Gerd Bruder
email: gerd.bruder@uni-wuerzburg.de

Frank Steinicke
email: frank.steinicke@uni-wuerzburg.de

Wolfgang Stuerzlinger
email: wolfgang@cse.yorku.ca

Touching the Void Revisited: Analyses of Touch Behavior On and Above Tabletop Surfaces

Keywords: Touch-sensitive systems, stereoscopic displays, 3D interaction

Recent developments in touch and display technologies made it possible to integrate touch-sensitive surfaces into stereoscopic three-dimensional (3D) displays. Although this combination provides a compelling user experience, interaction with stereoscopically displayed objects poses some fundamental challenges. If a user aims to select a 3D object, each eye sees a different perspective of the same scene. This results in two distinct projections on the display surface, which raises the question where users would touch in 3D or on the two-dimensional (2D) surface to indicate the selection. In this paper we analyze the relation between the 3D positions of stereoscopically displayed objects and the on-as well as off-surface touch areas. The results show that 2D touch interaction works better close to the screen but also that 3D interaction is more suitable beyond 10cm from the screen. Finally, we discuss implications for the development of future touch-sensitive interfaces with stereoscopic display.

Introduction

Recent exhibitions and the entertainment market have been dominated by two different technologies: (i) (multi-)touch-sensitive surfaces and (ii) stereoscopic threedimensional (3D) displays. Interestingly, these two technologies are orthogonal, as (multi-)touch is about input, whereas 3D stereoscopic display about output. Both technologies have the potential to provide more intuitive and natural interaction with a wide range of applications, including urban planning, architectural design, collaborative tabletops, or geo-spatial applications. First commercial hardware systems have recently been launched, e.g., [START_REF] De La Riviere | ilight 3D touch: a multiview multitouch surface for 3D content visualization and viewpoint sharing[END_REF], and interdisciplinary research projects explore interaction with stereoscopic content on 2D touch surfaces, e.g., [START_REF]iMUTS -Interscopic Multi-Touch Surfaces[END_REF][START_REF]InSTInCT -Touch-based interfaces for Interaction with 3D Content[END_REF]. Moreover, an increasing number of hardware solutions provide the means to sense human gestures and postures not only on surfaces, but also in 3D space, e.g., the Kinect, the ThreeGear system, or Leap Motion [START_REF]Leap Motion[END_REF]. The combination of these novel technologies provides enormous potential for a variety of new interaction concepts.

Until recently, research in the area of (multi-)touch interaction was mostly focused on monoscopically displayed data. For this, the ability to directly touch elements without additional input devices has been shown to be very appealing for novice as well as expert users. Also, passive haptics and multi-touch displays have both shown their potential to considerably improve the user experience [START_REF] Benko | Precise selection techniques for multi-touch screens[END_REF]. Touch surfaces build a consistent and pervasive illusion in perceptual and motor space that the twodimensional graphical elements on the surface can be touched. Yet, three-dimensional data limits this illusion of place and plausibility [START_REF] Slater | Place illusion and plausibility can lead to realistic behaviour in immersive virtual environments[END_REF]. Such 3D data sets are either displayed monoscopically, which has been shown to impair spatial perception and performance in common 3D tasks, or stereoscopically, which can cause objects to appear detached from the touch surface [START_REF] Mcintire | What is 3D good for? A review of human performance on stereoscopic 3D displays[END_REF][START_REF] Schöning | Bimanual interaction with interscopic multi-touch surfaces[END_REF][START_REF] Bruder | Effects of Visual Conflicts on 3D Selection Task Performance in Stereoscopic Display Environments[END_REF].

Stereoscopic display technology has been available for decades. Recently, it was revived due to the rise of 3D cinema, upcoming 3D televisions and 3D games. With stereoscopic displays, each eye sees a different perspective of the same scene through appropriate technology. This requires rendering of two distinct images on the display surface. When using stereoscopic technology to display each projection to only one eye, objects may be displayed with negative, zero, or positive parallax, corresponding to their appearance in front, at, or behind the screen. Objects with zero parallax appear attached to the projection screen and are perfectly suited for touch interaction. In contrast, it is more difficult to apply direct-touch interaction techniques to objects that appear in front of or behind the screen [START_REF] Hancock | Shallow-depth 3D interaction: design and evaluation of one-, two-and three-touch techniques[END_REF][START_REF] Pierce | Image plane interaction techniques in 3D immersive environments[END_REF][START_REF] Reisman | A screen-space formulation for 2D and 3D direct manipulation[END_REF]. In this paper we focus on the major challenge of touching objects that appear in front of the projection screen. Two methodologies can be used for touching such stereoscopic objects on a tabletop display:

1. If the touch-sensitive surface captures only direct contacts, the user has to penetrate the stereoscopically displayed object to touch the 2D surface behind it [START_REF] Valkov | Touching floating objects in projection-based virtual reality environments[END_REF][START_REF] Valkov | 2D touching of 3D stereoscopic objects[END_REF]. 2. Alternatively, if the system can capture finger movements in front of the screen, the user may virtually "touch" the object in mid-air, i.e., in 3D space [START_REF]Leap Motion[END_REF].

Due to the discrepancy between perceptual and motor space and the missing passive haptic feedback, both approaches provide natural feedback only for objects rendered with zero parallax. This poses the questions where users "touch" a stereoscopically displayed object in 3D space. Here, one issue is the well-documented issue of misperception of distances in virtual 3D scenes [START_REF] Loomis | Visual perception of egocentric distance in real and virtual environments[END_REF]. Another problem arises from potential touch locations on the 2D display surface, as there are two distinct projec-Fig. 1. Illustration of the main problem of touch interaction with stereoscopically displayed 3D data: (left) the user is either focused on her finger, which makes the selection ambiguous, or (right) on the object, which disturbs the visual perception of the finger. tions, one for each eye. If the user penetrates the object while focusing on her finger, the stereoscopic impression of the object is disturbed, since the user's eyes are not accommodated and converged to the projection screen's surface anymore. Thus, the left and right stereoscopic images of the object's projection appear blurred and can usually not be merged as illustrated in Figure 1 (left). However, focusing on the virtual object causes a disturbance of the stereoscopic perception of the user's finger, since her eyes are converged on the object's 3D position, see Figure 1 (right). If a 3D tracking system is used, the user can see a stereoscopic image while converging her eyes to her finger. Yet, due the vergence-accommodation conflict [START_REF] Bruder | Effects of Visual Conflicts on 3D Selection Task Performance in Stereoscopic Display Environments[END_REF][START_REF] Chan | Touching the void: Direct-touch interaction for intangible displays[END_REF], the virtual object will appear blurred in comparison to the real finger (see Figure 2).

In this paper we address the challenge of how to interact with stereoscopic content in front of a touch-sensitive tabletop surface. Towards this, we also analyze touch behavior when touch sensing is constrained to the 2D screen surface. In order to allow the user to select arbitrary objects, a certain area of the touch surface, which we refer to as on-surface target, must be assigned to each object. In the monoscopic case the mapping between an on-surface touch area and the intended object point in the virtual scene is straightforward. Yet, with stereoscopic projection this mapping is more problematical. In particular, since there are different projections for each eye, the question arises where users touch the surface when they try to "touch" a stereoscopic object. In principle, the user may touch anywhere on the surface to select a stereoscopically displayed object. However, according to our previous work [START_REF] Valkov | 2D touching of 3D stereoscopic objects[END_REF], the most likely alternatives that users try to touch are the (see Figure 4):

 midpoint (M) between the projections for both eyes,  projection for the dominant eye (D), or  projection for the non-dominant eye (N).

A precise approach to this mapping is important to ensure efficient interaction and correct selections, in particular in a densely populated virtual scene. First, we determine a precise on-display target area where users touch the screen to select a 3D object. Second, we compare this approach with systems where the user's finger can be tracked in 3D space, and where users virtually touch objects in mid-air 3D space. The results of this experiment provide guidelines for the choice of touch technologies, as Fig. 2. Illustration of the main problem of 3D mid-air "touch" interaction with stereoscopically displayed 3D data: If a 3D tracking system is used, the user can see a stereoscopic image while converging to her finger. Due to the vegence-accommodation conflict, the virtual object appears blurred in comparison to the finger [START_REF] Bruder | Effects of Visual Conflicts on 3D Selection Task Performance in Stereoscopic Display Environments[END_REF].

well as the optimal placement and parallax of interactive elements in stereoscopic touch environments.

In summary, our contributions are:

 An analysis of on-display touch areas for 3D target objects in stereoscopic touchsensitive tabletop setups.  A direct comparison of 2D touch and 3D mid-air selection precision.  Guidelines for designing user interfaces for stereoscopic touch-sensitive tabletops.

The remainder of this paper is structured as follows. Section 2 summarizes related work in touch interaction and stereoscopic display. Section 3 describes the experiments we conducted to identify 2D/3D touch behavior. Section 4 presents the results, which are discussed in Section 5. An example application using the derived guidelines is described in Section 6. Section 7 concludes the paper.

