Guillaume Levieux
email: guillaume.levieux@cnam.fr

Matthieu Montes
email: matthieu.montes@cnam.fr

Towards Real-Time Interactive Visualization Modes of Molecular Surfaces: Examples with Udock

Keywords: Protein-Protein Docking, Interactive Simulation, Molecular Visualization, Molecular Surfaces Index Terms: I.3.6 [Computer Graphics]: Methodology and Techniques-Interaction techniques J.3 [Life and Medical Sciences]: Biology and genetics-[K.8]: Personal Computing-Games

In the present work, we describe and discuss two interactive visualization techniques recently added to the Udock software. First, we propose to display a spherical projection of the protein's molecular surface properties. The resulting 2D map allows to get in one glance a global view of the protein. Second, we propose to let the user choose a specific level of detail when visualizing the protein's 3D surface. We describe a simple smoothing algorithm that achieves this goal in real time. These techniques are designed to enhance the usability of molecular visualization and interactive simulation software.

INTRODUCTION

Protein docking methods aim at predicting the geometry of macromolecular complexes that play a crucial role in biological processes. To perform such a task, molecular visualization is needed to extract all relevant information about the structural properties of the proteins to be docked. Since proteins are very complex objects, the total amount of information that needs to be visualized and integrated is very high. It is thus critical for molecular visualization software to provide the user with different visualization and interaction modes that are adapted to the kind of information the user is interested in.

The Udock software allows a quick and easy-to-handle exploration of the possible conformations of a protein complex [START_REF] Levieux | Udock, the interactive docking entertainment system[END_REF]. When performing interactive human-driven docking, the user needs to generate a complex 3D mental model of the given proteins' structures and properties in order to propose relevant geometries of their possible complexes. The generation of this mental model makes interactive docking a very complex cognitive task. Udock is designed to help the user to build such a mental model, using adapted representations and manipulation techniques. Proteins are rendered as solvent excluded surfaces with a smoothed coloring of the electrostatic potential, so that the user understands the global shape and electrostatic properties of the protein. In Udock, proteins are docked using an original harpoon-like technique and the collisions between proteins are simulated in real time by a physics engine [START_REF] Coumans | Bullet physics library. bulletphysics.org[END_REF], allowing the user to try different docking poses in a quick and intuitive fashion. Finally a force-field based [2] interaction score is computed and displayed in real time to guide the user during the whole interactive docking procedure.

The goal of Udock is to maximize its usability, as a molecular visualization and interactive simulation tool. The usability of a tool can be defined as follows: effectiveness: the ability of the users to complete a task using this tool, and the quality of the output, efficiency: the time or resources spent to complete the task and satisfaction: the subjective reaction of the user when handling the tool [START_REF] Standardization | Ergonomic Requirements for Office Work with Visual Display Terminals (VDTs): Part 11: Guidance on Usability[END_REF]. Our previous work with Udock was centered on effectiveness and satisfaction [START_REF] Levieux | Udock, the interactive docking entertainment system[END_REF]. In the present work, we focus on the efficiency aspect of usability. We want the user to be able to manage the type and quantity of visualized information to invest his cognitive resources in the most efficient way.

In the last version of Udock, we have developed new interactive modes of representation of a protein's global shape and of its surfaces properties that will be presented hereafter: 1. interactive 2D maps of protein surfaces and 2. interactive smoothing of the SES. These interactive and real-time modes of representation will enhance the usability of molecular visualization and interactive simulation software.

RELATED WORK

Many interactive visualization software are available to visualize protein surfaces in real time such as Pymol [START_REF] Delano | The pymol molecular graphics system[END_REF], VMD [START_REF] Humphrey | Vmd: visual molecular dynamics[END_REF] or Chimera [START_REF] Pettersen | ucsf chimera-a visualization system for exploratory research and analysis[END_REF]. They all provide standard representations of molecular surfaces such as Van Der Waals surface (or spacefill) and solvent excluded surface (SES or molecular surface [START_REF] Richards | Areas, volumes, packing and protein structure[END_REF]) with coloring options that highlight a particular property (atom types, [START_REF] Corey | Molecular models of amino acids, peptides, and proteins[END_REF], electrostatic, [START_REF] Weiner | Electrostatic potential molecular surfaces[END_REF], or hydrophobic potential [START_REF] Kurochkina | Hydrophobic potential by pairwise surface area sum[END_REF]).

In the Van der Waals surface representation, the atoms are depicted as spheres with a given radius (the Van der Waals radius). Its major drawback is that it does not represent the accessibility of the different atoms of the protein to potential other molecules with respect to the surrounding solvent. Hence, the work of Richards on solvent accessible surfaces (SAS) and solvent excluded surfaces (SES [START_REF] Richards | Areas, volumes, packing and protein structure[END_REF]) can address this issue. To construct the SAS, a spherical probe of a radius equivalent to a water molecule is rolled over the Van der Waals surface. The center of the probe defines the SAS, whether the surface of the probe defines the SES. The SES is the best choice of representation for molecular interactions since it shows the shape of the object and its accessible surface with respect to the surrounding solvent [START_REF] Krone | Parallel contour-buildup algorithm for the molecular surface[END_REF].

Due to the number of atoms constituting proteins, their surface is very complex to analyze and extracting relevant information about their shape is difficult. The most common effects used for enhancing the rendering are cast shadows and depth cueing but due to the complexity of the surface, they often fail to produce a high-level comprehensible picture of the shape to the user [START_REF] Tarini | Ambient occlusion and edge cueing for enhancing real time molecular visualization[END_REF].

In order to abstract the complexity of the shape, Duncan and Olson developed a spherical harmonics approximation of protein surfaces [START_REF] Duncan | Approximation and characterization of molecular surfaces[END_REF]. To compute a similar abstraction and focus on the global shape of the object, we use in Udock a rendering technique that allows the user to interactively reduce the surface's level of detail in real-time.

