Frederic Raber

Quan Nguyen
email: quan.nguyen@dfki.de

Michael Kipp
email: michael.kipp@hs-augsburg.de

Frederic Raberˡ

Quan Nguyenˡ

Michael Kipp²

MotionBender: A Gesture-Based Interaction Technique for Editing Motion Paths

Keywords: coordination, character animation, motion trajectory, Kinect, 3D user interfaces

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

Motivation

Character animation is required in many fields, from movie production and games to anthropomorphic interfaces. However, animation is a complex craft that requires high expertise. Current animation approaches can be divided into two categories. In interactive methods animation is created on-the-fly. The animator either moves a virtual model using his/her limbs or a controller. The model's motion is recorded during this animation process. In keyframe methods the animator defines several decisive "key" frames, using a 3D modeling tool. In-between frames are automatically computed by interpolation. This is the standard method in the industry. Neff et al. [5] presented an interactive system where this animation is done via the mouse. The movement of the mouse is mapped to respective joint movements by correlation maps. Kipp/Nguyen [START_REF] Kipp | Multitouch puppetry: creating coordinated 3D motion for an articulated arm[END_REF] explored the possibilities of a multitouch-surface to interactively animate the arm and hand of a puppet. There are several articles [START_REF] Min | Interactive generation of human animation with deformable motion models[END_REF][2] which describe keyframe methods, using a motion trajectory for visualizing and editing the animation of each joint.

In these solutions the trajectory is modified with conventional input devices like a mouse which have few degrees of freedom.

We propose a novel interaction technique called MotionBender (Figure 1) for editing motion paths by combining the possibilities of direct manipulation using a visionbased motion sensor (Kinect) and a simple button for switching between edit modes. The modes differentiate between selecting where to "grab" the path and moving these parts to "bend" the path into a new shape. For such a gesture-based interface, it is important that the user makes use of all degrees of freedom that such an interface offers. If s/he uses the system in a sequential way, one dimension at a time, the benefit against conventional input systems is decreased. We used a coordination measure in our evaluation to measure how coordinated the user's movement is.

Interaction technique

Overview

Our system is called MotionBender. It was created with a system like Min et al.'s [START_REF] Min | Interactive generation of human animation with deformable motion models[END_REF] in mind, providing the possibility for a sensor-based interactive editing of the motion paths. Like in Min et al.'s work, every joint (e.g. the wrist of one hand) has a motion path, describing the movement of this joint during the animation, as shown in Figure 1. The spheres represent the positions of a single joint at successive time points. The user selects two of the spheres on the curve (marked red) and moves them to another location in the virtual world. The surrounding parts of the motion path are automatically adjusted. After the user has finished, MotionBender recomputes the animation of the joint according to the new motion path. Figure 2 shows a user using MotionBender in a typical working situation. The corresponding video can be found online 1 . Fig. 2. User working with the MotionBender interface.

Workflow

We focus on the editing of a single motion path. The manipulation is done with a Kinect sensor and a two-button device in one hand. There are two modes: grab and bend. The user stands in front of the computer. In grab mode, the user selects two points on the motion path (marked red in Figure 1) where the path is "grabbed". This is done by clicking and holding the first button and moving the hands apart and together on a horizontal axis in front of the user. In bend mode, activated by clicking and holding the second button, the two selected spheres and their neighboring spheres follow the user's hand movements in 3D space (indicated by the blue arrows in Figure 1). Figure 3 shows how neighboring points (yellow) are moved in relation to the grabbed points (red).

Fig. 3. Effect of the neighbor function

To compute the change for the neighboring points, we differentiate between three types of points. Let us assume that points P1 and P2 have been moved by vectors V1 and V2.

 Points between P1 and P2 are moved by a vector which is a weighted interpolation between V1 and V2, where the weight corresponds on the distance to the points.  All other points, if they are less than M points away, are moved into the same direction as the nearest point moved by the user, but with a decreased distance. Formally, for every point P which is nearer to P1, its movement vector V will be calculated as V = k * V1, where k decreases linearly with increasing distance between P and P1.  All other points are not moved.