Background

Recently, many approaches for extending multi-touch interaction techniques to 3D applications with monoscopic display have been proposed [START_REF] Hancock | Shallow-depth 3D interaction: design and evaluation of one-, two-and three-touch techniques[END_REF][START_REF] Martinet | The design and evaluation of 3D positioning techniques for multi-touch displays[END_REF][START_REF] Pyryeskin | Investigating selection above a multi-touch surface[END_REF][START_REF] Reisman | A screen-space formulation for 2D and 3D direct manipulation[END_REF]. For instance, Hancock et al. [START_REF] Hancock | Shallow-depth 3D interaction: design and evaluation of one-, two-and three-touch techniques[END_REF] presented the concept of shallow-depth 3D, i.e., 3D interaction within a limited range, to extend interaction possibilities with digital 2D surfaces. However, direct touch interaction with stereoscopically displayed scenes introduces new challenges [START_REF] Schöning | Bimanual interaction with interscopic multi-touch surfaces[END_REF], since the displayed objects can float in front of or behind the interactive display surface. Müller-Tomfelde et al. presented anaglyph-or passive polarization-based stereoscopic visualization combined with FTIR-based touch detection on a multi-touch enabled wall [START_REF] Müller-Tomfelde | Building interactive multi-touch surfaces[END_REF], and discussed approaches based on mobile devices for addressing the formulated parallax problems. The parallax problem described in the introduction is known from the two-dimensional representation of the mouse cursor within a stereoscopic image [START_REF] Schöning | Bimanual interaction with interscopic multi-touch surfaces[END_REF]. While the mouse cursor can be displayed stereoscopically on top of objects [START_REF] Schöning | Bimanual interaction with interscopic multi-touch surfaces[END_REF] or monoscopically only for the dominant eye [START_REF] Teather | Pointing at 3D Target Projections with One-Eyed and Stereo Cursors[END_REF], movements of real objects in the physical space, e.g., the user's hands, cannot be constrained such that they appear only on top of virtual objects. Grossman and Wigdor [START_REF] Ferwerda | Psychophysics 101: How to run perception experiments in computer graphics[END_REF] provided an extensive review of the existing work on interactive surfaces and developed a taxonomy for this research. This framework takes the perceived and actual display space, the input space and the physical properties of an interactive surface into account. As shown in their work, 3D volumetric visualizations are rarely considered in combination with 2D direct surface input. Even on monoscopic touch surfaces, the size of the human fingers and the lack of sensing precision can make precise touch screen interactions difficult [START_REF] Hall | Factors affecting performance using touchentry systems: Tactual recognition fields and system accuracy[END_REF][START_REF] Vogel | Shift: A technique for operating pen-based interfaces using touch[END_REF]. Some approaches have addressed this issue, for example, by providing an adjustable [START_REF] Benko | Precise selection techniques for multi-touch screens[END_REF] or fixed cursor offset [START_REF] Potter | Improving the accuracy of touch screens: An experimental evaluation of three strategies[END_REF], by scaling the cursor motion [START_REF] Benko | Precise selection techniques for multi-touch screens[END_REF] or by extracting the orientation of the user's finger [START_REF] Hilliges | Interactions in the air: Adding further depth to interactive tabletops[END_REF].

Kinematics of Touch

The kinematics of point and grasp gestures and the underlying cognitive functions have been studied by many research groups [START_REF] Geniva | Attention for perception and action: task interference for action planning, but not for online control[END_REF][START_REF] Mackenzie | Threedimensional movement trajectories in Fitts' task: Implications for control[END_REF][START_REF] Whitney | The influence of visualmotion on fast reaching movements to a stationary object[END_REF]. For instance, it has been shown that total arm movement during grasping consists of two distinct component phases:

1. an initial, ballistic phase during which the user's attention is focused on the object to be grasped (or touched) and the motion is basically controlled by proprioceptive senses, and 2. a subsequent correction phase that reflects refinement and error-correction of the movement, incorporating visual feedback in order to minimize the error between the hand or finger, respectively, and the target [START_REF]ISO/DIS 9241-9 Ergonomic requirements for office work with visual display terminals (VDTs) -Part 9: Requirements for nonkeyboard input devices[END_REF].

Furthermore, MacKenzie et al. [START_REF] Kennedy | Simulator Sickness Questionnaire: an enhanced method for quantifying simulator sickness[END_REF] have investigated the real-time kinematics of limb movements in a Fitts' task and have shown that, while Fitts' law holds for the total limb-movement time, humans usually start sooner decelerating the overall motion, if the target seems to require more precision in the end phase. The changes of the kinematics and control of the reaching tasks within virtual environments have also been investigated [START_REF] Bruder | Effects of Visual Conflicts on 3D Selection Task Performance in Stereoscopic Display Environments[END_REF][START_REF] De La Riviere | ilight 3D touch: a multiview multitouch surface for 3D content visualization and viewpoint sharing[END_REF][START_REF] Reisman | A screen-space formulation for 2D and 3D direct manipulation[END_REF]. Valkov et al. [START_REF] Valkov | Touching floating objects in projection-based virtual reality environments[END_REF] showed that users are, within some range, insensitive to small misalignments between visually perceived stereoscopic positions and the sensed haptic feedback when touching a virtual object. They proposed to manipulate the stereoscopically displayed scene in such a way that the objects are moved towards the screen when the user reaches for them [START_REF] Valkov | Evaluation of depth perception for touch interaction with stereoscopic rendered objects[END_REF][START_REF] Valkov | Touching floating objects in projection-based virtual reality environments[END_REF]. However, the problem is that objects have to be shifted in space, which may lead to a disturbed perception of the virtual scene for larger manipulations.

3D Touch for 3D Objects

To enable direct "touch" selection of stereoscopically displayed 3D objects in space, 3D tracking technologies can capture a user's hand or finger motions in front of the display surface. Hilliges et al. [START_REF] Hilliges | Interactions in the air: Adding further depth to interactive tabletops[END_REF] investigated an extension of the interaction space beyond the touch surface. They tested two depth-sensing approaches to enrich multitouch interaction on a tabletop setup. Although 3D mid-air touch provides an intuitive interaction technique, touching an intangible object, i.e., touching the void [START_REF] Chan | Touching the void: Direct-touch interaction for intangible displays[END_REF], leads to potential confusion and a significant number of overshoot errors. This is due to a combination of three factors: depth perception is less accurate in virtual scenes than in the real world, see e.g., [START_REF] Sivak | Integration of visual information and motor output in reaching and grasping: the contributions of peripheral and central vision[END_REF], the introduced double vision, and also vergenceaccommodation conflicts. A few devices, such as the CyberGrasp, support haptic feedback when touching objects in space, but require extensive user instrumentation.

A similar option for direct touch interaction with stereoscopically rendered 3D objects is to separate the interactive surface from the projection screen, as proposed by Schmalstieg et al. [START_REF] Schmalstieg | Using transparent props for interaction with the virtual table[END_REF]. In their approach, the user is provided with a physical transparent prop, which can be moved in front of the object of interest. This object can then be manipulated via single-or multi-touch gestures, since it has almost zero parallax with respect to the prop.

2D Touch for 3D Objects

Recently, multi-touch devices with non-planar surfaces, such as cubic [START_REF] De La Riviere | Cubtile: a multi-touch cubic interface[END_REF] or spherical ones [START_REF] Benko | Sphere: multi-touch interactions on a spherical display[END_REF], were proposed. Other approaches are based on controlling the 3D position of a cursor through multiple touch points [START_REF] Benko | Balloon selection: A multi-finger technique for accurate lowfatigue 3D selection[END_REF][START_REF] Strothoff | Triangle cursor: Interactions with objects above the tabletop[END_REF]. These can specify 3D axes or points for indirect object manipulation. Interaction with objects with negative parallax on a multi-touch tabletop setup was addressed by Benko et al.'s balloon selection [START_REF] Benko | Balloon selection: A multi-finger technique for accurate lowfatigue 3D selection[END_REF], as well as Strothoff et al.'s triangle cursor [START_REF] Strothoff | Triangle cursor: Interactions with objects above the tabletop[END_REF], which use 2D touch gestures to specify height above the surface. Valkov et al. [START_REF] Valkov | 2D touching of 3D stereoscopic objects[END_REF] performed a user study, in which they displayed 3D objects stereoscopically either in front of or behind a large vertical projection screen. They recorded user behavior when instructed to touch the virtual 3D objects on the display surface. They identified that users tend to touch between the projections for the two eyes with an offset towards the projection for the dominant eye. However, the results suffered from a large variance between subjects. Hence, it is unclear how far these results can be applied to different setups, such as mobile screens or tabletops, where users have an easy frame of reference due to the bezel. Also, they may engage in different touch behavior due to physical support and gravity. So far, no comparative analysis exists for 2D and 3D touch interaction in stereoscopic tabletop setups. Thus, it remains unclear if 2D touch is a viable alternative to 3D mid-air touch.

Experiments

Here we describe our experiments in which we analyzed the touch behavior as well as the precision of 2D touch and 3D mid-air touches. We used a standard ISO 9241-9 selection task setup [START_REF] Kennedy | Simulator Sickness Questionnaire: an enhanced method for quantifying simulator sickness[END_REF] on a tabletop surface with 3D targets displayed at different heights above the surface, i.e., with different negative stereoscopic parallaxes.

