreting or over-diluting what was borrowed from other genres.

The Blurred Identity of the Model User

There is little evidence to indicate to what extent and for how long a user will take to engage with an i-doc. Indeed, in 2012, the New Writings department of France Télévisions was not able to provide a projection of audience figure before going online. This created problems when putting in place the technical and communicational framework of B4 given that the B4 designers wanted to appeal to the largest possible audience as is done in the long tradition of non-fiction films. In addition, the designers were well aware that the web site navigation of the i-doc needed a certain amount of media literacy often associated with an "expert" user of the Internet. From an editorial perspective, this meant that the i-doc was to be aimed at Internet-experiences users residing or interested in the social issues of inner-cities.

"We have a lack of feedback. We do not know what to expect from the public, or how many people want to participate. If I had to host the web site, it would very difficult to work out what size of server I needed. Are we going to have 6 000 people, or only two who will click on the same space at the same time?" (interview with a B4 developer, [START_REF] Gantier | L'expérience immersive du web documentaire : études de cas et pistes de réflexion[END_REF].

In this context, the experimental nature of an i-doc production is often used to justify the incertitude concerning the likely response of real-life user. It is therefore not surprising that few authors appear to have anticipated if the user will take to, for example, the gaming aspects of the navigation design.

Following [START_REF] Akrich | Sociologie de la traduction: textes fondateurs[END_REF], we argue that, typically, the designer has three basic choices in the portrayal of his or her Model User. First, the designer can base his or her choice essentially on personal experiences, drawn from the received wisdom of cultural "common sense". Secondly, the designer can focus on an ad hoc sample of user "representatives" taken from his or her private social circle. Thirdly, the designer can focus on a formalised user constructed from an empirical study in situ (e.g. ethnographic analysis, usability tests, ergonomic principles, etc.).

One key element of our findings is the identification of how the members of the design team use their personal experiences in a, more or less conscious, portrayal of the Model User. Another element of the findings showed how the design team regularly spoke about how users could make sense8 of the i-doc using the grammatically impersonal third person of the plural form, as in: "The people will go to the B4 screen, they'll start to watch the videos, they'll click here, then they'll, (…)". This type of discourse contributed to a projected vision of users portrayed as an amorphous, self-contradictory phenomenon without a clear identity. This vagueness about the future user opened up a space for the "imaginary" skills of designers, which in this case, were essentially focused on their own mode of operation and the demands of a computerized network media of the i-doc in the making. In other words, each member of the design team sought to legitimate his or her "naturalized" (taken for granted) vision of the future i-doc and its empowering possibilities through the artistic and editorial freedom the format affords.

This quasi-phenomenological approach to understanding discourse about the workflow process within a design team can be found in the account given by a film broadcaster who spoke of him starting from his own daily experience as an Internet user when he tries to understand habitual use of others on the Web: "Ask yourself what you do on the Web? Why do you click, or not, on a Facebook link sent by a friend? What type of link makes you react? " Given the impossibility of predicting the response of a typical Model User, the developer needs to take on board all the possible options when designing the web site. In an interview with a B4 developer, the person stated how some Internet users would probably directly look for key words in the text, others would prefer images. "In the case of B4, we raised the problem that some Internet users won't find it very easy to click on windows. So we included a display of themes and titles that are familiar to Inter-net users" [START_REF] Gantier | L'expérience immersive du web documentaire : études de cas et pistes de réflexion[END_REF]. In another interview, the graphic designer referred to the idea of a "basic Internet user" when describing how most Internet users had limited Web-media literacy skills [START_REF] Gantier | L'expérience immersive du web documentaire : études de cas et pistes de réflexion[END_REF]. A more hardline approach was taken by the i-doc producer who rejected heavy directive interactivity as a way to keep Internet users glued to the screen: "On the Web, as soon as you want to take me by the hand, I get turned off! I'm not interested in being told 'You are such and such a person, so login to live an incredible experience'" [START_REF] Gantier | L'expérience immersive du web documentaire : études de cas et pistes de réflexion[END_REF].