Experiment

Methods

Participants

One of the main goals of our work was to create a tool which can also be used by novices in character animation or similar fields. Therefore, we decided not to take experts for our evaluation, but lay people. We tested 21 participants. Two were taken out due to aborted runs. We thus analyzed 19 people (12 female, 7 male) of age 18-56 years (average 24). Two of the subjects are left-handed but use the mouse with their right hand. Participants were not paid.

Control Condition (Mouse Interface)

When trying to design the control condition, we started out with a conventional one-mouse interface, using a graphical widget to allow control over the various degrees of freedom. However, as Owen's work on bimanual curve manipulation [START_REF] Owen | When it gets more difficult, use both hands: exploring bimanual curve manipulation[END_REF] suggests, this would lead to an unfair comparison, as bimanual input is more efficient than unimanual. Since the Kinect also has more DOF, we had to extend this to make a fairer comparison. To allow the simultaneous usage of two hands, we designed an interface with two mice, one for each "grab point". As for the control mapping of each mouse, we made the x/y mouse motion control the x/y motion on the screen (frontal plane), whereas the scroll wheel would move along the depth axis (z axis). One could argue that the scroll wheel is usually used for up/down motion (y axis). However we deemed the mapping from x/y mouse motion to x/y motion in 3D space even more common, and since the mouse is usually pointing toward the screen the scroll wheel acts like physical metaphor of rolling with a wheel into the screen. In user studies, participants did not negatively comment on the scroll wheel mapping.

There are 12 trials, divided into two subsets A and B of 6 items each. Each subset was randomized in a controlled manner (e.g. reversed) to obtain new sets A' and B'. Each participant worked on either A or A' for one condition, and B or B' for the other condition. To avoid training effects on one condition, the first half of the subjects started using the condition "Kinect" and continued with the "Mouse" condition, and vice versa for the second half of the subjects.

Apparatus

For the experiment the user had to stand in front of a 22 inch screen in a distance of about two meters. For the two-mouse interface, the subject was sitting directly in front of the screen. The complete surface of the office desk was available for him to act upon with the mice (Figure 4). We are aware of the fact that the difference in the userscreen distance could give a slight advantage for the mouse condition, based on the assumption that for precision tasks it is better to be closer.

Task

The task was to match a given trajectory to a target trajectory (Figure 2). Each subject uses both interface versions and has to match six different trajectories for each interface. A trial is done if either the distance between given and target trajectory is below a certain threshold (successful match) or the maximum edit time (120 sec) has expired. In either case, the user directly continues with the next trial.

Procedure

The test procedure for all test candidates was as follows:

 At the start of the experiment the subject reads the written instructions.  After the subject finishes reading, the experimenter gives a demo of the interfaces and gives hints on solution strategies.  After a short question and answer session, the subject can practice on two sample trials, which will not be part of the main evaluation phase later. The sample trials and their order are always the same.  The main evaluation begins, where the subject is presented 6 trials to be matched in the first interface condition (Kinect or Mouse).  Phases 2-4 are repeated for the second condition (Mouse or Kinect)  The experiment ends with a short two-page questionnaire, where the subject expresses his/her experiences with both interfaces in a multiple-choice form on the first page, and gives free feedback about problems/issues on the second page

Measures

For our analysis, we computed three measures.

1. Completion time: Average time needed to complete a trial. 2. Mean squared error of all points which have not been matched yet.

3. Coordination: For comparing the movements regarding coordination, we use the measure defined by Kipp/Nguyen [START_REF] Kipp | Multitouch puppetry: creating coordinated 3D motion for an articulated arm[END_REF]. Coordination measures whether movement is performed in all three dimensions in parallel (high value) or sequentialized, i.e. performed along one dimension at a time (low value). Alternative coordination measures [1][7] are based on a fixed optimal solution path. For our problem, the optimal solution may be counterintuitive. The various selection possibilities for selecting grab points further complicate the matter. Therefore, such optimal-path metrics may yield misleading results. Kipp/Nguyen's measure is independent of optimal paths, taking only the raw movement vectors into account.