Participants

Ten male and five female subjects (ages 20-35, M=27.1, heights 158-193cm, M=178.3cm) participated in the experiment. Subjects were students or members of the Departments of computer science, media communication or human computerinteraction. Three subjects received class credit for participating in the experiment. All subjects were right-handed. We used the Porta and Dolman tests to determine the sighting dominant eye of subjects [START_REF] Mapp | What does the dominant eye dominate? A brief and somewhat contentious review[END_REF]. This revealed eight right-eye dominant sub-Fig. 3. Experiment setup: photo of a subject during the experiment (with illustrations). As illustrated on the screen, the target objects are arranged in a circle. jects (7 males, 1 female) and five left-eye dominant subjects (2 males, 3 females). The tests were inconclusive for two subjects (1 male, 1 female), for which the 2 tests indicated conflicting eye dominance. All subjects had normal or corrected to normal vision. One subject wore glasses and four subjects wore contact lenses during the experiment. None of the subjects reported known eye disorders, such as color weaknesses, amblyopia or known stereopsis disruptions. We measured the interpupillary distance (IPD) of each subject before the experiment, which revealed IPDs between 5.8cm and 7.0cm (M=6.4cm). We used each individual's IPD for stereoscopic display in the experiment. 14 subjects reported experience with stereoscopic 3D cinema, 14 reported experience with touch screens, and 8 had previously participated in a study involving touch surfaces. Subjects were naive to the experimental conditions. Subjects were allowed to take a break at any time between experiment trials in order to minimize effects of exhaustion or lack of concentration. The total time per subject including pre-questionnaires, instructions, training, experiment, breaks, post-questionnaires, and debriefing was about 1 hour.

Materials

For the experiment we used a 62 x 112cm multi-touch enabled active stereoscopic tabletop setup as described in [START_REF] Bruder | Effects of Visual Conflicts on 3D Selection Task Performance in Stereoscopic Display Environments[END_REF]. The system is shown in Figure 3 and uses rear diffuse illumination [START_REF] Mcintire | What is 3D good for? A review of human performance on stereoscopic 3D displays[END_REF] for multi-touch. For this, six high-power infrared (IR) LEDs illuminate the screen from behind. When an object, such as a finger or palm, comes in contact with the diffuse surface it reflects the IR light, which is then sensed by a camera. We use a 1024x768 PointGrey Dragonfly2 with a wide-angle lens and a matching IR band-pass filter at 30 frames per second. We use a modified version of the NUI Group's CCV software to detect touch input on a Mac Mini server. Our setup uses a matte diffusing screen with a gain of 1.6 for the stereoscopic back projection. We used a 1280x800 Optoma GT720 projector with a wide-angle lens and an active DLPbased shutter at 60Hz per eye. We used an optical WorldViz PPT X4 system with sub-millimeter precision and sub-centimeter accuracy to track the subject's finger and head in 3D, both for 3D "touch" detection as well as view-dependent rendering. For this, we attached wireless markers to the shutter glasses and another diffused IR LED on the tip of the index finger of the subject's dominant hand. We tracked and logged both head and fingertip movements during the experiment. The visual stimulus consisted of a 30cm deep box that matches the horizontal dimensions of the tabletop setup (see Figure 3). We matched the look of the scene to the visual stimuli used by Teather and Stuerzlinger [START_REF] Teather | Pointing at 3D targets in a stereo head-tracked virtual environment[END_REF][START_REF] Teather | Pointing at 3D Target Projections with One-Eyed and Stereo Cursors[END_REF]. The targets in the experiment were represented by spheres, which were arranged in a circle as illustrated in Figure 3. A circle consisted of 11 spheres rendered in white, with the active target sphere highlighted in blue. The targets highlighted in the order specified by ISO 9241-9 [START_REF]ISO/DIS 9241-9 Ergonomic requirements for office work with visual display terminals (VDTs) -Part 9: Requirements for nonkeyboard input devices[END_REF]. The center of each target sphere indicated the exact position where subjects were instructed to touch with their dominant hand in order to select a sphere. For 3D touch this was the 3D position, and for 2D touch the center of the 2D projection. The size, distance, and height of target spheres were constant within circles, but varied between circles. Target height was measured as positive height from the level screen surface. Subjects indicated target selection using a Razer Nostromo keypad with their nondominant hand. The virtual scene was rendered on an Intel Core i7 3.40GHz computer with 8GB of main memory, and an Nvidia Quadro 4000 graphics card.

3.3

Methods

The experiment used a 2 x 5 x 2 x 2 within-subjects design with the method of constant stimuli, in which the target positions and sizes are not related from one circle to the next, but presented randomly and uniformly distributed [START_REF] Ferwerda | Psychophysics 101: How to run perception experiments in computer graphics[END_REF]. The independent variables were selection technique (2D touch vs. 3D mid-air touch), target height (between 0cm and 20cm, in steps of 5cm), as well as target distance (16cm and 25cm) and target size (2cm and 3cm). Each circle represented a different index of difficulty (ID), with combinations of 2 distances and 2 sizes. The ID indicates overall task difficulty [START_REF] Fitts | The information capacity of the human motor system in controlling the amplitude of movement[END_REF]. It implies that the smaller and farther a target, the more difficult it is to select quickly and accurately. Our design thus uses four uniformly distributed IDs ranging from approximately 2.85bps to 3.75bps, representing an ecologically valuable range of difficulties for such a touch-enabled stereoscopic tabletop setup. As dependent variables we measured the on-as well as off-display touch areas for 3D target objects.

The experiment trials were divided into two blocks: one for the 2D and one for the 3D touch technique. We randomized their order between subjects. At the beginning of each block subjects were positioned standing in an upright posture in front of the tabletop surface as illustrated in Figure 3. To improve comparability, we compensated for the different heights of the subjects by adjusting a floor mat below the subject's feet, resulting in an (approximately) uniform eye height of 1.85cm for each subject during the experiment. The experiment started with task descriptions, which were presented via slides on the tabletop surface to reduce potential experimenter bias. Subjects completed 5 to 15 training trials with both techniques to ensure that they correctly understood the task and to minimize training effects. Training trials were excluded from the analysis.

In the experiment, subjects were instructed to touch the center of the target spheres as accurately as possible (either with 2D or 3D touch), for which they had as much time as needed. For this, subjects had to position the tip of the index finger of their dominant hand inside the 3D sphere for the 3D touch condition, or push their finger through the 3D sphere until it reached the 2D touch surface. Subjects did not receive feedback whether they "hit" their target, i.e., subjects were free to place their index finger in the real world where they perceived the virtual target to be. We did this to evaluate the often-reported systematical over-or underestimation of distances in virtual scenes, which can be observed even for short grasping-range distances [START_REF] Sivak | Integration of visual information and motor output in reaching and grasping: the contributions of peripheral and central vision[END_REF], as also tested in this experiment. Moreover, we wanted to evaluate the impact of such misperceptions on touch behavior in stereoscopic tabletop setups. We tracked the tip of the index finger in both 2D and 3D touch conditions. When subjects wanted to register the selection, they had to press a button with their non-dominant hand on the keypad. We recorded a distinct 2D and 3D touch position for each target location for each configuration of independent variables, with a total of 20 circles and 220 recorded touch positions per participant.

Results

In this section we summarize the results from the 2D and 3D touch experiment. We had to exclude two subjects from the analysis who obviously misunderstood the task. We analyzed these results with a repeated measure ANOVA and Tukey multiple comparisons at the 5% significance level (with Bonferonni correction).

2D Touch

For the 2D touch technique, we evaluated the judged 2D touch points on the surface relative to the potential projected target points, i.e., the midpoint (M) between the projections for both eyes, as well as the projection for the dominant (D), and the nondominant (N) eye, as illustrated in Figure 4. Figure 5 shows scatter plots of the distribution of the touch points from all trials in relation to the projected target centers for the dominant and non-dominant eye for the different heights of 0cm, 5cm, 10cm, 15cm and 20cm (bottom to top). We normalized the touch points in such a way that the dominant eye projection D is always shown on the left, and the non-dominant eye projection N is always shown on the right side of the plot. The touch points are displayed relatively to the distance between both projections. As it is illustrated in Figures 4 and5, we observed three different behaviors when subjects used the 2D touch technique. In particular, eight subjects touched towards the midpoint, i.e., the center between the dominant and non-dominant eye projections. This includes the two subjects for whom eye dominance estimates were inconclusive. We arranged these subjects into the group G M . Furthermore, three subjects touched towards the dominant eye projection D, which we refer to as group G D , and three subjects touched towards the non-dominant eye projection N, which we refer to as group G N . This points towards an approximately 50/50% split in terms of behaviors in the population, i.e., between group G M and the composite of groups G D and G N .

We found a significant main effect of the three groups (F(2,11)=71.267, p<.001, partial-eta 2 =.928) on the on-surface touch areas. Furthermore, we found a significant two-way interaction effect of the three groups and target heights (F(8,44)=45.251, p<.001, partial-eta 2 =.892) on the on-surface touch areas. The post-hoc test revealed that the on-surface target areas, see Figure 5, significantly (p<.001) vary for objects that are displayed at heights of 15cm or higher. For objects displayed at 10cm height group G D and G N vary significantly (p<.02). For objects displayed below 10cm we could not find any significant difference. As illustrated in Figure 5, for these heights the projections for the dominant and non-dominant eye are close together, and subjects touched almost the same on-screen target areas. Considering the on-surface touch areas, we found that on average the relative touch point for group G D was 0.97D+0.03N for projection points D∊ℝ 2 and N∊ℝ 2 , meaning the subjects in this group touched towards the projection for the dominant eye, but slightly inwards to the center. The relative touch point for group G N was 0.11D+0.89N, meaning the subjects in this group touched towards the projection for the non-dominant eye, again with a slight offset towards the center. Finally, for group G M we found that on average the relative touch point for this group was 0.504D+0.596N. We could not find any significant difference for the different heights, i.e., the touch behaviors were consistent throughout the tested heights.