In short, the findings show a blurring between the innovative nature (clicking on windows) and the "common sense" banality (clicking on a familiar display of themes and titles) of the site. This blurred definition of the Model User reveals a paradox about the nature of the designers' vision of the "typical" Internet user.

"The person who is interested in i-docs today, is someone who uses the Internet every day when looking for information. It is not my mother who will only go on the Internet to see if I have sent an email, or to buy train tickets online. Plus, we are banking on the fact that there is no bandwidth problem, nor of users misunderstanding what we have done" (interview with a B4 graphic designer, [START_REF] Gantier | L'expérience immersive du web documentaire : études de cas et pistes de réflexion[END_REF].

In conclusion, the exogenous dimension of the Model User depends on the designer's professional culture, on the one hand, and the mode of practice of the design team, on the other hand. The differences between the members of the team can lead to very different criteria about the defining features of the i-doc. This can lead to blurring, if not a confusion, about the operational identity of the Model User, notably about what can be expected of the person in terms of acquitted media literacy skill.

The Interfacial Dimension of the Model User

The interfacial sub-dimension of the Model User encapsulates the ways in which a user would make sense of the interactional design of an emerging i-doc. This dimension questions the different professional pathways and preoccupations of the members of the design team. It seeks to pinpoint how designers' preconceptions about the media literacy level of the "average" Internet user can formally be presented in an i-doc design.

The Issues in Design Interaction

I-doc "interaction design9 " defines scenarios of actions that facilitate users' sensemaking process. To do this, "suggestions" from the designers were communicated to users via an intermediary technical artefact. To do this, the artefact affords "handles" to tangibly frame user interactional experience (cf. [START_REF] Bouchardon | Littérature numérique: le récit interactif[END_REF]. Concerning the B4 i-doc, three modes of navigation co-exist in order to view the 96 videos. First, there is a horizontal navigation focused on the characters. Second, the user has access to the videos by a vertical navigation highlighting different film styles. Finally, the user can choose a transversal navigation to choose the videos grouped according to various themes (see Figure 1, below). We call Interfacial User that dimension of the Model User concerned with the interaction modes with the i-doc. The interaction consists of a dialectic between the logic of user's freedom of choice and that of the more coercive navigation paths of the i-doc. These challenges of these two logics manifest themselves during the design of the film interface when design team members negotiate which functions and which semiotic signs should be displayed on-screen when ing, for example, a video player, an instrumental function, or a "paratext10 ". Without going into the technical details, it seems useful to examine the different approaches that arise in the negotiation between the B4 design team members.

An interview with the filmmaker and graphic designer duo put forward arguments for an "open" laissez-faire navigation mode based on an intuitive and spontaneous navigation logic [START_REF] Gantier | L'expérience immersive du web documentaire : études de cas et pistes de réflexion[END_REF]. They argued that one should avoid, as much as possible, adding in "paratexts" to influence users' navigation. It is up to the user to infer the navigation logic according to "clues" embedded in the graphic interface. This design approach can be descried as a type of "hermeneutic" navigation in that the user proceeds in a trial and error way. The B4 developer and producer duo had a more nuanced approach and wanted "semi-guided" navigation. Their aim was to minimally guide the user's sense-making process (Labour, 2014) by offering different types of navigation (content indexation in the video reader, suggestion of continuing the sense-making process in different domains after the user had watched the videos module, etc.). A more hard-lined approach was taken by the broadcaster who advocated a guided navigation that adhered to current Web usage norms and that was adapted to the media literacy skills of a "typical" Internet user of today. The table, below, summarizes the different points de view of the B4 design team on the navigation question. "I come from a Web development institution, so the aim for me is, above all, not to lose Internet users by making sure everything is immediately identifiable. The difficulty for me is to grasp what the filmmaker wants, given that he did not want to start from scratch by saying to me 'here's the map of the web site, now get on with the job'!" Producer Semiguided with guidelines Graphic design guidelines in the video reader "One must guide the Internet user by giving him either a horizontal, or a vertical navigation? On this point, I was practically the only one, and against them all, at the start (…) as I put myself in the place of the Internet user who viewed the first film by chance: 'What will incite him to look at a second film? Up to what point will the person freely and willingly engage himself?".