Results

Completion time

Figure 5 shows the average completion times over all participants and motion path samples. A paired t-test proved that subjects are significantly faster with the Kinect (M=79.36; SD=23.9) compared with the mouse (M=83.71; SD=19.02): t(18)=3.89, p<0.01. To test for gender, we analyzed males and females separately. While the performance for the mouse was statistically equal, we found that, for the Kinect, male participants were much faster (M=65.24) than female ones (M=87.59) which we found to be highly significant (t(17), p-value<0.001).

Coordination

Using the above mentioned coordination measure [START_REF] Kipp | Multitouch puppetry: creating coordinated 3D motion for an articulated arm[END_REF] we compared the performance for the left and right hand separately over all participants and devices (see Figure 6). In both cases the Kinect interface encourages significantly higher coordination: For the mouse device the coordination values are almost zero (left hand 0.009, right hand 0.007), whereas the Kinect interface reaches a value of 0.29 for the left hand and 0.26 for the right hand.

Questionnaire

In a post-experiment questionnaire we asked the participants questions which discriminated between Kinect and mouse on a 5-point differential scale where the middle position was labeled "both or equal". This questionnaire contained 17 questions regarding usability and user experience. For analysis, we transformed this to an interval between -2 for mouse and +2 for Kinect. We summed up ratings for each question over all subjects and computed a chi square statistic, comparing the ratings against the expected neutral value of zero. Only three questions reached significance, all three in favor of the Kinect interface.

Discussion

The results show that our MotionBender interface is objectively more efficient and subjectively better liked. Performance was significantly better, movement was clearly more coordinated and participants preferred it over the mouse interface and enjoyed it more. We also found statistical evidence that male users seem to profit more from our gesture-based technique than female users. Due to the low number of participants this may be an artifact and must be validated in future studies, which might also reveal a possible cause of that effect. However, time improvements were relatively small, compared to the overall task time. Our experiment design might have favored the mouse interface due to the closer user-screen distance. However, even if this was the case it only strengthens the results.

The Kinect was more coordinated than mouse. For mouse, users mostly acted separately, so they stopped moving the mouse when adjusting the depth position by rotating the scroll wheel. Some found this a limiting factor, others liked the fact that no unintentional movement along the z-axis was possible. The left hand was more coordinated than the right one in the mouse condition. We often observed that people are acting serially in matching the two selected points. They concentrated on the first (left), neglecting the second one initially. On the sensor-based interface, there is less of a difference. This might also be a result of the reduced cognitive difficulty with the direct-manipulation concept. Some problems emerged because subjects had problems to visually understand the 3D scene. Some suggested that a 3D stereoscopic view could reduce problems. We had two views on the scene, a top and a front view. Some users were confused by these two different views, and temporarily lost orientation.

In one interesting case, a subject had general problems with precise hand movements. She found the Kinect interface much easier to use because due to the wholebody motion some focus was taken away from the precision of her hand movements.

Conclusion

We presented an interaction technique called MotionBender, suitable for Character Animation. We used the concept of motion paths for the visualization of the animation and provided an editing method for post processing the animation by grabbing and bending the motion path. For this, we used a combination of a motion sensor (Kinect) and a simple button controller (wireless mouse) as input devices. The technique was intended to be efficient, intuitive and enjoyable. We validated our technique against a mouse-based interface and found MotionBender to be faster, easier to use and more enjoyable. Although that a comparison of our technique against a single-mouse interface subjectively appears to be unfair, that question should be checked in a future work. We also found that our technique encouraged highly coordinated movement. A gender effect indicates that this technique particularly appeals to male users but this finding needs further study. Future work must examine how this technique could fit into the larger character animation workflow and whether experts would prefer this technique over the accepted industry standards.

Fig. 1 .

 1 Fig. 1. In character animation, the movement of a joint, like the hand/wrist is visualized as a motion path. With our technique the user can modify this path by grabbing it (two red spheres) and bending it. The goals of our editing technique are:

Fig. 4 .

 4 Fig. 4. The Kinect setup (left) and the two-mouse interface (right).

Fig. 5 .