However, we observed a trend of target height on the standard deviations of the horizontal distributions (x-axis) of touch points for all groups as shown in Figure 5. For 0cm target height we found a mean standard deviation (SD) of 0.29cm, for 5cm SD 0.32cm, for 10cm SD 0.42cm, for 15cm SD 0.52cm, and for 20cm SD 0.61cm. For the vertical distribution (y-axis) of touch points and at 0cm target height we found a mean SD of 0.20cm, for 5cm SD 0.20cm, for 10cm SD 0.25cm, for 15cm SD 0.29cm, and for 20cm SD 0.30cm.

In summary, the results for the 2D touch technique show a significant effect for the different user groups on the on-surface touch area over the range of tested heights. These on-surface touch areas vary significantly for objects displayed at heights of 10cm and higher.

3D Touch

We analyzed the tracked physical 3D "touch" points where subjects judged the perceived center of the mid-air target spheres for the 3D touch technique in terms of their deviation from their actual position in the 3D virtual scene. Figure 6 shows scatter plots of the distribution of judged target positions in relation to the 3D target centers for the different target heights over all trials. The red dots indicate the center positions of the spheres as judged by the subjects. The black wireframe spheres illustrate the actual position and size of the objects. We normalized the judged positions relative to the optical view angle towards the target center. We found no significant difference in the judged positions for the three groups identified in Section 4.1 and pooled the data. We analyzed the effect of target height on the subjects' judgments. We found a significant main effect of target height on the distances of judged positions from the displayed target centers. Mauchly's test indicated that the assumption of sphericity had been violated for effects of height on the distances of judged positions (chi 2 (9)=62.388, p<.001), therefore degrees of freedom were corrected using Greenhouse-Geisser estimates of sphericity (epsilon=.302). The results show that the distances of judged positions significantly differs for heights (F(1.21,15.725)=12.846, p<.002, partial-eta 2 =.497).

A Tukey post-hoc test with Bonferroni correction revealed that subjects estimated the target centers significantly closer to the actual displayed target centers for the 0cm targets in comparison to targets displayed at 20cm height (p<.002). For all other heights the results suggest that the higher the targets are displayed, the larger are the deviations. Pooling over all subjects, we observed mean distances to target centers of M=0.56cm (SD=0.27cm) for 0cm target height, M=0.88cm (SD=0.53cm) for 5cm, M=0.97cm (SD=0.61cm) for 10cm, M=1.32cm (SD=0.93cm) for 15cm, and M=1.90cm (SD=1.48cm) for 20cm. The results suggest that the physical constraints provided by the touch surface at 0cm height reduced judgment errors for objects at zero parallax relative to the other heights. We found no significant difference when comparing to the results for the 2D touch technique at 0cm target height as presented in Section 4.1.

As it can be seen in Figure 6, subjects made larger errors along the view axis than along the orthogonal axes. For the mid-air target positions we found a mean standard deviation of 1.43cm along the optical line-of-sight, a mean SD of 0.36cm parallel to the touch surface, and a mean SD of 0.50cm orthogonal to the other axes. Furthermore, these deviations increased with increasing target heights. For the different target heights above the surface we observed standard deviations of judged positions along the optical line-of-sight of SD=2.20cm (for 20cm target height), SD=1.52cm (15cm), SD=1.05cm (10cm), and SD=0.94cm (5cm). On the other hand, we observed standard deviations of judged positions orthogonal to the view axis parallel to the touch surface of only SD=0.49cm (20cm), SD=0.39cm (15cm), SD=0.30cm (10cm), and SD=0.27cm (5cm). Finally, we found standard deviations of judged positions orthogonal to the other axes of only SD=0.70cm (20cm), SD=0.55cm (15cm), SD=0.41cm (10cm), and SD=0.35cm (5cm). We further analyzed the data to determine whether deviations in judged target positions result from under-or overestimation of distances from the observer to the mid-air targets [START_REF] Bruder | Effects of Visual Conflicts on 3D Selection Task Performance in Stereoscopic Display Environments[END_REF][START_REF] Chan | Touching the void: Direct-touch interaction for intangible displays[END_REF]. We observed a mean distance underestimation of 0.25% (SD=2.93%). Surprisingly, we found a distance overestimation of M=0.4% (SD=2.00%) and M=1.0% (SD=2.25%) for heights of 5cm and 10cm, respectively. Yet, we found an underestimation of M=-0.54% (SD=2.67%) and M=-0.98% (SD=4.18%) for heights of 15cm and 20cm, respectively. In summary, the results for the 3D touch technique show a significant effect of stereoscopic parallax over the range of tested heights on the precision and accuracy of judging the position of a target object.

Discussion

Our results provide interesting guidelines on how touch interaction in 3D stereoscopic tabletop setups should be realized. First of all and in contrast to previous work [START_REF] Valkov | 2D touching of 3D stereoscopic objects[END_REF], our results show evidence for a twofold diversity of 2D touch behaviors of users. As shown in Figure 5, roughly half of the subjects in our study touched through the virtual object towards the center between the projections, and the other half touched towards projections determined by a single eye. The second group roughly splits in half again depending if they touch the projection for the dominant or non-dominant eye.

Our results differ from the findings by Valkov et al. [START_REF] Valkov | 2D touching of 3D stereoscopic objects[END_REF]. Using a setup with a large vertical projection plane they observed that subjects touched towards the center projection, with a slight offset towards the dominant eye. With 3 subjects touching towards the dominant eye, and 3 subjects towards the non-dominant eye in our study, user behavior in tabletop environments cannot be explained by this model. As a guideline, we suggest that the center between the projection for the left and right eye can be used to detect selections of objects stereoscopically displayed with less than 10cm height, since we did not observe significant differences between subjects at such heights. In order to reliably detect selections for objects higher above the screen, i.e., with larger parallaxes, our results suggest that for each user a calibration would be required. Our results confirm that this approach is highly beneficial, since subjects touched consistently for all heights towards the dominant, center, or non-dominant projection.

For practical considerations and to evaluate the ecological validity of using the 2D touch technique for selections of targets at a height between 0cm and 10cm, we computed the minimal on-surface touch area that supports 95% correct detection of all 2D touch points in our experiment. Due to the similar distributions of touch points between the three behavior groups for these heights shown in Figure 5, we determined the average minimal 95% on-surface region over all participants. Our results show that an elliptical area with horizontal and vertical diameter of 1.64cm and 1.07cm with a center in the middle between the two projections is sufficient for 95% correct detection. This rule-of-thumb heuristic for on-surface target areas is easy to implement and ecologically valuable considering the fat finger problem [START_REF] Hall | Factors affecting performance using touchentry systems: Tactual recognition fields and system accuracy[END_REF][START_REF] Vogel | Shift: A technique for operating pen-based interfaces using touch[END_REF]. Due to this problem objects require a relatively large size of between 1.05cm to 2.6cm for reliable acquisition, even in monoscopic touch-enabled tabletop environments.

The results of our second experimental condition reveal that distinct differences exist between the 3D mid-air touch technique and the 2D touch technique. These differences impact the relative performance and applicability for interaction with objects displayed stereoscopically at different heights above the surface. We found no behavior groups or effects of eye dominance on the distribution of judged 3D target positions. Our results show that target height has an effect on precision and accuracy of 3D selections, with large errors mainly along the optical line-of-sight, which we believe to correlate with distance misperception. For 3D objects displayed close to the display surface up to 10cm, touching objects in 2D on the surface by touching "through" the stereoscopic impression is more accurate than 3D mid-air touching. Considering that much research has shown that 3D mid-air touches of virtual objects suffer from low accuracy and precision due to visual conflicts, including vergenceaccommodation mismatch, diplopia, and distance misperception [START_REF] Bruder | Effects of Visual Conflicts on 3D Selection Task Performance in Stereoscopic Display Environments[END_REF][START_REF] Chan | Touching the void: Direct-touch interaction for intangible displays[END_REF], it is a promising finding that the reduction of 3D selection tasks to 2D input with the 2D touch technique can improve performance for tabletop surface with stereoscopically displayed objects. However, the results also show that the accuracy for 2D touching of objects displayed above the screen decreases significantly for large negative parallax. The findings are encouraging for stereoscopic visualization on (multi-)touch surfaces. They suggest that virtual objects do not have to be constrained exactly at the zeroparallax level, but may deviate up to 10cm before 2D touch accuracy is significantly degraded [START_REF] Valkov | Touching floating objects in projection-based virtual reality environments[END_REF][START_REF] Valkov | 2D touching of 3D stereoscopic objects[END_REF]. For such distances, the 2D touch technique is a good choice and instrumenting users with gloves or 3D markers can be avoided. Overall, our results show that it is possible to leverage stereoscopic cues in tabletop setups for an improved spatial cognition.

As a guideline for future tabletop setups with direct 2D touch input, the results suggest that touch-enabled 3D objects should not be displayed above an interactive display surface at more than about 10cm height. Above that, the disadvantages outperform the benefits and 3D interaction techniques should be used in that region, as they will provide more accurate interaction possibilities.

Example Application: Stereoscopic 3D Widgets

Our experiments have shown that the 2D touch technique has enormous potential as a new interaction paradigm for stereoscopic multi-touch surfaces as long as the objects are displayed with less than 10cm above the surface. In this region our 2D touch technique is the more accurate choice. While this constraint appears to limit the application scenarios in which one could use the 2D touch technique, it also ensures a simple implementation for interaction, in particular, a clear definition of on-display target areas as described in Section 5. Moreover, the size and scale of many virtual objects used in actual tabletop applications suit this constraint. For instance, 3D widgets can be displayed stereoscopically on any multi-touch surface and provide the user with a natural haptic feedback experience when she virtually touches them.