Different other approaches have been developed to reduce the complexity of protein surfaces for visualization purposes, in particular using 2D projections of molecular surfaces. These 2D maps were developed to characterize global surface properties (normal vectors, principal curvature [START_REF] Duncan | Approximation and characterization of molecular surfaces[END_REF]), compare the binding sites of proteins [START_REF] Fanning | Molecular cartography of globular proteins with application to antigenic sites[END_REF] [16] [START_REF] Yang | Protein surface representation and analysis by dimension reduction[END_REF] or monitor the conformational changes of proteins occurring upon ligand binding [START_REF] Koromyslova | Deciphering fine molecular details of proteins structure and function with a protein surface topography (pst) method[END_REF] or during molecular dynamics simulations [START_REF] Koehl | Automatic alignment of genus-zero surfaces[END_REF] [9] [START_REF] Todd | Foldsynth: A physics-based interactive visualisation platform for proteins and other molecular strands[END_REF].

In Udock, we use 2D maps of the molecular surface of proteins for real-time interactive analysis of their properties and interactions. These 2D maps provide a global view of the whole protein surface properties and are complementary to classical molecular surface representations. When performing interactive protein docking, the user has to identify the optimal geometry of a protein-protein complex. This complicated cognitive task relies on an accurate analysis of the structure and properties of the two proteins to be docked in order to identify the regions over the surface that would constitute the protein-protein interface. This analysis is performed using molecular visualization software that display the molecules in perspective projection.

MAPS OF PROTEIN SURFACES

Conformal Mercator With Depth Conformal Mercator Without Depth

However, when visualizing the whole structure of a protein, the perspective projection has the major drawback to never show the entirety of the structure: the backside of the protein is always hidden. To visualize this hidden part, the user needs to change his viewing position by manipulating the virtual camera. The backside of the molecule will then become visible, but at the expense of another part of the protein that will become the new hidden backside. As a result, the user has to constantly rely on memorization and manipulation when trying to analyze and integrate the information about the structure and properties of the whole protein.

To overcome this limitation, we propose to let the user switch to a spherical projection of the surface of the protein. This projection maps the entirety of the surface of the protein to a single 2D image, allowing the user to see the whole surface at one glance (fig. 1). In Udock, we use a triangulated mesh of the SES to visualize the protein. When the user switches to spherical projection, we use shaders to render a spherical projection of the SES as follows.

Vertex Shader In the vertex shader, we compute each vertex's spherical coordinates (r, θ , φ ), with the origin of the sphere at the barycenter of the protein. Then, we transform these coordinates using different map projections. Currently, the user may choose between direct polar projection, Mercator conformal projection or Gall-Peters equal-area projection. As proteins are not totally spherical objects, some rays starting from the barycenter will hit its surface multiple times. When transforming the vertex, we compute z = 1r so that in that case, the depth culling keeps the farthest surface point. We also darken the vertex colour based on its depth to add a basic depth shading (fig. 1).

Geometry Shader Some SES triangles will have one vertex with θ slightly over 0 and another vertex with θ slightly below 2π, resulting in large rasterized triangles covering a big part of the screen. In the geometry shader, we duplicate these triangles and offset each x component so that one triangle extends beyond the left of the screen and the other one extends beyond the right of the screen. These new triangles are then clipped by the pipeline.

Each type of projection allows to see the whole surface of the protein but at the cost of distortion of certain key structural features that gets stronger as we reach the edges of the map. To overcome the increased distortion problem at the edges of the map, the user can manipulate the rotation of the projected molecule. This allows to define the part of the surface that will be projected at the center of the 2D map and that will thus be less distorted.

SURFACE SMOOTHING

One way to provide users with a useful abstraction of a protein structure for molecular docking purposes is to hide the atoms that are deep inside the molecule, and thus less prone to interact with those of other proteins. The SAS and the SES provide such a representation.

In order to go further in this abstraction process, we developed an interactive surface smoothing method that will let the user define the level of detail of the surface in real-time. This allows the user to switch his focus from the whole shape of the protein to specific details of the surface (fig 2).

There exists documented ways to compute such a smoothed surface, like spherical harmonics approximation [START_REF] Duncan | Approximation and characterization of molecular surfaces[END_REF]. We currently rely on a basic three-passes approach.

Pass 1 -Direct polar projection First, we render the direct polar projection of the protein in a single pass, using the vertex shader and geometry shader described in the previous section, and store the depth buffer value in a texture. We use a direct polar projection because it corresponds to the most direct mapping between polar and texture coordinates.

Pass 2 -Depth blurring In the second pass, we render a single quad covering the whole screen, mapped with the depth texture obtained during the previous pass. We use a fragment shader to blur the depth texture by simply computing, for each fragment, the mean of the surrounding depth values, within a given radius. The size of the radius determines the strength of the smoothing. Since proteins are not totally spherical, if we use a constant radius, the fragments that are close to the center will be less smoothed than the fragments that are far from the center. The smoothing radius is thus scaled with regard to the depth of the given fragment.

No SES Smoothing

Medium SES Smoothing Heavy SES Smoothing We use these coordinates to perform a lookup in the blurred depth texture, which gives us the distance at which a ray starting from the barycenter of the molecule and going to the current vertex would hit the smoothed SES. We use this distance to move the current vertex on the smoothed SES.

It is to note that the most time consuming pass is the smoothing pass, since for each pixel, we need to compute the sum of the surrounding pixels, and thus perform many texture lookups. However, we speed up this process by computing the sum of only a portion of these pixels, by varying the step size of the sum. The resulting smoothing is less accurate but can be computed in real time. Also, it is to note that we do not need to compute pass 1 and pass 2 for each rendered frame, but only when the user changes the smoothing radius. Once the blurred depth texture is computed and stored, we only need to render the smoothed SES with pass 3.