 5 Fig. 5. Average completion times over all subjects.

Fig. 6 .

 6 Fig. 6. In terms of coordination Kinect (green) significantly outperforms mouse (blue).



  Which interface was easier to use? (M=.21; chi²(2, n=19) = 11.47; p<0.005)  Which device was more fun to use? (M=.84; chi² (2, n=19) = 8.58; p<0.05)  Which device do you prefer? (M=.47; chi² (2, n=19) = 11.47; p<0.005)

http://tiny.cc/MotionBender


Jaedong Lee
email: jdlee@korea.ac.kr

Sangyong Lee

Gerard J Kim
email: gjkim@korea.ac.kr

Funneling and Saltation Effects for Tactile Interaction with "Detached" Out of the Body Virtual Objects

Keywords: Phantom sensation, Illusory feedback, Funneling, Saltation, Vibrotactile feedback, Multimodal feedback

In a previous work, we confirmed the existing effects of "Out of the Body" tactile illusion for virtual and augmented objects through funneling and saltation. However, it required a virtual imagery to be attached to the user for directly extending one's body. This paper aims at investigating similar phantom tactile sensations exist when the virtual object is visually detached from the user's body. Two usability experiments were conducted to verify the hypothesized phantom tactile effects: one for funneling and the other, saltation. Our results have shown that in addition to the perception of the phantom sensations with the "detached" visual feedback, the interaction experience was significantly enriched (vs. when without explicit visual feedback). We also discovered for the first time that for funneling, phantom sensations can be elicited without any visual feedback at all. The findings can be applied to the tactile interaction design using minimal number of actuators on a variety of media platforms including the mobile, holography and augmented reality.

Introduction

Tactile feedback has become almost indispensable in improving interaction experience. Realization of tactile feedback by using vibration devices is one inexpensive and practical method. However, due to its size and mechanics, a single vibrator scheme is most often employed and it is only able to convey simplistic on-off type of events. Instead, as a way to improve the tactile experience, a more advanced form of vibro-tactile feedback most often involves an array of vibrators [START_REF] Borst | Enhancing VR-based Visualization with a 2D Vibrotactile Array[END_REF][START_REF] Borst | Haptic Controller Design and Palm-sized Vibrotactile Array[END_REF] that brings about mechanical and cost complications and a constraint that a relatively significant area of the body has to be in full contact with the array.

One way that researchers have considered to overcome this problem is to create illusory (or pseudo) tactile feedback and combining it with the corresponding visual or auditory feedback [START_REF] Israr | Tactile Brush: Drawing on Skin with a Tactile Grid Display[END_REF]. For example, in a previous work, we confirmed the existing effects of "Out of the Body" tactile illusion for virtual and augmented objects through saltation and funneling. However, it required a virtual imagery to be attached to the user for directly extending and connecting one's body. In other words, this is undesirable because the user must have both hands/fingers in contact with the virtual object (see Figure 1). Thus, this paper aims at reproducing the same "Out of the Body" phantom sensation effect with "floating" (i.e. no virtual extension) dynamic virtual object detached from the body (see Figure 3). In particular, we consider two phantom tactile sensation phenomena, namely funneling [START_REF] Bekesy | Funneling in the nervous system and its role in loudness and sensation intensity on the skin[END_REF] and saltation [START_REF] Geldard | The Cutaneous Rabbit: A perceptual Illusion[END_REF].

Funneling (Saltation) refers to the illusory tactile sensation occurring away from the actual places of timed (simultaneous) vibratory stimulations. The intended location of the phantom sensation can be changed by modulating the intensity (Funneling) or inter-stimulus time interval (Saltation). Funneling and saltation have often been applied to reduce the number of tactile actuators in tactile interaction design [START_REF] Hoggan | Mobile multi-actuator tactile displays[END_REF][START_REF] Richter | The PhantomStation: Towards funneling remote tactile feedback on interactive surfaces[END_REF]. Recently, researchers have discovered such phantom sensations can be extended to the "Out of the Body" [START_REF] Miyazaki | The "Cutaneous Rabbit" hopping out of the body[END_REF] and for "Out of the Body" virtual objects [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF] (see Figure 1), thus making it possible to generate phantom tactile sensations as if coming from an external object (both real and virtual). Possible applications of such a phenomenon are shown in Figure 2. A medium for extending and connecting the body parts is required, virtual or real. The phantom sensation is more evident with the physical medium (e.g. felt at 3cm from the left) than the virtual (felt at 1.3cm with the same stimulation) [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF].