In order to evaluate the quality of the 3D touch technique in a real-world application, we adapted a simple visualization application for virtual caravans (see Figure 7). With this application customers can evaluate various types of caravans with several different features. The 3D widgets on the menu plane allow users to change the visual appearance of the caravan, lighting parameters, turn on signals, headlamps etc. We implemented the on-surface target areas of these 3D widgets as described in Section 5. The highest widgets, i.e., the 3D buttons on the menu panel, are displayed about 10cm above the surface. We used the same physical setup as described in Section 3.2. For this application we used the Unity3D game engine for the generation and rendering of the virtual scene. Unity3D provides a simple development environment for virtual scenes, animations and interactions. In order to synchronize virtual camera objects with the movements of a user, we integrated the MiddleVR for Unity software framework. MiddleVR supports streaming of motion data from our tracking system to Unity3D using the Virtual Reality Peripheral Network (VRPN) protocol. With this we stream head poses to Unity3D, resulting in a correct perspective from the user's point of view at all times.

We presented this application to four users, and made several interesting observations. First, all users acknowledged the stereoscopic display when viewing the 3D scene. Second, most users immediately understood that the menu panel with the 3D Fig. 7. User interacting with a virtual scene in a stereoscopic multi-touch tabletop setup using touch-enabled 3D widgets. The widgets in the graphical user interface were rendered with negative parallax of up to 10cm height. widgets provides a means to interact with the setup. Surprisingly, when users tried to "touch" the 3D widgets, they adapted their actions to the affordances provides by the widget. For instance, when they pressed the toggle switch, usually they touched its lifted part, although we did not distinguish between touch positions on the surface. We see this as further indication that stereoscopic display in combination with a touch-enabled surface does indeed support the notion of 3D physical interaction elements. Finally, none of the users complained about non-reactive 3D widgets, which might have occurred if they missed the on-surface target areas. This suggests that the shape and size of the on-surface touch areas, as determined by our above study, is sufficient for using stereoscopic 3D widgets in tabletop setups.

Conclusion and Future Work

In this paper we evaluated and compared 2D touch and 3D touch interaction techniques for scenes on touch-sensitive tabletop setups with stereoscopic display. We analyzed the differences of 3D mid-air touch input and a technique based on reducing the 3D touch problem to two dimensions by having users touch "through" the stereoscopic impression of 3D objects, resulting in a 2D touch on the display surface. We identified two separate classes of user behavior, with one group that touches the center between the projections, whereas the other touches the projection for the dominant or non-dominant eye. The results of the experiment show a strong interaction effect between input technique and the stereoscopic parallax of virtual objects.

The main contributions of this work are:

 We identified two separate classes of user behavior when touching "through" stereoscopically displayed objects.  We compared precision and accuracy of 2D/3D direct touch input, which revealed that the 2D touch technique is a viable alternative to 3D touch interaction for object selection up to about 10cm height from the display surface.  We determined on-surface target regions that support a simple implementation of the 2D touch technique. This enables intuitive touch input for 3D objects and widgets in stereoscopic 3D tabletop applications.

The results are encouraging for stereoscopic visualization in future touch-enabled tabletop setups, since no additional instrumentation and tracking technology is needed for objects with a small stereoscopic parallax. An interesting question for future work is if the results can be applied to portable setups, where the orientation of the touchsensitive surface varies during interaction. We plan to further pursue these topics to provide compelling user experiences and effective user interfaces for touch-sensitive stereoscopic display surfaces. Moreover, we plan to investigate also how the 2D and 3D touch methods compare in terms of the speed-accuracy tradeoff.

Fig. 4 .

 4 Fig. 4. Illustration of finger movement trails for eye user groups touching towards the dominant eye projection (D), non-dominant eye projection (N), or towards the midpoint. The trails have been normalized and are displayed here for a right-eye dominant user.

Fig. 5 .

 5 Fig. 5. Scatter plots of relative touch points between the dominant (D) and non-dominant (N) eye projections of the projected target centers on the surface for the 2D touch technique. Black crosses indicate the two projection centers. Black circles indicate the approximate projected target areas for the dominant and non-dominant eye. Top to bottom rows show results for 20cm, 15cm, 10cm, 5cm, and 0cm target heights. The left column shows subject behavior for dominant-eye touches (3 subjects), the middle for center-eye touches (8 subjects), and the right for non-dominant-eye touches (3 subjects). Note that the distance between the projection centers depends on the target height.

Fig. 6 .

 6 Fig. 6. Scatter plots of judged positions of the 3D target centers for the 3D touch technique over all subjects. Black wireframe spheres indicate the targets. The diagonal arrow illustrates the normalized view angle. The five diagrams show results for 20cm, 15cm, 10cm, 5cm, and 0cm target heights.

Acknowledgements

This work was partly supported by grants from the Deutsche Forschungsgemeinschaft and the Natural Sciences and Engineering Research Council of Canada.


Wallis Motta
email: w.motta@ucl.ac.uk

Ava Gen Schieck

Holger Schnädelbach
email: holger.schnadelbach@nottingham.ac.uk

Efstathia Kostopoulou

Moritz Behrens
email: moritz.behrens@ucl.ac.uk

Steve North
email: steve.north@nottingham.ac.uk

Lei Ye
email: lei.ye@nottingham.ac.uk

Ava Fatah
email: ava.fatah@ucl.ac.uk

Gen Schieck

Efstathia Kostopou- Lou

Considering Communities, Diversity and the Production of Locality in the Design of Networked Urban Screens

Keywords: Diversity, communities, ethnography, workshops, in the wild, urban screens

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

Introduction

When studying the deployment of interactive screens in urban public settings [START_REF] Redhead | NNUB: the neighbourhood nub digital noticeboard system[END_REF][START_REF] Peltonen | It's Mine, Don't Touch!: interactions at a large multi-touch display in a city centre[END_REF][START_REF] Satchell | Suburban nostalgia: the community building potential of urban screens[END_REF][START_REF] Taylor | Programming video art for urban screens in public space[END_REF] it is important to consider not only their immediate use and spatial location [START_REF] Behrens | Exploring the Effect of Spatial Layout on Mediated Urban Interactions[END_REF][START_REF] O'hara | Understanding collective play in an urban screen game[END_REF][START_REF] Taylor | Programming video art for urban screens in public space[END_REF], but also their assimilation within the wider socio-cultural context of the locale. This is of great importance because the interplay between these spatial and cultural elements can strongly influence the long-term success of these technologies. One way in which this interplay can be addressed is by investigating the production of locality [START_REF] Appadurai | Modernity at Large: Cultural Dimensions of Globalization[END_REF][START_REF] Dourish | Re-space-ing place: 'place' and 'space' ten years on[END_REF][START_REF] Taylor | Social interaction around a rural community photo display[END_REF]. This refers to the processes that communities use to build a sense of place; in other words, how the communities create cultural conventions to control both space and people's behavior within them in ways that allow individuals to develop their own identities in relation to both the locale and other people [START_REF] Appadurai | Modernity at Large: Cultural Dimensions of Globalization[END_REF]. By defining who has access to what spaces, at what times, under what conditions and based upon which narratives or behaviors, a community can define itself in terms of its conventions, practices, values and aspirations, hence providing a shared social identity. Place building activities are varied, and they are as much about appropriation as they are about the negotiation and control of space when interacting with other people. For instance, Dant and Deacon observed that homeless people living in hostels emphasized control over what they were allowed to do, such as being able to make a cup of tea whenever they wished, as a way to transform an impersonal space into a place they felt they belonged to [START_REF] Dant | Hostels to homes: The rehousing of single homeless people[END_REF]. Similarly, some residents in the London neighborhoods where we worked emphasize a degree of control over certain public spaces (e.g. Library halls) to install art exhibitions for public display. In these two cases, certain practical actions (such as making tea or periodically renewing art exhibits) enable social exchanges and conviviality, which translate into a feeling of commonality that underpins the wider social structures that support the community. The analysis of place building is of particular interest in locales where there is a large heterogeneity in ethnicity, nationality, language, religion, educational level and income. In these highly diverse contexts [START_REF] Vertovec | Towards post--multiculturalism? Changing communities, conditions and contexts of diversity[END_REF] a sense of place does not arise implicitly as a simple product of long held traditions; instead, the local inhabitants must explicitly create it through active efforts of place building [START_REF] Appadurai | Modernity at Large: Cultural Dimensions of Globalization[END_REF].

The literature on urban screens has explored the crossroads between communities, public space, technical factors and social interaction around interfaces [START_REF] Fatah Gen | Research in the Wild: Exploring the potential of Networked Urban Screens for Communities and Culture[END_REF][START_REF] Memarovic | Using public displays to stimulate passive engagement, active engagement, and discovery in public spaces[END_REF][START_REF] North | Tension Space Analysis: Exploring Community Requirements for Networked Urban Screens[END_REF][START_REF] Satchell | Suburban nostalgia: the community building potential of urban screens[END_REF][START_REF] Taylor | Social interaction around a rural community photo display[END_REF]. Although some works address the social aspects of screen use, the study of how wider socio-cultural variables influence long-term community support of urban screens remains an open question. In general, there has been an emphasis on social interactions mediated through screen content on shorter timescales [START_REF] O'hara | Understanding collective play in an urban screen game[END_REF][START_REF] Peltonen | It's Mine, Don't Touch!: interactions at a large multi-touch display in a city centre[END_REF][START_REF] Taylor | Programming video art for urban screens in public space[END_REF], without the consideration of longer term, wider scope socio-cultural variables. Only very few longitudinal case studies have taken a wider contextual approach such as [START_REF] Redhead | NNUB: the neighbourhood nub digital noticeboard system[END_REF][START_REF] Taylor | Social interaction around a rural community photo display[END_REF].