DISCUSSION AND FUTURE WORKS

Map projections There exists many map projections that could be used in Udock. We currently let the user choose between three of them, each one having its advantages and drawbacks (fig 3). Direct polar projection distorts local shape and surface area but has the most direct mapping between polar coordinates and texture coordinates. Mercator conformal projection maintains the local shape but distorts local surface area. Indeed, as illustrated in figure 3, the atoms appear more spherical in the Mercator map (shape preserving projection) than in the other maps , but as we reach the edge of the map, the atoms are getting bigger and bigger as their sur-

Direct Polar Conformal Mercator

Equal Area Gall-Peters face area gets more and more distorted. The Gall-Peters equal-area projection maintains local surface area but distorts local shape. As illustrated in figure 3, the atoms maintain a consistent surface area, but few seem to be spherical. Conformal Mercator map seems to be the best choice for visual inspection of protein structures with interactive protein-protein docking in sight since it is mainly about local shape matching, but this matter needs to be further investigated.

Information on the map To date, the smoothed electrostatic potential of the protein surface is displayed on the map. When the colour is shaded depending on the depth of the surface, the user gets more information about the shape of the surface. In the future, we plan to display additional information such as hydrophobic potential, atom types or secondary structures. We will let the user interactively select different layers of information to be added to the projected image.

Manipulation techniques

The map projections we use in Udock suffer from distortion at their edges. To overcome this drawback, let the user chose which part of the surface will be projected This figure presents the modifications of a 2D Conformal Mercator map when applying a rotation to the acetycholinesterase protein (PDB ID 1FFS). We rotate the protein around the y axis, passing through polar coordinates (0, -π/2) and (0, +π/2). We have then rendered the 2D Conformal Mercator map without clearing the frame buffer, to display the corresponding trajectories. We can clearly see the rotation axis and non intuitive rotation of the pixels. at the image's center where the information is the less distorted. However, manipulating projections interactively is not straightforward. As illustrated in figure 4, applying a rotation to the protein before its projection often modifies the 2D map in a non intuitive fashion: the pixels of the map will often be translated in diverse directions. For instance, if the proteins rotation is applied along the axis passing through the north and south pole of the sphere, then it only offsets the longitude of each point, translating the 2D map on the x axis. This is a straightforward manipulation. But any other rotation will modify the latitude of the surface points, and the resulting map modification becomes less intuitive. Indeed, when any point of the surface lowers its latitude, the corresponding opposite surface point raises its latitude. In the 2D map, each surface point y coordinate is a function of its latitude, and thus some points will start to go up on the y axis while others will going down at the same time. Figure 4 depicts these trajectories, and we clearly can see the rotation axis, that differently modifies the two halves of the 2D map. We thus need to define an adapted interaction scheme for spherical projections.

Interactive smoothing In the current version of Udock, the shape of the SES can be smoothed interactively with the electrostatic potential of the original SES. As a result, since we move the vertices without recomputing the electrostatic potential on the smoothed surface, the displayed potential is less accurate. We plan to investigate how to smooth in real time the information displayed on the surface.

Usability One of the major next steps of this work is to evaluate the usability of both the 2D maps and the smoothed surface rendering. We postulate that it will be easier for the user to manipulate and mentally model the shape and properties of the studied proteins. To evaluate this, we propose to measure the performance of the users on interactive protein docking on reference proteins with known interaction geometry. By comparing the quality of the docking poses generated with the same amount of time, with and without the use of surface maps and surface smoothing, we may be able to determine whether these rendering techniques are really helpful to the users. We also plan to evaluate the usability of the system using subjective assessment like the System Usability Scale [START_REF] Brooke | Sus-a quick and dirty usability scale[END_REF].

CONCLUSION

In the present work, we propose to display projections of a proteins structure and surface properties in order to alleviate the cognitive and manipulatory load required to create a mental model of its possible interaction geometries with a given partner. Of course, allowing such a global view of a proteins surface and its properties comes with certain drawbacks, like shape or surface distortions and highlights a need for specific manipulation techniques.

We have also proposed to allow the user to dynamically smooth the SES of the protein, letting him choose his desired level of detail of the surface. We have described a very simple algorithm that allows to create and display such a smoothed surface in real time.

The focus of this research is to enhance the usability of molecular visualization systems. The presented manipulation and visualization modes are designed to make a step towards this goal. Besides pushing further the abstraction of the representation of protein surfaces and of their properties, the next step of this research is to experimentally validate the impact of such techniques in terms of usability.

Figure 1 :

 1 Figure 1: Acetylcholinesterase (PDB ID: 1FFS) Conformal Mercator Maps

Figure 2 :

 2 Figure 2: Barnase (left -PDB ID: 1BRS, chain A)/ Barstar (right -PDB ID: 1BRS, chain D) Solvent Excluded Surface at different smoothing levels

Figure 3 :

 3 Figure 3: Barnase (PDB ID: 1BRS, chain A), Different Spherical Projections

Figure 4 :

 4 Figure 4: Manipulation issue of Spherical Projections


L Freitas Dutra

S Chafei

A D Tran Le

T Langlet

Mechanical behavior of a cementitious composite incorporating flax fibers treated with citric acid

Keywords: Cementitious composite, flax fibers, bio-based materials, mechanical performance

The building and construction sector is one of the sectors most concerned by the challenges of sustainable development. Indeed, building (construction) represents about 40% of CO2 emissions in developed countries, 37% of energy consumption and 40% of waste produced. In this perspective, the development of building materials and devices that emit less CO2, consume less non-renewable resources and are as widely recyclable as possible has been the subject of numerous studies. This work relates to the treatment of flax fibers with citric acid in order to improve the behavior of a cementitious mortar. The effect of the treatment on the kinetics and the water absorption rate of the fibers was first evaluated, then its influence on the properties of the fibrous mortar was studied. The treatment reduced the setting time of the cementitious mixtures and significantly increased the flexural strength compared to the raw fiber composites. SEM and wettability test confirmed the treatment efficiency to reduce the hydrophilic nature of the treated fibers.