It has been found that the extent or the controllability of the effect is diminished when extended to "Out of the Body" and even more so when a virtual object is used as the medium extending one's body [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF][START_REF] Miyazaki | The "Cutaneous Rabbit" hopping out of the body[END_REF]. Consequently, we seek and experi-mentally investigate the possible synergistic effects by associating it with "dynamic" visual feedback to improve the tactile experience and controllability, possibly even without the medium (real or virtual) that connects the body parts (i.e. tactile interaction with virtual objects completely detached from the body). If validated, such a phenomenon can be applied to tactically interacting with holographic objects hovering in the air (Figure 2 This paper is organized as follows. In the next section, we first review previous research literatures related to phantom tactile sensation, multi-sensory integration and their application to practical interaction design. Then, we describe the two validation experiments and report the results. Finally, we conclude the paper with a discussion and directions for future research.

Related Work

Funneling and saltation are the two major perceptual illusion techniques for vibrotactile feedback. Funneling refers to stimulating the skin at two (or more) different locations simultaneously with different amplitudes and eliciting phantom sensations in the space between [START_REF] Alles | Information transmission by phantom sensations[END_REF][START_REF] Bekesy | Funneling in the nervous system and its role in loudness and sensation intensity on the skin[END_REF]. Several researchers have applied this phenomenon to human interfaces [START_REF] Barghout | Spatial resolution of vibrotactile perception on the human forearm when exploiting funneling illusion[END_REF][START_REF] Israr | Tactile Brush: Drawing on Skin with a Tactile Grid Display[END_REF][START_REF] Mizukami | Tactile information transmission by apparent movement phenomenon using shape-memory alloy device[END_REF][START_REF] Rahal | Investigating the influence of temporal intensity changes on the apparent movement phenomenon[END_REF][START_REF] Raisamo | Evaluating the effect of temporal parameters for vibrotactile saltatory patterns[END_REF]. For instance, Hoggan et al. experimented with using three vibrators on a mobile device to emulate a tactile progress indicator [START_REF] Hoggan | Mobile multi-actuator tactile displays[END_REF]. Tan et al. applied saltation to implement a tactile chair using a 3 x 3 tactile array for a pattern recognition application [START_REF] Tan | Tactual displays for wearable computing[END_REF].