The present paper argues that this wider social context matters if one wants to understand the different perspectives and behaviors surrounding long-term community support for public interactive screens. To study the ways in which such wider cultural context affects screen node use, we explore how urban, networked screens are integrated within the cultural practices of a highly diverse urban locale in London. We examine the social aspects of screen use through an analysis of place building practices, and inquire into the reasons why certain people emphasize certain uses for the screens as opposed to others. Our research takes a targeted, longitudinal approach to understand a complex range of social, technical and interactional issues [START_REF] Behrens | Exploring the Effect of Spatial Layout on Mediated Urban Interactions[END_REF][START_REF] Fatah Gen | Research in the Wild: Exploring the potential of Networked Urban Screens for Communities and Culture[END_REF][START_REF] North | Tension Space Analysis: Exploring Community Requirements for Networked Urban Screens[END_REF].

Our data gathering methodology includes ethnography and workshops in the wild. The main contribution of this work is to show how action, space and sociality mutually impinge upon one another forming a wider cultural logic, which in turn has a great influence on how a network of public interactive screens is experienced, treated, understood and embraced in practice.

The structure of the paper is as follows. First, we describe our methodology in Section 2. In Section 3 we discuss the integration of screen nodes within pre-existing place building activities. We provide further analysis and conclusions in Section 4.

Methodology

Our project involves a network of four interactive touch screens designed to encourage public participation and to explore how networked urban screens can augment urban experience to support communities and culture [START_REF] Fatah Gen | Research in the Wild: Exploring the potential of Networked Urban Screens for Communities and Culture[END_REF]. There were three main screen node applications discussed with our participants during workshops and ethnography: Slideshow, SoundShape and ScreenGram. Slideshow simply loops through a set of images. SoundShape enables people connected remotely to create a collaborative musical pattern. ScreenGram leverages common technologies (e.g. Twitter) to enable users to upload images to the screen nodes. For further description see [START_REF] North | Tension Space Analysis: Exploring Community Requirements for Networked Urban Screens[END_REF].

Diversity is a fundamental consideration of this project: two of these screens are located in London, which is home to people from ~179 countries and speaking ~300 languages [START_REF] Vertovec | Towards post--multiculturalism? Changing communities, conditions and contexts of diversity[END_REF]. Furthermore, the borough where these are located (Waltham Forest) is one of the most ethnically diverse in Britain [22]. One of our screen sites is The Mill in Walthamstow, a non-profit organization that runs a building that has become a hub for local communities; the second one, Leytonstone Public Library, fulfills a similar social role. Amongst many other uses, locals use these two sites to hold business meetings, playgroups and art exhibitions, and also to give and/or take free courses. Both The Mill and Leytonstone Library are popular public places where people organize and enact initiatives that actively build a sense of local identity (a sense of place) for both their neighborhoods and the borough itself. Both sites bring together a diverse cross-section of local residents to pursue varied social activities, and hence represent a natural sample of people present in the locality.

In order to engage with a wide range of social groups within these diverse communities, we carried out ethnography and conducted two workshops in Leytonstone and four in Walthamstow. Rather than defining our groups in terms of statistical or demographic properties, we chose to target "grassroots" groups that were already being sustained through various degrees of involvement with the locality. This enabled us to interact with people in their usual social configurations and pre-established networks of relationships. A total of 70 people from the locality participated in the workshops. These people came from a wide variety of cultural backgrounds and age brackets; with roughly equal numbers of toddlers, children, teenagers, adults and seniors. In addition to the workshops, we also carried out ethnography, consisting of observations during the various stages of implementation for over a year, and conducted 64 semi-structured interviews with locals belonging to different social groups. In all our research encounters we discussed a broad selection of issues that included the locale, the screen, and issues relating to a sense of community and belonging.

Each one of the workshops was built around the activities of one or more local groups with which the researchers established contact after engaging ethnographically with the locale for ~8 weeks. These groups emerged from social meeting grounds such as churches, local schools, activism societies and hobbyist communities. Each group was chosen to be not only as homogeneous as possible, but also to fit with other groups so that, in aggregate, it would provide as wide a view into the dynamic of the locale as possible. Although we did not specifically create our groups on this basis, as the research progressed we found that age provided a convenient means to identify sets of groups which manifested consistent behaviors. Hence, regarding the analysis of our data, we chose to divide the whole population into three age-related groups: children/teenagers, adults/young adults, and seniors. This classification, although mainly intended to facilitate data analysis, is supported by the social dynamics of our sites. For instance, a group of children in the locale that participated in our workshops also participate in playgroups with their parents. Another example of a local group that participated in our workshops is The Recycled Teens, a group of seniors that gathers every Tuesday in a senior residence to sing old tunes, watch old films and discuss current affairs in relation to past events. Of course, some groups are not as homogeneous as these. An example of this kind is a knitting group, that includes participants ranging from children as young as 6 to others over 65 years old. In between creating scarves and hats, they discuss local, national and international politics. They use their knitting to support and draw attention to causes. Also this group participated in one of our workshops. In the following section we discuss how our informants integrated screen nodes within these pre-existing place-building activities. We will follow the three age-centered categories we defined earlier in this section.

3

Embedding Screen Nodes within Place Building Activities

Screens approached through competitive physical action

One of the ways in which children and teens generate a sense of belonging, community and place is by playing in public spaces after school hours. These include playgrounds, parks, markets, and sites like The Mill and Leytonstone Library. It is here where they meet friends that can become an integral part of their life. Discovering and mastering the spatial landscape and its community helps children and teenagers build their personal and social identity, while at the same time, helping them feel their neighborhoods as actually theirs. In a way, children and teens seek to assert certain degree of control over their spaces and their social experience within, eventually claiming these places as an extension of their own selves. To a great extent, this is done through a highly embodied, physical engagement. This means that a park or public space only becomes theirs in action [START_REF] Warnier | A praxeological approach to subjectivation in a material world[END_REF]: by wandering around, playing in it, and interacting with friends and family both in the space and with the space. From both ethnographic observations and workshops we noticed that, when interacting with the screen node, children and teens use the same action-centered approach that they use when they make public spaces theirs. They like to explore the opportunities provided by the technology: the sensitivity of the touch foil, the fluidity in application response, and their ability to exert control over the system. Whether it is by simply moving the pointer or by systematically exploring the possibilities in touch response, their approach is dominated by the actions that they can perform rather than by the explicit interactions that the developer envisaged for the application running in the node. Children and teens hence use the node in novel ways, inventing competitive games that rely on physical interaction (e.g. free pointer motion) and play them with others. In this way they create their own uses for the screen, which are quite independent from those intended by the application designers (who in our project imagined mainly collaborative, rather than competitive applications). In this case, users invent competitive games because it is easier for them to embed competition within their playground politics, as compared with the abstract collaboration ideal presented by the application itself. In both of our sites children and young teens attend playgroups where they learn to share the space with others; they enjoy books, toys and resources that are not theirs, but which they can borrow and play with. These objects and spaces are not seen as alien, but as personal, in that "public property may become viewed as private possessions and thereby potentially contribute to a sense of self" [START_REF] Belk | Possessions and the extended self[END_REF]. Children frame the screen node using a similar collective ownership approach, and they treat it as personally enjoyable, yet community-owned. For these participants, the screen node is experienced in the same way as a park or a playgroup, and social behavior is enacted accordingly.

Screens used as springboards for local culture

When constructing a sense of belonging, adults emphasize less the physicality of the environment and take a more instrumental view. They are interested in using the screen node to create what they consider a positive environment by improving education, increasing historical awareness or fostering better communication flows between different groups within their neighborhoods. This place building perspective has resulted in the formation of many of initiatives, from the creation of grass-roots business improvement districts, to art trails and community centers. In this context, generating a "Walthamstow/Leytonstone culture" is relevant to adults as an intellectual, symbolic and material practice that not only enables the production of a sense of place, but also promotes "the human capacity to expand worlds towards other potentially distant horizons and more complex outcomes of life" [START_REF] Miller | Tales from Facebook[END_REF]. For our adult participants, place building is an opportunity for the purposeful pursuit of change in the locality; it is a scaffold [START_REF] Greenfield | A Theory of the Teacher in the Learning Activities of Everyday Life[END_REF], an instrument to mediate between their imagined view of their locality and the reality in which it stands. Hence, our adult participants see their place building activities as a springboard to generate local culture and bridge the gap between what their locale is and what it could become; a stage for the display of the unrealized potential development of the locale. For instance, those explicitly engaged in transformative activities (such as local activism) wanted to use it to extend their efforts, informing and educating the local population about relevant projects. Members of the Knitting Group, expressed this view, urging researchers to upload to ScreenGram images of knitted characters they were selling to raise funds for their local community center. Local artists thought that the node could extend the available physical exhibition spaces by imagining it as a window to display digital art. The managers of The Mill imagined the node as a virtual notice board, extending their own functions and enabling users to connect with each other even if the building itself was closed. Even those adults not involved in specific community-building projects identified the node as an opportunity for dissemination of community ideas, particularly as outlets of local useful information: recommendations on what to do in the area, news on what is happening and mechanisms to connect with other locals.