Introduction

The use of flax fiber has a very ancient origin for the creation of objects and clothing, but today there is a renewed interest in this material that has developed due to its good mechanical properties. This aspect opens new frontiers for the use of flax fiber as a low density, ecological and cost-effective material that can be used as reinforcements in composite materials, improving the mechanical properties and also the life cycle of the final product. However, attempts to incorporate plant fibers into cementitious materials have yielded mixed results because they are accompanied by a decrease in the workability of the mixture, a delay in setting and hardening, and a reduction in mechanical performance after 28 days. These shortcomings reflect the physicochemical incompatibility between the cementitious matrix and the plant fibers, which motivates many works aiming at stripping the fibers by chemical or physical treatments, or covering them with a protective coating with respect to the cementitious medium [START_REF] Vo | Treatments of plant biomass for cementitious building materials -A review[END_REF].

The adaptation of the reinforcement/cement matrix couple was also sought by modifying the cement matrix [START_REF] Vo | Treatments of plant biomass for cementitious building materials -A review[END_REF][START_REF] Chafei | Influence de différents traitements sur les comportements rhéologique et mécanique d'un composite cimentaire mortier-fibres de lin[END_REF][START_REF] Ramli | Strength and durability of coconut-fiber-reinforced concrete in aggressive environments[END_REF].

A chemical treatment of flax fibers with a citric acid solution is proposed in order to partially clean the fiber from its surface impurities. Its impact was evaluated on the properties of the fibers and those of the cementitious composite in the fresh and hardened states.

Flax fibers

Fibers treatment

Flax was produced in Normandy, France in 2018. The physical and mechanical properties of the raw fibers are given in Table 1. Fibers of length 24 mm were used in the preparation of the mixtures. Citric acid powder was used for the preparation of the solution for the treatment of the fibers. The fibers were immersed in a 5% citric acid solution for 2 hours, rinsed with tap water and air dried for 24 hours.

Kinetics and the rate of saturation in water

The water uptake kinetics of FB and FA fibers were followed up to 24 h. The test consists of immersing 5 samples of 1 g of each type of fiber in water using tea tongs for 5, 20, 30 minutes and 24 h. The fibers are then superficially wiped with absorbent paper and their masses are then measured. The water absorption rate at these times is calculated with the equation 1.

(1)

Figure 1 shows the water absorption kinetics of raw and treated fibers. It can be seen that raw fibers can absorb up to 133% of their dry mass. Their saturation is reached after 20 minutes of immersion. Following the citric acid treatment, a considerable decrease of 41% of the water saturation rate of the treated fibers after 24 h of immersion is noted.

The time required for water saturation remained unchanged. This could be explained by the fact that the treatment partially cleaned the surface of the fibers by reducing the level of hydrophilic compounds. As hemicellulose and other non-cellulosic impurities are removed from the fibers after the acid treatment, it is expected that absorption decreases.

Scanning electron microscopy (SEM)

Images of untreated and treated fibers were acquired using a Quanta 200 field emission gun environmental scanning electron microscope (ESEM, FEI Company, USA) in "low vacuum" mode at the pressure of 0.5 torr and an accelerating voltage of 5kV.

Figure 2 shows SEM images of untreated (a) and treated fibers (b). It can be seen from Figure 2a that raw flax fibers are formed by elementary fibers held together. They are mainly composed by wax, pectin, lignin, and hemicellulose. After treatment with citric acid (Figure 2b), in addition to the modification of the fiber surface, it can be seen the individualization of the fibers which indicates that substances related to cellulose were removed. This is congruous with results presented by [START_REF] Lazorenko | Effect of pre-treatment of flax tows on mechanical properties and microstructure of natural fiber reinforced geopolymer composites[END_REF] who used alkaline treatment on flax fibers.

Contact angle

Contact angle can be used to characterize the wettability of materials: the smaller the contact angle between a solid and a liquid, the more the liquid wets the solid, which reflects a certain affinity between the two elements. Thus, an angle between 0° and 90° characterizes a hydrophilic surface. Conversely, an angle between 90° and 180° means that the surface is hydrophobic.

The contact angle between the fiber and a water has been performed by using a drop shape analyzer. A water droplet was deposited on the fiber with a syringe. The profile of the droplet was filmed against the light and the contact angle between the fiber and the droplet can be measured using an image processing software.

Images of testing samples untreated and treated can be found in Figure 3. From these results, it can be concluded that the use of citric acid significantly increased the contact angle (from 44° to 90°) providing a more hydrophobic fiber surface. These findings corroborate the decrease in water absorption described in the previous sections.

Preparation of mortars

The binder used is a portland cement CEM I 52,5 N in compliance with the NF EN 196-1 standard [START_REF]Méthodes d'essais des ciments -Partie 1 : détermination des résistances[END_REF]. The sand is a 0/4 compliant with the NF P 18-545 standard [START_REF][END_REF].

The flax was produced in Normandy in 2018. Three formulations were made: mortar incorporating raw fibers, mortar incorporating treated fibers and control mortar, called MFB, MFA and MT respectively. Twelve 4×4×16 cm3 specimens were prepared for each formulation for mechanical tests at 7 and 28 days. They were demolded after 48 hours and stored in a storage room at 20°C and 50% humidity until the mechanical tests.

Setting time

The setting time was measured with a Vicat device according to EN 196-3 [START_REF]Méthodes d'essai des ciments -Partie 3: Détermination du temps de prise et de la stabilité[END_REF].