Miyazaki has discovered the saltation could be extended to body-worn (e.g. handheld) objects and to create "Out of the Body" tactile experience [START_REF] Miyazaki | The "Cutaneous Rabbit" hopping out of the body[END_REF]. Furthermore, Lee et al. has confirmed the same phenomenon existed for virtual objects but with reduced effects and less precise controllability [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF]. Other researchers have investigated different ways and effects to apply phantom sensation by employing different stimulation interpolation methods [START_REF] Alles | Information transmission by phantom sensations[END_REF], varying the values of ISI's [START_REF] Cholewiak | The generation of vibro-tactile patterns on a linear array: Influences of body site, space, and time[END_REF][START_REF] Geldard | Saltation in Somesthesis[END_REF][START_REF] Geldard | Sensory Saltation: Metastability in the Perceptual World[END_REF], relative vibration amplitudes [START_REF] Alles | Information transmission by phantom sensations[END_REF][START_REF] Rahal | Investigating the influence of temporal intensity changes on the apparent movement phenomenon[END_REF], stimulation duration and frequency [START_REF] Cholewiak | The generation of vibro-tactile patterns on a linear array: Influences of body site, space, and time[END_REF][START_REF] Sherrick | The localization of low-and high frequency vibrotactile stimuli[END_REF], interstimulation distances [START_REF] Raisamo | Evaluating the effect of temporal parameters for vibrotactile saltatory patterns[END_REF], and even applying saltation to non-continuous skin (e.g. from the right arm to the left, fingertip to fingertip) [START_REF] Eimer | Cutaneous saltation within and across arms: A new measure of the saltation illusion in somatosensation[END_REF][START_REF] Warren | Electrotactile stimuli delivered across fingertips inducing the Cutaneous Rabbit Effect[END_REF]. Note that in our previous work, only minimal static visual feedback was used, namely the virtual "ruler" representing only the medium bridging the two body points (rather than the actual visual representation of the tactile event) at which the actual vibratory stimulations were given, to recreate the original "Out of the Body" phenomenon. No detailed studies have been reported regarding phantom tactile feedback with dynamic or detached visual feedback. Also note that aforementioned works [START_REF] Barghout | Spatial resolution of vibrotactile perception on the human forearm when exploiting funneling illusion[END_REF][START_REF] Hoggan | Mobile multi-actuator tactile displays[END_REF][START_REF] Israr | Tactile Brush: Drawing on Skin with a Tactile Grid Display[END_REF][START_REF] Mizukami | Tactile information transmission by apparent movement phenomenon using shape-memory alloy device[END_REF][START_REF] Rahal | Investigating the influence of temporal intensity changes on the apparent movement phenomenon[END_REF][START_REF] Raisamo | Evaluating the effect of temporal parameters for vibrotactile saltatory patterns[END_REF][START_REF] Tan | Tactual displays for wearable computing[END_REF] that have applied tactile phantom sensations to human interfaces did it so directly to the human skin and had not investigated the use of the "Out of the Body" phenomenon nor the issue of thereby minimizing the number of tactile actuators.

Interestingly, Flach et al. [START_REF] Flach | The cutaneous rabbit revisited[END_REF] and Kilgard [START_REF] Kilgard | Anticipated stimuli across skin[END_REF] have found that the phantom sensation was much influenced by the subject's focus of attention, anticipation and/or the line of sight. This strongly hints the possibility of further synergistic effects with more apparent visual effects associated with the intended phantom sensation.

In fact, the synergistic sensory integration is not new. It is generally accepted that multi-sensory feedback is additively helpful to interactive task performance [START_REF] Lee | Assessing the benefits of multimodal feedback on dual-task performance under demanding conditions[END_REF]. This is only true provided when the respective modality feedback is consistent in its content and timing with one another [START_REF] Oviatt | Multimodal interfaces. The human-computer interaction handbook: Fundamentals, evolving technologies and emerging applications[END_REF]. Many synergistic multimodal interaction systems have been devised and studied employing gestures [START_REF] Bolt | Put-That-There, Voice and Gesture at the Graphics Interface[END_REF], voice [START_REF] Corradini | A map-based system using speech and 3D gestures for pervasive computing[END_REF], proprioception [START_REF] Mine | Moving Objects in Space: Exploiting Proprioception in Virtual-Environment Interaction[END_REF], speech/audio [START_REF] Grasso | The integrality of speech in multimodal interfaces[END_REF], and force feedback [START_REF] Richard | A comparison of Haptic, Visual and Auditive Force Feedback for Deformable Virtual Objects[END_REF]. Aside from just improving task performance, multisensory interactions can also modify user perception, as illustrated by the famous McGurk effect. The McGurk effect is a perceptual phenomenon in which vision alters speech perception [START_REF] Mcgurk | Hearing Lips and Seeing Voices[END_REF]. Simple visual tricks can easily alter the body image that is created by the proprioceptive sense [START_REF] Ramachandran | Phantoms in the Brain[END_REF]. Although the best known cross modal effects are those of vision influencing other modalities, visual perception can be altered by other modalities as well [START_REF] Waller | Factors Affecting the Perception of Inter Object Distances in Virtual Environments[END_REF].