As shown above, most adults in our workshops and interviews reported that they expected the screen node to embody a higher purpose. They approached the screen as an extension of their pre-existing place building endeavors, uploading photos or short messages that were relevant to their community. For many, the mere physical presence of the node itself legitimized previous efforts for place building, showing them to have been efficacious, as new resources for the realization of the community had been produced. These locals presented the node in local events with pride, as a symbol of their achievements to bring the community closer to its unrealized potential.

Screens used as collective memory fixing mechanisms

One important way in which seniors generate a sense of place is through remembrance narratives involving the past of the neighborhood and their lives within it. This activity is one of anchoring [START_REF] Giddens | The Construction of Society[END_REF], in which subjects bring the past to inform the present, and ensure that valuable historical lessons remain with us. Although this is particularly visible when they interact with each other during community group meetings, these narratives are also important in other contexts. Whether sharing these stories with children in local schools or with local historical societies, seniors enjoy talking about the past in public spaces. Given this predilection for reminiscence, our senior participants enjoyed the node applications that enabled them to preserve and share memories. Hence, they quickly identified the ScreenGram and Slideshow applications as a potential collective photoalbum, which could help preserve and rework memories, acting as a vehicle for their remembrance narratives. In addition, since the screen node is located within one of the social hubs in which our senior participants congregate, it can also serve as a focus point to talk about these memories. Hence, the visual elements of the screen facilitate the verbal storytelling with which they usually accompany photographs, and which they traditionally employ to build a sense of belonging.

Although seniors were not able to interact easily with the screen node (researchers helped them upload images and display them in the node), they enjoyed the social opportunities provided by using the node to share images in their community group. Rather than a superficial feeling derived from watching a large number of generic images, these participants preferred to dwell for longer in specific images that held importance for lives of others in the neighborhood; they preferred the images that better allowed them to retain collective memories, rather than those that simply had aesthetic appeal. They stressed that they would have liked to have a pause button to dwell for longer on specific images and discuss them, rather than having an automatic timed loop. Capturing an image in a photograph, and re-capturing the moment by pausing its display on the screen node, enhances the process of building intimate connections with the place and with other people. Hence, for these users remembrance is less about simple archiving and more about reclaiming the experiences of others as own, and enabling their own experiences to be reclaimed in the same way by others. Hence, our senior participants expressed a need to discuss specific images that participants of other ages did not require, and which directly connected with their place building efforts. Screen nodes helped seniors share their place building perceptions with each other, reinforcing their commonalities and leading to a more rewarding sense of place identity based on collective participation and mutual reinforcement.

Discussion and conclusions

When interacting with the screen nodes, most participants (irrespective of their age) enact performances that connect them to their existing place building practices. These performances consist of patterns of behavior "whose repetitions situate actors in time and space, structuring both individual and group identities" [START_REF] Kapchan | Performance[END_REF]. Although these performances are based on repetition, mimicry and reproduction of social interaction in the neighborhood, they are also varied: to an extent, each performance is unique, allowing locals the expression of individuality even when following normative patterns of interaction. On the one hand, the repetition associated with each performance serves to deeply embed patterns of interaction and produce a shared sense of place. On the other, the freedom that each performance affords the individual can become a vehicle for the exploration of the relationships between the individual and the community. Hence, these performances are both agents of social change and aids to reproduce current social structures of interaction. Although anchoring, scaffolding and action based approaches are universal human experiences regardless of age, we found that the behavior of our participants towards the screen could be understood (albeit in a simplified manner) by considering one of these approaches as dominant. We posit that this simplification can provide useful guidance when designing applications for public interactive screens, particularly regarding their longer-term integration with the social dynamics of a locale.

In this paper, we emphasized the commonalities between regular place building practices and the performances that locals enacted when interacting with screen nodes. To understand this performance, simply conducting interviews and recording the spoken word is not enough, as it misses the sociological and cultural milieu in which the performance acquires its meaning and cultural significance. Hence, we argue that understanding the wider context through ethnography and workshops in the wild provide an invaluable tool to make sense of the ways in which technologies become embedded within the place building practices of a locale.

Place building is directed towards the generation of a structure of feeling [START_REF] Williams | The Long Revolution[END_REF]; a deep subconscious familiarity with our everyday living spaces. Hence, it is not a purely emotional construction: it is also physical, leading to a deep-seated bodily experience in which the residents belong in the space and are able to navigate it effortlessly. It depends not only on the values, beliefs and customs of locals; it also depends on the unique ways in which their pre-existing place building practices create a bridge between their culture and the locale in which they live. In this sense, place building takes the spatial analysis of interaction with users and extends it with cultural dimensions, making it a useful addition to the tools that can be applied when considering the deployment of a technology in a given public setting.

This work is funded by the UK DE Program grant EP/I031839/1 and EP/I031413/1. We would like to specially thank our host venues and research participants.


Daniel Costa
email: dancosta@di.fc.ul.pt

Nádia Fernandes
email: nadiaf@di.fc.ul.pt

Sofia Neves

Carlos Duarte

Raquel Hijón-Neira

Luís Carriço

Web Accessibility in Africa: a Study of Three African Domains

Keywords: Web Science, Web accessibility, automated evaluation

Being the most used method for dissemination of information, especially for public services, it is of paramount importance that the Web is made accessible as to allow all its users to access the content of its pages. In this paper, we evaluated 2250 Governmental Web pages from each one of three different African countries (i.e., Angola, Mozambique and South Africa). This report compares the accessibility quality and the level of structural complexity of these African countries government's Web pages. We found that hand coded pages tend to have larger number of HTML elements and also to present higher number of accessibility problems. Finally, it suggests some recommendations to repair the most common problems in these pages.

Introduction

In many countries, the Web is the main vehicle used by governments to spread information, education, allow civic participation and other public services. It also is an important medium for receiving and providing information and interacting with society. Therefore, it is essential that the Web is accessible in order to provide equal access and equal opportunity to people with or without disabilities. Besides, an accessible Web has the potential to help people with disabilities and the elderly to participate more actively in society.

The United Nations (UN) estimates that approximately 10% of the world's population are persons with disabilities [START_REF] Kuzma | Global e-government Web Accessibility: An Empirical Examination of EU, Asian and African Sites[END_REF]. It is difficult to estimate how many people are affected by Web accessibility problems, nevertheless, if we move forward to an ideal situation, where only a reduced percentage of the population faces accessibility barriers, then technology is serving society in the right way.

The importance of Web accessibility is increasing in the international context, and especially in the European Union [START_REF]eAccessibility -Opening up the Information Society[END_REF]. In Europe, more and more countries have legislation requiring that government Web sites be accessible. In contrast, developing countries in Africa have less stringent laws, if any [START_REF] Kuzma | Global e-government Web Accessibility: An Empirical Examination of EU, Asian and African Sites[END_REF]. Governments worldwide have several stimuli to adopt accessibility. Demonstration of social responsibility by provisioning information and services to all citizens is one of them.

In this paper, we present a report of the state of Web Accessibility in three countries located in the African continent. The evaluation of accessibility we describe is based on the Web Accessibility Guidelines (WCAG) 2.0 [START_REF] Cooper | Techniques for WCAG 2.0 -Techniques and Failures for Web Content Accessibility Guidelines 2.0. W3C Note[END_REF].

Web Content Accessibility Guidelines 2.0

To help creating accessible Web pages, WCAG 2.0 defines guidelines that encourage designers/developers to craft Web pages according to a set of best practices. These guidelines are also used for accessibility evaluation.

WCAG 2.0 contains several guidelines written as testable sentences and chosen to address specific problems related with accessibility. Each guideline has a testable success criterion, which is supported by techniques that can be true or false when testing Web content against them.

Although, it is possible to use the guidelines to manually evaluate Web pages, due to the nature of this study (i.e., the large number of Web pages evaluated) we used an automated evaluation tool: QualWeb [START_REF] Fernandes | Evaluating the accessibility of rich internet applications[END_REF].

QualWeb

QualWeb is a Web automatic accessibility evaluation tool. The main advantage of this tool is the in browser context evaluation [START_REF] Fernandes | Evaluating Web Accessibility at different processing phases[END_REF], i.e., after the Web browser processes the Web page and all resources are loaded. To this end, the Webkit-based Phantom1 headless browser is used, allowing us to assess the page's code after browser processing. In terms of techniques, QualWeb covers 51% of the HTML and 73% of the CSS techniques.

An additional distinguishing feature of this tool is the ability to find different states of the Web page [START_REF] Fernandes | Evaluating the accessibility of rich internet applications[END_REF]. This means QualWeb is capable of interacting with DOM elements and detecting changes to the DOM of a page. QualWeb stores a new state if more than content is replaced after interaction (e.g., introduction of new HTML elements on the DOM tree). We consider the total number of states found, the level of complexity of a Web page as this reflects the dynamism we can find on the current state of the Web.

Experimental Study

For this study, the first step was to obtain a list of governmental Web pages for each of the three countries: Angola, Mozambique and South Africa. Starting from each of the main government's pages, we used a Crawler to look for clickable elements in it. Every time a clickable element redirected to another URL on the same domain name (gov.ao for Angola; gov.mz for Mozambique; or gov.za for South Africa), this new URL was kept as an object to be evaluated and the algorithm continued to execute.