The presence of fibers in the mortar hinders the penetration of the Vicat needle which could lead to incorrect results. Therefore, another protocol was followed:  The raw or treated fibers were first immersed in water for 48 hours with the same fiber/water ratio of the mortar formulations;  Then, the water containing the FB and FA extractables was recovered and used to make cementitious pastes called PFB and PFA respectively;  A control cementitious paste (CP) was also prepared with tap prepared with tap water.

Mechanical tests

The three-point bending test was carried out on the mortar specimens at 7 and 28 days of curing in accordance with NF EN 196-1 [START_REF]Méthodes d'essais des ciments -Partie 1 : détermination des résistances[END_REF], using an electromechanical press of 50 kN capacity operating at a speed of 50 N/s.

The compression test was performed with another press of higher capacity. The loading speed was 2400 N/s.

3

Results and discussion

Setting time

The setting times of cementitious pastes made with fiber extractables (PCFB and PCFA) or with tap water (PC) are shown in Figure 4. For the PC paste, it can be noted a time of about 2.5 hours. A significant delay of about 60% was noted for PCFB compared to PC. Indeed, the non-cellulosic polysaccharides present on the surface of the fibers, in particular the pectins, are known for their role as chelators of calcium [START_REF] Morvan | Building flax fibres: more than one brick in the walls[END_REF], an ion necessary for the advancement of the cement hydration reaction [START_REF] Khadraoui | Dependence of the properties of cementitious composites on the nature of the hydraulic binder coating the reinforcing flax fibers[END_REF]; this could explain the delay noted. For the PCFA paste, there is a decrease in the delay of about 24% compared to PCFB. This also confirms the partial elimination of the polysaccharide treatment responsible for this delay.

Mechanical tests

Figure 5 shows the three-point bending behavior of the cementitious composites and the control mortar. Comparing the curves of the composite incorporating raw fibers MFB and that of the control mortar MT, it can be noted very different behaviors. In the case of MT, the behavioral domain of the material ends with an abrupt failure which corresponds to the abrupt propagation the first matrix crack. In the case of MFB, the first matrix crack (point (δP1, P1)) appears for the same value of displacement as MT, the force and displacement continue to grow after this first damage until a maximum load value P2 (associated displacement δP2). The range of the curve between δP1 and δP2 is the range of extension of the damage by microcracks in the matrix and the resumption of the loading by the fibers.

From the point (δP2, P2), the cracks propagate in the composite in a controlled manner due to the presence of the fibers bridging the two lips of the macrocrack (if the loading rate is slow enough). A somewhat different behavior is observed for MFA: the post-pick phase is much larger although the maximum load is similar to that of MFB. The larger deformation is explained by a better load transfer from the matrix to the fibers due to a better adhesion between the fibers and the matrix The bending stresses obtained for these different formulations are presented in Erreur ! Source du renvoi introuvable.. A considerable increase was noted for MFB in comparison with the control mortar MT. It is more important at 7 days (about 29%) than at 28 days (10.5%).

The treatment of the fibers with citric acid resulted in even greater increases compared to MT which could be due to a better surface quality of these fibers leading to a good fiber-matrix adhesion as early as the seventh day of curing.

Conclusion

Pure cementitious matrices that exhibit brittle behavior change to more ductile behavior when added with plant fibers. In this work, the influence of a treatment based on citric acid on the mechanical properties of a mortar incorporating these fibers was studied.

The treatment significantly reduced the water saturation rate of the fibers as well as the setting time of the cementitious mixtures. SEM and wettability test confirmed that the treatment was effective in changing the surface of the fiber from hydrophilic to hydrophobic. Mortar with treated fibers (MFA) exhibited better flexural and compressive strengths compared to the mortar with untreated fibers (MFB). Therefore, the surface modification of plant fibers could improve the composite in complementary ways: increasing their mechanical properties, improving the adhesion of the fibers to the matrix to optimize the resistance of the fiber/matrix transition zone and finally limiting the problems related to the high water absorption of the fibers.

Considering that this treatment slows down the degradation of the fibers within the composite without stopping it, other ways to improve the durability of these plant fiber-reinforced cementitious composites are being explored.

Figure 1 :

 1 Figure 1: Water absorption kinetics of raw and treated flax fibers

Figure 2 :

 2 Figure 2 : Scanning electron microscopy images of flax fiber bundles untreated (A) and treated with citric acid (B). The magnification is 500× on the left column and 700× and the right column.

Figure 3 :

 3 Figure 3: Contact angle for raw (A) and treated flax fiber (B). 2.5 Preparation of mortars

Figure 4 :

 4 Figure 4: Setting time of cementitious materials.

Figure 5 :

 5 Figure 5: Three-point bending behaviour of the different formulations.

Figure 6 :

 6 Figure 6: Flexural stresses of mortars

Table 1 : Physical and mechanical properties of flax fibers

 1

	Diameter (µm)	22,75 ± 6
	Density	1,52
	Thermal conductivity (W.m-1.K-1)	0,041
	Modulus of elasticity (GPa)	44 ± 21
	Tensile strength (MPa)	849 ± 482
	Ultimate deformation (%)	1,78 ± 0,6

Acknowledgements

Authors thank Michael Lejeune from LPMC at University of Picardy Jules Verne for contact angle measurements and Arash Jamali at the Platform of Electron Microscopy of the University of Picardy Jules Verne for providing facilities and assisting in the SEM Analysis.


Zahen Malla Osman
email: zahen.malla_osman@cnam.fr

Jérôme Dupire
email: dupire@cnam.fr

X Sign Language (xSL) Forum: Considering Deafness as a Language Rather Than an Impairment

Keywords: Deaf, Internet forum, online, video, communication, interaction, sign language. I

In this paper, we describe a video based, text free, online forum called xSL Forum. The aim of this work is to provide a tool for signing deaf people, allowing them to communicate using sign language(s). In contrast to the widely used, real time, video chat system, xSL Forum does not require people to be present at the same moment, facing their webcam, in order to communicate. Beyond a digital library, xSL Forum can be used as an asynchronous communication tool and thus, can be useful for variety of other applications such as entertainment, education, or administration.