Broadcaster

Guided Navigation

Conform to the norms of current usage "The 'clickism" is codified. One must totally guide the Internet user and not be totally enigmatic".

In short, the interfacial dimension of the Model User looks at the different ways in which a user could make sense of the interactional design of the i-doc. This leads to asking questions such as what were the different pathways proposed by the different members of the design team. How are user's media literary skills formally portrayed in the i-doc design in a way that it can facilitate giving both voice as authorship (with the design team) and voice as participation (with other users) (see Nash 2014: 6).

The Interactional Dimension of the Model user

The interactional dimension affords a central place and role to the Model User. In such a case, how does the i-doc design formally portray our concept of "participative interactivity"? What were the different action modes made available to the future Internet user wanting to interact with the i-doc i.e. voice-as-authorship (Nash 2014: 5). What can the design team of the i-doc put in place to facilitate empowering participation of Internet users?

The "Contributive Interactivity" Mode

The participative nature of an i-doc is based on a particular idea of what constitutes an "innovative" editorial format. The department of New Writings of France Télévisions heavily promoted an innovative editorial approach that encourages user intervention through a network of empowering interactive artefacts. The objective is to empower Internet users by encouraging them to introduce their own data onto the film interface and in so doing, the i-doc goes beyond just giving users access to existing author-filmmaker content.

In this light, the aim of B4 designers was to empower its Model Reader as a "reader-writer". To do this, the Internet user was allowed to send tweets that were immediately displayed on the stairwell wall of the block of apartments portrayed on-screen. In this way the flow of real time information of the social network tool Tweeter was directly displayed on the i-doc web site for all to see and comment. The technical framework of B4 enabled Internet users to read and reply to messages of fellow Internet users who had put tweets on the wall. Internet users' tweets attempted to semiotically mime the writings on the wall, akin to graffiti that embellish the urban décor of certain inner cities. The introduction of new data from Internet users effectively modifies the editorial content and the aesthetic form of the i-doc. From this angle, the i-doc is both a machine (superstructure) and a mechanism (infrastructure) "in the sense that it provides different handles for a public that can now, within certain limits, also become actors of a partially prefabricated i-doc" (Fourmentraux, 2010: 39-40).

This mode of bottom-up user contribution was designed to give Internet users a place to freely voice their opinions within the diagesis (interior view of the fictional world) of an i-doc interface. We describe these series of tweets as "contributive interactivity" that nurtures user participation empowerment as can be found in the interactivity modes of Net Art (Fourmentraux, 2010: 39). If this interactive contribution function encapsulated the wish to empower the Model User, its application posed a number of questions. Up to what point can one empower any and every Internet user to write what he or she wants on the interface of an i-doc? In response to this question, the design team agreed to create an editorial filter between real time tweets of users and its immediate display on the B4 web site. This was to be done by asking a community manager to mediate (filter) between what would be displayed on the B4 and what Internet users would send. From a production perspective, it was also necessary to develop an automatic management pro-tocol to cope with users' tweets. These two suggestions were not put in place due to a lack of human and material resources.

Besides the financial costs, the contributive interactivity mode had aesthetic consequences. The semiotic challenge was to transform users' authorship voices, as expressed via a Tweeter thread, into an appropriate artistic typographical, layout form that could be assimilated into the graphics of the web site.

The Mediated Circulation of Facebook

The widespread nature of Facebook and its social implications has been largely discussed in the literature. We will therefore not discuss the circulation and the monetization of personal data based on neither users' profile, nor the sociotechnical features of Facebook in portraying personal space. Instead, we examine the possibility of putting user comments of B4 on Facebook and the discursive intertextuality between Facebook and B4. The aim on including Facebook was to empower users' "voice-as-social participation" through electronic media (Nash, 2014: 6). This begs the following question: What were the design implications for organizing mediated interactions via a social network like Facebook?