Experiment I: Effects of Funneling with "Detached" Visual Object

Purpose and Hypothesis

In the first experiment, we have compared the tactile experience of funneling for a virtual object, between (1) when it was associated with a (dynamic) visual presentation detached from the body and (2) when no visual presentation is given at all (as a reference). In a usual application setting, virtual objects will normally be rendered without a part that visually extends or connects user's body parts (Figure 2). Our interest is first to assess whether funneling elicits phantom sensation, its extent and effects to the overall interaction experience. It is well expected that no dislocated phantom tactile experience will be elicited without any visual feedback (Figure 4, bottom right). We still test for it as a base case. Also it has been shown through prior research [START_REF] Miyazaki | The "Cutaneous Rabbit" hopping out of the body[END_REF] that singular vibration (e.g. with a single vibrator) cannot create any localized phantom sensation for "virtual" object external to the body either. To reiterate, we are interested in and hypothesize the existence and quality of the phantom tactile sensation of the virtual objects even when it does not directly attached to the user body. We also expect that results for the detached "Out of the Body" virtual objects with the hypothesis that results will mostly extend to detached "Out of the Body" physical objects

Experimental Design and Set up

To create the funneling based phantom sensation, the user was given simultaneous tactile stimulations to one's two index fingers, one in the right and the other in the left. The two fingertips were tracked using small markers (25mm x 25mm) by a head mounted camera and a "detached" augmented reality video imagery was presented to the user through a 47 inch monitor (nominal viewing distance: ~60cm). As for the augmented visual feedback, a small moving object ("a bouncing basketball") was rendered at the intended location of sensation between the two fingers (see Figure 3).

The OSGART [START_REF]ARToolKit For OpenSceneGraph[END_REF] was used to recognize/track the small markers and generate the augmented video imagery. A webcam was worn on the head mounted fixture to produce a view close to one according to the actual line of sight. The user was asked to maintain a nominal distance (8cm) between the fingers (using an 8cm wide marking on the table), but was allowed small movements for natural and comfortable interaction. The inter-finger distance of 8cm was used and set equal to the experimental conditions used in [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF][START_REF] Miyazaki | The "Cutaneous Rabbit" hopping out of the body[END_REF]. A common flat coin-type vibrator (11mm in diameter) was used (taped to the respective fingertips) and controlled by an Arduino board [2] (and interfaced to and synchronized with the OSGART based experiment software). For detailed specifications refer to [START_REF] Jung | Perceived magnitude and power consumption of vibration feedback in mobile devices[END_REF]. It is controlled by a voltage input using a pulse width modulation signal with an amplitude between 0 to 5V, which in turn produces vibrations with frequency between 0 and 250 Hz and associated amplitudes between 0 to 2G (measured in acceleration, or 0 to 18μm in position) respectively. According to [START_REF] Jung | Perceived magnitude and power consumption of vibration feedback in mobile devices[END_REF][START_REF] Sherrick | A scale for rate of tactual vibration[END_REF], these values are well above the human's normal detection threshold (about 6 ~ 45db).

The experiment was designed as a 2x5 factor within-subject. The two factors were (1) inclusion of the visual feedback (with or without), and (2) intended locations of tactile illusion (five locations between the fingers labeled P1 ~ P5). Four survey questions were answered in a 7 Likert scale asking of the various aspects of the phantom tactile experience.

Detailed Procedure

Twenty paid subjects (15 men and 5 women) participated in the experiment with the mean age of 25.5. After collecting one's basic background information, the subject was briefed about the purpose of the experiment and instructions for the experimental task. A short training (3 minutes) was given for the subject to get familiarized to the experimental process. In addition to the head mounted fixture for the camera, the subjects wore ear muffs to prevent any bias from the sounds of the vibration. The ear muff was tested to make sure so that no sound could be heard during the experiment, and did not affect the outcome of the experiment. The levels of stimulations were given with the intention to create phantom and real sensations at 5 equi-distanced locations between two fingers. The Linear variation of stimulus amplitudes methods of Alles [START_REF] Alles | Information transmission by phantom sensations[END_REF] (Figure 4) was used with the stimulation duration set at 200ms. Preliminary studies and prior research has also confirmed that the aforementioned linear method and stimulation duration exhibited the best effect [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF]. The visual feedback appeared at the intended location of sensation, at the time of the stimulations, stayed for 200 milliseconds (same as the tactile stimulation duration) and disappeared. Each subject experienced, in a balanced order, a total of 60 positional feedbacks in all the 2 x 5 (10) conditions (6 repetitions each) with 10 second inter-stimulus rest interval, lasting about 40 minutes. For each condition, two exact same stimulation patterns were given, then subjects were asked to indicate the place of phantom sensations in terms of the five prescribed positions. We use the symbols L1~L5 to indicate the subject's response (as distinguished from the actual intended locations of sensations). The subjects were explicitly asked to report the place of tactile (e.g. rather than visual) sensation right after experiencing the stimulation. In addition, after all trials, they were asked to answer a short survey about their subject feelings (questions shown in Table 1).