Using this method, we collected a sample of 2250 government Web pages, from each country.

Afterwards we performed the evaluation itself, on each one of these 2250 Web pages per country. Every Web page was assessed with the QualWeb evaluator to check for conformance with WCAG 2.0 HTML and CSS techniques. The evaluation produced a list of Warnings, Passes and Fails that are analysed in the results section. In the interest of classifying the complexity of the evaluated Web pages, the QualWeb feature allowing the identification of different page states was used to determine the total number of states in the pages evaluated.

Results

Our evaluation yielded differences in the HTML documents in terms of number of HTML elements, between domains of different countries (Figure 1). The pages of South Africa (za) present a higher number of elements with an average of 846.37 elements per page, followed by pages of Angola (ao) with an average of 360.17 elements per page and finally by the pages of Mozambique (mz) with an average of 344.60 elements per page. Figure 2 presents how the evaluation outcomes (fail, pass and warning) differ between the African countries' Web pages. A failure occurs in the cases where the evaluator can detect automatically and unambiguously if a given HTML element has an accessibility problem. A pass ensues from elements that, unambiguously, are classified as having no accessibility problems. Warnings are raised when the evaluator can partially detect accessibility problems, but which might require additional inspection (often by experts). Table 1 presents the percentage of outcomes (pass, fail and warning) by country. Inspecting these results with additional detail, the Web pages have the following evaluation outcomes:

 Fail: Even though the compliance with accessibility techniques is quite different in all three countries, the common factor between the Web pages of Mozambique and South Africa is that fails are slightly above 50%. In addition, the Angolan Web pages are just above 40% for fails.

Evaluation by technique

In the following analysis, we will focus on the accessibility results by technique, identifying the more compliant and the more infringed techniques for each country. Figure 3 shows the techniques where occurred passes and their average. All three countries present higher pass values for techniques C23 and C19. The third higher pass value is C8 for South Africa, C9 for Mozambique and C21 for Angola. These techniques evaluate the following conditions:  C23if div elements in main content have background colour;  C19whether text is incorrectly altered to "look" as if it has an align right or centre;  C8for paragraphs and headings, looks for a wrong usage of extra spaces between letters to simulate the letter spacing property;  C9whereas decorative images are specified in CSS rules and therefore removable when disabling CSS;  C21checks if the line-height property is used with relative values and if these values range between the ones recommended.

The first observation that can be made is that HTML techniques present lower values of pass comparatively with CSS techniques. This can be explained by the fact that CSS techniques are more specific than HTML ones, which means that an automated evaluation can more easily determine pass for these, while HTML return higher number of warnings. The average number of possible problems and problems (fails and warnings) per technique is presented in Figure 4. All three countries present higher values in techniques C15 and C7. For South Africa and Angola, the subsequent high value technique is H30, while for Mozambique is H73. Respectively these techniques evaluate the following conditions:  C15if anchor and input form components present a visual alteration when interacted with;  C7whether anchor elements are followed by a span tag with a textual description of the link hidden by a CSS rule;  H30if the link text describes the purpose of the anchor;  H73checks the correct usage of the summary attribute in tables.

From these results, we can deduce the most common elements with potential accessibility problems. In South Africa and Angola these are anchors or input form components, and in Mozambique tables are added to these.

Incompliance with certain techniques is more pronounced in some countries. For instance:

 H33if a title attribute supplements a link, is a more common problem in South Africa (average of 24.61), comparing with the other countries (average of 2.95 for Angola and 1.50 for Mozambique);  C23which presents an average of 9 elements with problems for Angola, being negligible in the other two countries;  H39verifies the usage of caption elements to associate data tables captions with data tables, shows the same behaviour as H33, with an average of 39.58 for South Africa (average of 1.10 for Angola and 11.42 for Mozambique).

Level of Complexity

We found that the average complexity for the three different domains is approximately 1. The results gathered for Mozambique and South Africa show that the highest level of complexity is 2 (found in 2 Web pages). For the Angolan Government pages, the highest level is 3 (found in 3 Web pages), while 17 pages had level 2.

From these results we can conclude that, for these countries, dynamic changes to the governmental pages layout or interaction elements (thus excluding changes to their content) is not common. When these changes are required, a new page will be loaded, instead of changing the DOM.

Discussion

We found there are differences between the three African domains government's Web pages accessibility quality. The South African Government's Web pages have a larger number of HTML elements, but also present a larger percentage of elements raising fails and warnings, comparatively with the Web pages from Angola and Mozambique. This goes towards the conclusions of Lopes et al [START_REF] Lopes | Macroscopic characterisations of Web accessibility[END_REF], where it was found that the size of the pages influences its quality (i.e., smaller Web pages have less accessibility problems than bigger ones).

Concerning the techniques, it was observed that CSS techniques have a greater influence on the positive accessibility values for all the countries domains than HTML techniques. Techniques C23, C21, C19, and C8 were found to be the ones with highest compliance levels.

When considering potential accessibility problems (fails and warnings), we perceived that they also have higher values in CSS techniques but the difference to HTML techniques is not as pronounced as we found when analysing passes. The techniques most often violated were C7, C15, H30 and H73. It is interesting to note that what we observed for one of the techniques with more problems, H30 (which verifies if the link text describes the purpose of the anchor), is consistent with what was already seen in a previous accessibility study of two hundred of the most used Web pages in the entire world [START_REF] Fernandes | Evaluating Web Accessibility at different processing phases[END_REF].

The majority of the HTML problems found are related with the accessibility quality of tables, specifically when they do not have captions and summary elements, and if links do not have text descriptions. If those were carefully reviewed and redone the accessibility quality of the pages would considerably improve.

The results show that government Web pages would greatly benefit from reviewing their CSS, since the majority of their problems are located in techniques C7 and C15, especially for the South African government's Web pages. Problems with these techniques can be solved by adding a description of the link given in the anchor element, inside a span tag and hidden by a span CSS, as recommended by the WCAG 2.0 description. For technique C15, the solution would be to ensure that every anchor link and input box changes its colour whenever it is interacted with. People would greatly benefit from this visual aid and contrary to technique C7, it is much easier to enforce. Correcting these situations would help separate the normal paragraph's text and the interactive text in the anchor element, as well as help signalling which form input element is selected at a specific instant when it is being interacted with.

After finishing the automated evaluation, we performed a manual inspection of some of the government's pages from each country. This inspection was performed following the indications of the WCAG 2.0. For the South Africa's Web pages we observed that: the limitations of the several divisions of the pages was not always clear; link elements were confused with parts of the text; the general structure was quite similar to a newspaper and did not denote a lot of accessibility concerns. For Mozambique's Web pages, decorative images do not have either alt or title attributes when they should have them with empty values; some colours are also perceived as being too bright; table captions were also almost inexistent; there are also some flash objects directly embedded without any textual descriptions. For Angolan Web pages, since they generally follow the same structure, they all could benefit from adding captions to tables and textual descriptions to images and anchor elements. We can see that some of the issues found manually confirm the findings of the automated evaluation.

It was also possible to detect that Angola and Mozambique's Web pages benefited from tools that help code generation (such as Flyout and Plone, respectively). On the other hand, South African pages, taking into account the quantity of comments in the code and its specificity, were probably manually coded. This probably contributed to the bigger number of CSS problems, because code generators avoid several CSS problems, such as the use of relative font-sizes.

Regarding the level of complexity of the Web pages, we found that dynamic changes to the pages' DOM are mainly used to change the content of the pages and not to add new elements to the page (i.e., less structural complexity). In what concerns the accessibility quality, the slightly higher complexity found in Angolan Web pages does not reflect any significant change in the overall accessibility score.

Conclusion

The Web is the main vehicle used by many governments to spread information, education, allow civic participation and other public services. If these pages are not accessible they fail to reach their target population.

In this paper we evaluated 2250 Governmental Web pages from each one of three different African countries: Angola, Mozambique and South Africa. This report shows that the South Africa Government Web pages have more elements than the other countries but have less quality in terms of accessibility. The Angolan Government Web pages scores the best ratio of passes when comparing with the other countries. Mozambique's pages have the lower rating of fails and warnings combined. Regarding the level of structural complexity, we did not find major differences between the different countries' Web pages.

A manual inspection of a sample of the pages suggested that Angolan and Mozambican Web pages might have benefited from the support of code generation tools during their development, while this is not so clear in South African Web pages. The accessibility evaluation, concomitantly, has shown more accessibility problems in South African pages, with some of these problems being in some cases more easily addressed and prevented with the use of code generation tools.

This overall view of the current state of accessibility in these African governments Web pages by WCAG 2.0 techniques facilitates establishing a set of recommendations to repair the most common problems.

Fig. 1 .

 1 Fig. 1. Average number of elements per page for each country's governmental pages.

Fig. 3 .

 3 Fig. 3. Average number of passes by technique per country.

Fig. 4 .

 4 Fig. 4. Average number of fails and warnings (possible problems) by affected technique.

Table 1 .

 1 Distribution of evaluation outcomes (absolute values and percentages) by country.

	 Pass: Angola's governmental Web pages register the highest percentage of passing
	elements, reaching over 40%. Mozambique ratio decreases to around 37% and South
	Africa registers the lowest value, around 19%.

Fig. 2. Average number of HTML elements by evaluation outcome by country.

PhantomJS: http://phantomjs.org/