A strong impetus for including a contributive interactivity function within B4 came from its broadcaster who repeatedly requested that the i-doc contains such a feature in spite of the fact the author-filmmaker felt that users' "taggings on the walls" demeaned the aesthetic quality of his work. For the author-filmmaker, Facebook appeared to be a better place to present user reactions to the videos. The idoc should therefore remain a closed author-filmmaker generated place, while Facebook could be a place for user exchanges and community building. Given this, the auctorial work of the B4 consisted of defining an intermediary mediation framework to frame users' participation modes between B4 and Facebook.

When wanting to include a tool like Facebook in the design process, however, it assumes that the Model User is an informed Internet user who communicates regularly on electronic social networks. It implies that prior to consulting B4, Internet users have a Facebook account and are familiar with the action of "like" as a way to share their tastes to a community. In point of fact, the design issue concerning contributive interactivity goes beyond Facebook. Whether, or not, the contributive interactivity of B4 is externalized to Facebook, it does not fundamentally alter the design issue of what to do with what users communicate as a way to empower Internet users.

It turned out that the negotiations between the design team members were effectively focalized on the discursive nature of user comments and the value judgments they could contain (user approval, thanks, encouragement, questionings, criticisms, insults, etc.). The B4 design team sharply criticized the electronic ("virtual") social networks that tend not to participate in social and political debates raised by documentary films. These networks went against the aim of the authorfilmmaker of encouraging public debate through his videos.

The counter-argument to the author-filmmaker was that the dialogue dimension of the contributive interactivity mode should be associated with the principle of "virality", a typical feature of electronic networks. This form of communication could also be linked to a marketing strategy that uses electronic social networks to attract Internet users to a web site. Such a strategy goes beyond merely creating socialization links into a community. It also aims at creating a large audience of "people that you can contact and who will 'advertise' whatever you want" (Georges, 2009: 8). From this perspective, an i-doc that converges with a social network, finds itself sharing a similar communicational logic as the service industry (restaurants, tourists sites, businesses) where consumers are encouraged to comment on the quality of the service rendered as a way of creating "loyal" and regular customers. This is paradoxical given that many documentary filmmakers claim not to have a profit-making agenda in mind, nor wanting to create "loyal customers". There is consensus, however, about the need for users to identify, and make sense, a distinguishing quality in what they were asked to see and hear. One of the challenges of the graphic designer is thus to create a link between giving a clear visual distinguishing identity to an artefact and the practical necessity of respecting semiotico-cultural ("common sense") conventions of a given public.

In this case, the challenge was to create a clear link between the B4 Facebook page and the B4 i-doc. The design solution opted for affording Internet users a choice of "go-between" signs (signes passeurs, in French, see Souchier and Jeanneret, 2002) when wanting to recommend the B4 i-doc to a social network "friend". Indeed, Facebook appears to have a number of potent go-between signs based on a convincing semiotic logic, i.e. a meaningful sign linked to a recognized (and often implicit) social convention. For example, the apparently successful thumbs-up Facebook "like" icon appears to be based on an ancestral cultural symbol going back to gladiatorial combats of ancient Rome (Corbeil, 2004[START_REF] Engagement | Thumbstruck: The Semiotics of Liking via the "Phaticon" Semiotic Review[END_REF]. The icon directs the user to his or her personal Facebook screen-page.

"Its small size rests on a complex technical writing technique that allows precise user gestures to produce important sense-making effects and economic returns by mobilizing a significant amount of people who click" [START_REF] Candel | Signes passeurs et signes du web: le bouton like, ou les ressorts d'un clic. Manuel d'analyse du web en sciences humaines et sociales[END_REF].

Another well-known "go-between sign" to Internet users is the grey, encircled Facebook letter "F" (relooked by a B4 graphic designer), on the top right-hand side of the B4 screen, see Fig. 1, above. The sign was aimed at guiding the Internet user to the B4 Facebook screen-page administered by the production team. Internet users can thus go to the Facebook B4 space in order to become part of the greater B4 "community" and/or update news thread on their personal Facebook "wall".