INTRODUCTION

Hearing loss, as a disability, is often misunderstood by the hearing world. It is often an "invisible" disability: a hearing aid may be the only visible indication. Furthermore, many hearing people do not realize that deaf people often have difficulty learning spoken and written languages, as the ability to listen to the sounds of a language is an important part of learning how to read and write. Indeed, in France, for example, an estimated 80% of deaf people are illiterate [START_REF] Gillot | The rights of deafs[END_REF].

In our daily lives, communication, when it is not oral, is mainly based on the written modality: newspapers, public displays, and television. All of which require the ability of reading (Figure 1). The captioning of movies, television shows and news broadcasts is an important advance, but is not useful for all deaf people. Professional environments have the same characteristics, nor is the World Wide Web an exception to this rule: text dominates on the internet and people cannot reasonably be expected to navigate it efficiently without mastering a written language.

A useful means of increasing the accessibility of textual content for deaf people is to propose a signed alternative in the form of a video. As far as we know, this solution is neither automatic nor systematic. If, technically, this is quite achievable, we still find very few sites that deploy this solution. Where it does existing, it is mostly found on specialized sites, initially intended for deaf people. The main obstacle to this alternative modality is the production cost.

So much so that even the dedicated websites or the sites which include deaf people in their audience offer this signed alternative only for their menu options, and switch to text mode (read and/or write) for all or part of their content, including dynamic content (Figure 2). When not in a native sign language form, the contents of the site are translated into a sign language according to the capacity of the structure and the constraints of the production schedule and budget.

Traditionally, websites have these common characteristics: (1) a high dynamicity of contents, and (2) an extensive use of text, which poses a problem for its appropriation by deaf users. These characteristics are also shared by online forums.

This paper is organized as follows: Section II presents motivations behind this work. Section III presents related work. Section IV describes the x Sign Language (xSL) Forum, including the software and hardware architecture, the modes of navigation and interaction, and the evaluations. Section V provides an outlook for future work. Section VI summarizes and concludes our paper.

II. MOTIVATIONS

The online forum has become a major communication tool in the landscape of the modern World Wide Web. It offers many advantages; such as allowing the participation of a wide community in a single conversation, whether for sharing information or seeking a solution; communicating through a forum allows people to instantly broadcast to all users.

Figure 1. Example of a general forum intended for the deaf community [START_REF]Deaf4Life forum[END_REF] Asynchronous communication dispenses with the need to be available simultaneously in order to communicate, by passing the constraints of time zone difference in communications between international communities. Finally, the online forum is, by virtue of the persistence of its contents, a perennial source of data.

Although the model of the online forum has proved its value and purely textual access, it is still very difficult to deaf users to benefit from it.

We decided to design a tool that will give access to deaf people to such an online forum, without the difficulties posed by their lack of knowledge of written languages.

III. RELATED WORK

Over the previous decade, the technology sector has proposed ideas towards facilitating communication between deaf and hearing people.

Video chat systems, such as the free software ooVoo, Skype or Live Messenger, allow signing deaf users to communicate remotely, one-to-one or one-to-many, using their webcams. This system compensates for phone use in oral synchronous mode but has the same weaknesses, namely: (1) the need for the participants to be available at the same time; (2) the need to own the equipment, i.e., a computer equipped with a webcam; and (3) a lack of a permanent record of the information exchanged during the communication.

From this starting point, many research projects studied how real-time video systems may be implemented, evaluated them in different contexts and looked at how it such a system could become a useful tool for deaf people.

The Mak-Messenger project [START_REF] Ohene-Djan | Mak-Messenger and Finger-Chat, Communications Technonlogies to Assist in the Teaching of Signed Languages to the Deaf and Hearing[END_REF] has been deployed in the field of education to help deaf students learn sign language. This application allows for the broadcasting of messages in the form of signs between users using an interface similar to a conventional chat. This technology can be used to address the educational needs of deaf people.

The Mobile American Sign Language (ASL) project [START_REF] Cavender | MobileASL : Intelligibility of sign language video as constrained by mobile phone technology[END_REF] studied the limits of wireless video communications using mobile phones. This was done to assess the extent of the constraints imposed by mobile devices in terms of size and video quality.

The Learning Management System (LMS) [START_REF] Drigas | An elearning management system for the deaf people[END_REF] uses video in Greek Sign Language to translate any text in a learning environment. This system is designed specifically for deaf people who want to improve the mastery of a language. It offers a bilingual interface (video and text) and a real-time video chat system.

All of these projects use video for synchronous communication (in real time) between users and do not allow any communication in delayed time.

Researchers at the University of Washington created a project for enabling American Sign Language to flourish in Science, Technology, Engineering, and Management (ASL-STEM forum) [START_REF] Cavender | Asl-stem forum: enabling sign language to grow through online collaboration[END_REF]. This project provides a space for exchange, for the deaf to refine their comprehension of concepts used in science courses at university and provide a translation into American SL. This forum offers a mixed interface, text, and video; which is consistent with the context of use (higher education).

The ASL STEM Forum is primarily intended for the use of tertiary level students, which implies that the users' mastery of reading and writing is high. This is the only project that includes asynchronous video communication between users. However, it is not designed to be general purpose, nor is it adaptable to another field of interest, like gardening or sports. Moreover, it is designed for a group that has already mastered a written language and wants to discuss the relationship between that and a sign language.

xSL Forum is designed to fill this gap, allowing signing users to discuss a wide range of topics in their native language (sign language), without the need to engage in reading or writing.

IV. XSL FORUM

The main motivation of this work is to offer the whole signing community a communication tool that is usable, accessible, efficient, and up-to-date; one that provides the same easy-of-use and functionality as text-based, online forums. All its content has to be accessible for signing people, especially if they do not know a written language.