The link, however, between the B4 Facebook screen-pages (personal space page and B4 page) creates a breach between "watertight" levels of the Facebook system. There is no direct exchange between screen-pages. It is up to the Internet user to move from one page to another. It also needs be said that it is an appointed administrator who posts information on the B4 Facebook page. Users can annotate or comment, but not post information directly on the B4 Facebook. Seen from this perspective, the Facebook system does not satisfy the aims of B4 to empower users by encouraging them to freely voice issues of what is important to them in terms of the B4 videos.

The Shared Unity of Choice

The issue of contributive interactivity becomes particularly complicated when the question is asked at what level, users can share the i-doc content with other online users. To answer the question one needs to define the minimal semantic unity of what should be disseminated on the network. Three basic options were possible. The first involves sharing the i-doc as an integrative whole. The B4 experience is then lived in its total globalness. The second option concerns the dissemination of the 96 videos as independent, stand-alone units. The B4 experience is thus presented in a modular form. The third option offers the possibility of commenting on fellow users opinions of the B4 videos. The idea, here, was that Internet users could be interested in what fellow users had to say about B4 content as much as, if not more so than, the original content itself. This may be particularly true for fans who visit the B4 site wanting to see what other people felt about the issues raised by the videos. The B4 experience of facilitating this type of user dialogue was aimed at encouraging the empowerment process through an online agora type of "community" building.

A unitary and integrative logic (Option 1) contrasts to a fragmentary and modular logic (Option 2). A modular logic is defined by an ontological network of elements that allows the user to have access to information in a fragmented way through metadata that describes the content in HTML tags. The challenge, here, is to reference deep links of the web site in order to improve its identification on the Web, notably when users conduct online searchers. This approach differs to a unitary logic (Option 1) that sees an i-doc as a totality of complex undissociable elements. This involves viewing the videos in the foreground while keeping in the background an overall a Gestalt visual mood of the inner-city residential environment. The hypothesis of designers was that viewing the same video with the B4 background will not have the same empowering effect as viewing the video, for example, on a more "impersonal" Youtube or Facebook context.

If links are primordial in a sharing logic, the i-doc then becomes the central element of a reticular system. In what manner were data and information constructs then disseminated? What mediation mode could be introduced so that a Facebooktype function becomes "naturalized", i.e. "invisible" to the Internet user? In response to these questions, we hypothesize that a potentially tension exists between a centrifugal and centripetal logic within a larger reticular navigation system. A centripetal logic draws Internet users to the B4 site via the intermediary shared link, for example of a Facebook "friend". Yet, paradoxically, when the Internet user clicks on the B4 "like" icon, the interface creates a centrifugal logic that propulses the Internet user towards his or her own Facebook personal wall (and not to other B4 users). There is a risk, here, that such external links lose Internet users even before they finish viewing the videos. The sharing function could thus have the counter-productive effect of getting newly arrived users to the B4 web site to immediately go onto Facebook without viewing any videos. Instead of Facebook bringing users to the B4 i-doc it could take people away from watching the videos in a B4 environment.

The Offline Mediation

In order to produce an editorially high quality publication, the B4 producer sought to organize an amateur photographic competition on the theme of inner-cities11 (see Figure 2 below). This was done according to the precepts of transmedia logic of developing offline events parallel to online events in B4. The objective was to create a special symbolic "virtual museum" in which Internet users expose photographs linked to the film universe. The museum could be updated in real time so that users can see their photographic works of life in inner-cities. This action created a dialectic tension between the wish to encourage free and creative expression, on the one hand, and the (legal, moral) need to have an editorial control on what content is published on the B4 site. How can one then organize the contributions of Internet users without deforming the artistic quality of B4? Once again the issue of human (manual) and/or machine (automatic) filtering of Internet users' contributions is raised. The thorny editorial question is poised between having limited selected content of "recognized quality", on the one hand, and encouraging a maximum amount of mass participation without distinguishing "low" or "high" quality user input, on the other hand. This creates a dual paradox. The selecting of users' comments may disempower those whose comments were rejected, but in doing so help empower the lucky few who publish online. Likewise, putting a maximum of comments online may discourage "high quality" seeking users, not wanting to sift through what they may deem as "low quality" comments, but in doing so the mass publication policy could empower other users as socially recognized readers-writers regardless of the apparent quality of their texts.

In spite of claims from some, that "Web 2.0" ushers in the Golden Age of the reader-writer, time has shown that the editorial creation of quality content remains socially selective. For Rouquette (2009: 55) it is not enough for Internet users be intellectually and technically able to create usable and popular content on the Internet, they need, above all, to be part of a certain "macro-social environment". Such an environment allows marginal pioneering practices to be transformed into everyday habits and to become taken for granted cultural practices. The interactional dimension of the Model User is thus situated at the confluence of the actions of "enlightened" amateur and the demands of new documentary writing techniques.

Fig. 2 Screen capture of the amateur photographic competition

To summarize, the interactional dimension affords a central place and role to the Model User in the way the i-doc design formally portrays participative interactivity based on a view on how to empower Internet users. This approach leads to asking questions such as what were the different modes of action made available to the user wanting to make sense of an i-doc? What did the upstream members of the design team of an i-doc put in place to facilitate the active participation of Internet users?

The Rival Editorial Models

The expression "to put online" means, in computer terms, being connected to a network of computers or other devices12 ; the term thus implies a network computers and, more often than not, people behind the machines. In this sense, a transmedia production attempts to combine a linear television documentary with an interactive content for the Web and/or offline cultural events. This type of cultural production drives its organizers to think about how it will present its message to the public via a "media plan". This invariably leads to establishing a timeline of when and how to disseminate its message. The challenge for the producer is to create bridges between the different media in order to attain different audiences. If in a very competitive context, each media functions according to its own personal logic, the utopic idea of a transmedia action remains a mirage of wanting to federate different rivals around a common media framework.

Regarding the B4 i-doc, there were intense negotiations with the broadcaster about when go on online. On one level, there were legal, logistic and organizational question to be resolved. On another level, questions about when to go online revealed a hidden and symptomatic dimension of the Model User. When one deals with such questions, one is effectively defining the way in which the i-doc will meet its public. Behind the negotiations are the issues of when an i-doc will be watched, how often and, more generally, the mode of "consumption". Will the Model User watch the i-doc once or several times? Must one renew the filminterface in order not to tire regular users with the same screen page? Will the Model User alternate between virtual reality of the Web and face to face social practice offline? These questions show the incertitude around creative digital works that seek more and more to attract Internet users.

These questions also explain the need for establishing communication strategies around media-event scenarios either to create expectations and/or to frustrate future Internet users. This invariably means that one seeks to attain a target audience or to maintain the link with a particularly volatile online audience. In some ways, these communication strategies can be compared to organizing a television program schedule so to coincide with events in the everyday life of the user.

Based on grassroots observation, i-doc film editors have observed that the life cycle of an i-doc is particularly short. Once the connection peak has been attained, often during the first few days of the launch, the visitor curve drops dramatically to become practically non-existent. How then can one extend the life of an i-doc after being put online?

One solution is to create active and updated editorial presence on the i-doc web site as a way to ensure a "loyal" audience over a longish period. For the moment, this strategy has still to prove its worth. Other alternatives of involving Internet users were also being experimented. A first scenario was that of creating a series of regular events to meet up with Internet users. A second scenario aims at synchronizing a whole program of online events with a key element in the news. A third scenario seeks to spread the content of the video over two scheduled seasons. A fourth scenario focuses on editorially updating the web site of the i-doc and coordinating it with offline events. The table, below, summarizes these different scenarios.

Conclusion

Our case study brought to the fore three complementary dimensions to the B4 Model User, which we named exogenous, interfacial and interactional. The exogenous dimension is characterized by a wide diversity of i-doc formats that appeal to, or repel, those (filmmakers, producers, web developers, graphic artists, television broadcasters, etc.) involved in a documentary design process. This dimension takes cognizance of the reality that each member of the design team comes with his or her own agenda about who the user is and what he or she wants. Depending on the designers' professional culture, and the design mode of practice, the defining qualities of one i-doc from another can thus strongly differ.