"xSL" stands for "x Sign Language". The "x" means that the tool is not linked to a particular SL (Sign Language). It can be used with French SL (FSL), American SL (ASL), British SL (BSL), etc. Thus, all signing people, without any consideration for nationality or location, can use it without any modification for its functionalities.

Most computer applications nowadays are localized: they benefit from translation and some possible adjustments depending on the culture and the language mastered by potential users in a particular part of the world.

Our approach is to consider developing an application originally designed for signing people. No written text is presented on the pages of the forum and we do not ask the user to enter any kind of text when using the xSL forum.

Forum access is possible even if the signing person does not know how to read or write. Knowledge of sign language (French or otherwise) is the only requirement to use this tool.

The xSL forum is an asynchronous tool. Users simply record a message in video form (a question, for example), and then come back later to see the answer(s). Beyond the constraints mentioned earlier, this feature can become a major advantage in certain contexts, e.g., to compensate for a large time difference between two interlocutors (a French one and a Japanese one).

xSL Forum can also be seen as a digital library with a capacity for long-term memory. Therefore, we can consider its use in specific contexts, such as: education (online courses), or administration (explanation of procedures, individual experiences, frequently asked questions, etc.).

A. Software and Hardware Architecture

The hardware architecture is standard for the World Wide Web landscape. It is a server (64bits PC with an Intel Xeon, 2GB of RAM and a 500 GB hard drive), on which are installed: an Apache server with PHP and a MySQL database (Figures 3 and4). The software architecture consists of three principal parts:

• Flash Media Server

The video streams are operated by a software component installed on the server side, Adobe Flash Media Server (FMS) [START_REF]Flash Media Server[END_REF]. This software supports streaming and recording video streams within web pages on the forum. On the client side, a small module coded in Flash Action Script 3 ensures the communication with FMS. Although Flash is not free and make us dependant on Adobe, we chose to use this technology to the extent that, at the time of development, it was the only solution available that allowed us to test the client behavior, independently of the browser used. Furthermore, many websites are using this technology and many users have already installed the Flash plug-in on their browser. It is a form of standardization in the management of video streams. FMS is not free in its standard version, but a free version for developers is available. It is merely limited in number of simultaneous connections from clients. Although the inclusion of FMS in the global architecture was quite complex, this choice allows the administrator/owner of the forum to keep a hand on the video content, in order to manage, for instance, privacy and restricted access to it. It seems that this would not have been fully achievable if we had decided to use another technology such as the YouTube API.

• Flux BB

The software architecture of the website is derived from FluxBB [START_REF] Flux | [END_REF]. It is an open source forum (GNU) developed in PHP, that we modified and added to. Common features have been preserved but all interfaces (GUIs) have been adapted to the specificities of video. The text input interface used for posting messages was replaced by a video recording interface (Figure 5).

• FFmpeg This is a library for manipulating video [10], which offers, for example, the ability to convert videos from one format to another. We use FFmpeg to dynamically extract images from videos that are posted on the forum. These images are used as thumbnails, which will be displayed as visual tips to recognize the messages of different users. We decided to extract the 10th picture of a movie and use it as a thumbnail to avoid the issues encountered if an earlier picture was chosen (blank or black ones, due to the streaming process).

B. Navigation and Interaction

One of the challenges of this development task was to find solutions for keeping interaction fluid, including exploration of the site contents and ease the manipulation and playback of videos. Interaction is exclusively via the mouse; the keyboard is never required. This feature will make its use on mobile platforms much less restrictive (no need of any keyboard) and more appropriate. The hierarchy of the xSL Forum consists of three levels: categories, discussions and messages. The home page shows the categories hosted within the xSL Forum. By clicking on the thumbnail of one of these categories, users can access the various discussions (Figure 6). Similarly, by clicking on a thumbnail for a discussion, users can view the various messages exchanged in this discussion (Figure 7).

The pages on xSL forum are designed in such a way as to preserve simplicity while being visually pleasing and intuitive at the same time. Several design choices were implemented to meet these criteria.

First, we systematically combine two images to describe a given piece of content. For example, on a category page (Figure 6), the "category" tab in the top-left consists of an icon and a thumbnail, which reflects the title of the category. This combination allows us to offer two formats, one static and one dynamic, to take into account varying levels of navigation expertise. When users mouse over the thumbnail, the corresponding explanation video is played.

We also implemented a color code to complete the specificity of each level of the hierarchy of the forum: categories are green, topics are orange, and discussion messages are yellow. This encoding can also be retrieved via the tabs presented in the top left of the interface, which effectively completes the double (icon/thumbnail) presentation described above. These tabs allow the user to locate him/her-self through the existing topics and categories. A video (from a topic or a message) is played, when the cursor is placed over its thumbnail. To achieve this, our tool calls a script that executes a JavaScript code into a PHP file, using AJAX asynchronous mechanisms. This code implements a PHP/Flash playback that connects with the FMS to obtain the requested video and broadcast it (Figure 4). The requested video then appears next to the thumbnail in a window size of 200×160 pixels (Figure 8). This size was considered the most efficient as a result of an experiment to evaluate different sizes of videos on a web page by deaf signing users. The study was conducted with seven deaf users. They were asked to access a webpage with a 15'' laptop and choose which sizes of video frames should be considered wide enough to understand the signed message.