The different professional agenda within a design team can result in a clash of design methodologies, resulting in an inconsistent workflow process. It is in this context that the exogenous dimension of the Model user underlines the challenge of clearly establishing the identity and the media literacy skills expected of an idoc Model User. In doing this, it places the issue of how to identify the place and the role of the projected Model User at the heart of designing user friendly navigation paths (interfacial dimension of the Model User) in order to encourage empowering user participation (interactional dimension of the Model User).

The embedment of the three dimensions of the B4 Model User is complex. The complexity highlights the challenge of connecting up different levels of the design process in pursuance of an efficient and effective user-centered design. This connection is no mean feat. Amongst other things, it necessitates establishing a clearly shared vision of the Model User within the design team that can be adjusted to the workflow process and the changing environment. One such vision is that of Gaudenzi's interactional and systematic approach to designing for a "living documentary" [START_REF] Gaudenzi | The Living Documentary: from representing reality to co-creating reality in digital interactive documentary[END_REF], i.e., a relational object that comes to life when interacting with users.

For Gaudenzi (2013: 73-81) i-docs, as relational objects, represent "artefacts that link technologies and subjects and that create themselves through such interaction". This is based on the hypothesis that a relational artefact, in its core essence, cannot be understood "as a finite form but needs to be addressed through the complex series of relations that form it and that it forms" (Gaudenzi, 2013: 73-81). This implies that the Internet user moves from externally controlling the artefact to playing a more integral role within the i-doc universe. At the present moment, the idea of a "living documentary" [START_REF] Gaudenzi | The Living Documentary: from representing reality to co-creating reality in digital interactive documentary[END_REF] is more an ideal to strive for than a roadmap for designers. If such a systematic and analytic vision of the i-doc is appealing, our approach, however, seeks to take a critical distance from an overly theoretical approach to user empowerment via mediated interaction design.

The methodological rigor put in place in the case study of B4 (see above), such as the cross-matching of data sources, does not exclude recognizing its limits. One limit of our findings is the very fact of having examined in depth an in vivo situation in a given time and place. What would be useful is the examination of comparable situations through other case studies, and/or by establishing a mass questionnaire based on our findings. These measures could contribute to generalizing key elements of our study. A second limit of our study is its participantobservation research approach. It may well be that the position as a freelance film editor, or even the personality, of the researcher, may have produced an overly distorted portrayal of the team design process. It would thus seem useful that other participant-observation approaches conducted from other positions, such as that of the graphic designer, the film maker, the community manager, etc. A third limit of our approach is that we did not share our findings with the B4 research team. This may have helped to consolidate our interpretation of our data. This was not done for essentially logistic reasons (lack of accessibility to the design team members after the publication of B4, lack of time and resources).

Given these limits and in looking into the near future, a clearer knowledge of the apparent antinomies and paradoxes of a designated Model User would help designers formally portray their Model User. In this context, the Actor-Network Theory (Akkrich, et al., 2006) could be usefully revisited in terms of [START_REF] Gaudenzi | The Living Documentary: from representing reality to co-creating reality in digital interactive documentary[END_REF] "living documentary". In both cases, human actors are considered in conjunction with system-generated actors, but each approach does this in its own way. This brings up other questions. How can the notion of "voice", as a central element of user empowerment, be enhanced in an i-doc and its online social networks? Should all voices be heard, or should they be editorially guided, or even "filtered", as a way to create a community-building identity? Who decides the criteria of an i-doc related online community identity? What are the ethical values of such a community? What role can user communities be encouraged to play in the life cycle of an i-doc even before its launch in the public sphere? Such questions put on the spot the traditional linear and sequential logic of the audiovisual domain (pre-production, production, post-production). It calls on them to take on board the importance of giving a place and role to users at the onset of the design process.