The available sizes ranged from 75×60 to 200×160 pixels (Figure 9). The comprehension of the message was good or acceptable on the three widest video frames, ranging from 125×100 to 200×160 pixels. However, all users reported that the widest one (200×160 pixels) was the most comfortable. If the user moves the cursor off the thumbnail, the video will stop and the window of the player will disappear (Figure 10). • Recording videos (posting messages) Our current version allows users to create threads or leave messages, while the administrator controls the creation of new categories. If a user wishes to create a discussion or leave a message, he/she has to click on the icon on the bottom right of the screen (Figures 6 and7). A new screen appears that implements a video recording interface (made with Flash Action Script 3) (Figure 5), that includes four features: record, stop, play, and validate. The recorder can record Flash videos using the webcam of the user's computer, in conjunction with the FMS server-side module. Once the video is validated, a PHP script is called. FFmpeg will extract a thumbnail, and then the server saves the extracted image and video to the database (Figure 3).

C. Evaluations

Beyond the experiments outlined above, we offered the use of the xSL forum to several deaf people.

Our first observation was that this type of tool (an online discussion forum) is not very well known by among the deaf community. This is consistent with our discussion in the introduction, where we assessed the typical form of online (textual) forums as having a low level of accessibility for this population. Therefore, their use and operation remain unclear.

However, after explaining the operation and purposes, all those contacted expressed great enthusiasm, and considered the many contexts of use of the xSL Forum, its use in the educational context was the most frequently mentioned.

We are about to start a formal evaluation of the xSL Forum. The aim is to evaluate the human-computer interfaces and ergonomics of the navigation and interaction. For this, we are preparing an experiment that consists of showing part of a movie to a set of deaf people and allowing them to discuss it remotely using the xSL Forum. We will prepare a variety of topics for this purpose. After a period of time has passed, questionnaires will be given to the users in order to elicit their feelings, criticisms, and suggestions regarding the tool.

V. FUTURE WORK

A new version of the xSL forum is under development. We are working on the following improvements:

Users should be able to access their account securely, so we are looking into authentication tools based on face recognition, such as [START_REF]Pam Face Authentication[END_REF]. The integration of these tools will help us meet our initial constraints, i.e. not to have to handle text.

We are also replacing FMS with the RED5 media server [START_REF]RED5 Media Server[END_REF], a royalty free audio/video streaming engine. We are also considering the integration of video using HTML5 on the client side, but its implementation in the browser is still limited. This will provide a free forum independent from proprietary technologies; the ultimate being to provide a tool that is open source and royalty free.

We are trying to integrate help videos with interactive icons, to be read in the same way throughout the forum (with the mouse over action).

Video playback will also be improved by providing a "full screen" mode with control buttons (pause, play, timeline). These controls do not exist in the current version and yet they are indispensable, especially in the case of long videos.

We are planning to evaluate this tool with deaf communities. We also intend to test it with people who practice Cued Speech, as it can be used in the same way as sign language in the xSL Forum.

We are also planning to develop and deploy a version of the website for mobile platforms (phones and tablets). A specific application, IOS and Android, will ideally complete the whole set. These kinds of platforms suffers from the need for a real or virtual keyboard. When the keyboard is a physical one, it makes the device bigger; and when it is virtual, it poses serious constraints and ergonomics issues for the user. The xSL Forum is a great laboratory for experimenting with non-verbal interaction, based on visual cues, images, videos and gesture recognition. Initially linked to the deafness context, all the experiments and results could be tested and applied in a more general context, i.e. toward all users (hearing or deaf).

Another important discussion has been raised: the way search engines deal with video indexation and the use case offered by the xSL Forum.

Finally, the aim of this project is for the xSL Forum to be used to bring together different communities, including deaf, hearing-impaired, and hearing people. We, therefore, wish to collaborate with teams working on issues of recognition and automatic translation from sign language into written language and vice versa. This kind of collaboration would lead to a true bilingual access to xSL Forum and total accessibility.

VI. CONCLUSION

As far as we are aware, an online forum using videos as its main modality of communication for signing people -deaf or hearing-did not exist before the xSL Forum. It allows users to simply exchange experiences and knowledge in various fields, according to previously unexploited temporalities in video mode (asynchronous communication and message archiving).

Beyond the important contribution to the deaf community, xSL Forum allows us to devise and propose non-textual interactions for situations in which text is not or is no longer be available. This forum can be used in contexts in which the modality or text is not very usable (mobile devices, embedded, no keyboard, etc.).

Figure 2 .

 2 Figure 2. Example of a website with alternative videos [3]

Figure 3 .

 3 Figure 3. Recording a video (posting a message)

Figure 4 .

 4 Figure 4. Playing a video (reading a message)

Figure 5 .

 5 Figure 5. Video record interface

Figure 6 .

 6 Figure 6. Different discussions in a category

Figure 7 .

 7 Figure 7. A particular discussion • Playing videos (reading messages)A video (from a topic or a message) is played, when the cursor is placed over its thumbnail. To achieve this, our tool calls a script that executes a JavaScript code into a PHP file, using AJAX asynchronous mechanisms. This code implements a PHP/Flash playback that connects with the FMS to obtain the requested video and broadcast it (Figure4). The requested video then appears next to the thumbnail in a window size of 200×160 pixels (Figure8). This size was considered the most efficient as a result of an experiment to evaluate different sizes of videos on a web page by deaf signing users. The study was conducted with seven deaf users. They were asked to access a webpage with a 15'' laptop and choose which sizes of video frames should be considered wide enough to understand the signed message.The available sizes ranged from 75×60 to 200×160 pixels (Figure9). The comprehension of the message was good or acceptable on the three widest video frames, ranging from 125×100 to 200×160 pixels. However, all users reported that the widest one (200×160 pixels) was the most comfortable. If the user moves the cursor off the thumbnail, the video will stop and the window of the player will disappear (Figure10).

Figure 8 .Figure 9 .

 89 Figure 8. Cursor over a thumbnail: the video appears (played)

Figure 10 .

 10 Figure 10. Cursor out of thumbnails

ACHI 2015 : The Eighth International Conference on Advances in Computer-Human Interactions

ACKNOWLEDGMENT

We would like to thank Laure Giry, Katharine Neil, Lubna Odeh, and all the people who accepted to give feedback on our study.