Questions about the development of i-docs could be inspired by the iterative and incremental methods of the video game industry and software engineering (e.g., the 12 principles of the Manifesto for Agile Software Development, [START_REF] Beck | Manifesto for Agile Software Development[END_REF], notably in their capacity to integrate authoring issues through selforganization, team work and flexible responses to change. In the coming years, it is difficult to envisage living and empowering non-fiction films and i-docs without such features and the challenges that such a development entails for designers.

Fig. 1

 1 Fig. 1 Screen capture of the home page of B4 Windows of a block of apartment

Table 1

 1 Tridimensionality of the Model User

	Conceptualized categories	Definition	User dimensions

Table 2

 2

	.1 Portrayal of i-doc formats by the B4 design team (continued)
	Design team	Portrayals of i-doc formats	Verbatim
	Author-filmmaker	An empowering space of editorial and artistic free-dom for an author to ex-press a personal point of view in a non-linear narra-tive.	"A cinematographic adaptation of the novel La vie mode d'emploi would be too repetitive. The succession of about 20 portraits, from seven different angles, even if it were inventive, would quickly become tiresome".

Table 2 .2

 2 Portrayal of the i-doc format by the B4 design team

Table 3 .1

 3 Issues in the interaction design of B4

	Design team	Prescribed navigation	Communicational logic	Verbatim
				"The initial idea is that we had to under-
				stand very rapidly the vertical and hori-
			Gaming dimen-	zontal navigation principle. The formal
	Author,	Intuitive	sion to discover-	principle (horizontal and vertical) must
	Filmmaker,	navigation,	ing how the web	create positive user expectations. So
	Graphic	trial and	site functions, not	when the Internet users clicks, she must
	Designer	error logic.	necessary to view	quickly understand the navigation prin-
			all the videos.	ciple. This means you don't have to in-
				form her about what it means to go left,
				right, top or bottom".
	Developer	Semi-guided nav-igation with guidelines.	Information organized with-is out informing the user how to navigate.

Table 4

 4 Scenarios to extend the life the B4 i-doc

	Schedule for putting online	Site content	Media events	Offline events	Editorial models
		15
	Scenario 1	characters (120 video	Weekly (May and June 2012)	None	Series of regular events
		modules)
		15	Put all online	French
	Scenario 2	characters (120 video	at the same time (April	presidential election	Totality of the videos
		modules)	2012)
		10
		characters
		(80 video	Every two
	Scenario 3	modules) + 5 characters modules) (40 video	2012) months (April and September	Coordinated with the photographic competition	Successive versions of the program
	Scenario 4	12 characters (96 video modules)	All put online at the same 2012) time (April	Announcing the results competition photographic of the	Whole program

The B4 i-doc was directed by Jean-François Ribot and co-produced by France Télévisions and Mosaique films in June 2012, see http://www.francetv.fr/nouvelles-ecritures/banlieue-b4 (viewed

The term refers to a problem-solving sense-making process (ex. "What is going on in this idoc?") that consists of connecting personally significant meaningful informational constructs into a coherent, integral whole(cf. Labour, 2014). This view of sense-making is in line with what[START_REF] Hymes | Ethnography, linguistics, narrative inequality: Toward an understanding of voice[END_REF]

: 9) described as the human process of "making sense out of disparate experiences using reason to maintain a sphere of integrity in an immediate world".

Bill Moggridge and Bill Verplank coined the term "interaction design" in 1986 to underline "user experience" rather than just focusing on artefacts and the "design of user interface" of computer engineers. This approach emphasizes users' needs, desires and pleasure.

According toDupuy (2008: 23-24) the web-based "paratext" allows a user to categorize the text into its genre, and to organize the different elements of the (main) "text".

The photographic competition was organized with partner institutions (e.g. Academie-desbanlieue), social organizations (e.g. La fonderie) and the media (e.g. Respectmag, 20 minutes).

see http://whatis.techtarget.com/search/query?q=online (consulted

[START_REF] Engagement | Thumbstruck: The Semiotics of Liking via the "Phaticon" Semiotic Review[END_REF]