Table 1. The four survey questions regarding the subjective feel for the phantom sensation answered in 7 Likert scale.

Q1

Were you able to perceive phantom sensation?

(1: Not at all ~ 7: Very well) Q2

When you perceive phantom sensation, did visual feedback affect you?

(1: Not at all ~ 7: Very much) Q3

How confidence are you about your answer to Q1? (1: Not confident at all ~ 7: Very confident) Q4

How long did it take you to perceive the phantom sensation if any?

(1: Instantly ~ 7: Few seconds)

Results

Figure 5(a) and (b) each shows the sensed/perceived locations of the tactile sensation (vertical axis), elicited by funneling, as reported by the users vs. the intended locations (horizontal axis) of sensation with visual effects and without. To our surprise, even without a virtually mediating object, phantom sensations were perceived at all five intended locations (Figure 5(b)). This is a first time discovery to our knowledge. There is still clear marked difference in the accuracy (or variance) for the intermediate locations, P2~P4. Note that P1 and P5 are where the vibrators are actually located, thus, a correct perception even without visual effect is naturally expected. Also note that the perceived locations were different among each other with statistical significances (see Table 2). Thus, a high localization controllability (~± 4mm) at approximately 2cm resolution was possible. Consistently to the statistical results, subjects reported that when no visual effects were given, it was difficult to differentiate between P1 and P2 (and similarly for P4 and P5), where the vibration motors were actually placed. ANOVA revealed statistically significant differences existed in the senses locations between when the visual effect was given and when it was not, at all five locations except at the middle, L3 (Figure 6 and Table 3). Note that with funneling, when equal stimulation strengths are given at the two finger tips and it is plausible to think that it would be easier (that is, no help needed with the visual effect) to perceive the phantom sensation to come from the middle and make the proper response. Fig. 6. A pair-wise comparison of the perceived locations between when visual effect is given (square) and not (circle). Star marks indicate those with statistically significant differences. Table 3. Statistical differences (p-values) in the "differences" of perceived locations between when with associated visual effect (e.g. L1) and without (L1').

L1-L1'

L2-L2' L3-L3' L4-L4' L5-L5' 0.013 < 0.001 0.352 < 0.001 0.013

Figure 7 shows the number of correct answers (i.e. correct when the perceived location matches the intended within a pre-specified threshold) in terms of score out of 100. Similarly to the above analysis, performances were generally worse without the associated visual effects for intermediate locations P2 and P4 being confused with P1 and P5 respectively. We emphasize that while distinguishing of five distinct points within the 8cm distance was possible with visual feedback or without, the accuracy (how close the perceived is to the intended location of sensation) is expectedly lower when the visual feedback is absent. Finally, Figure 8 shows the responses to the four survey questions, which are mostly consistent with the quantitative analysis. Subjects were conscious of the helpful effects of the visual feedback and confident of their phantom sensations. Experiment II: Effects of Saltation with "Detached" Visual Object

Purpose and Hypothesis

The second experiment is mostly similar to the first except that the "detached" visual feedback effect to saltation was tested instead. We hypothesize for the existence and the improved quality of the phantom tactile sensation of the virtual objects when coupled with visual effects.

Experimental Design and Set up

Again the basic experimental design and set up is mostly identical to the first one. Since saltation was used the vibro-tactile stimulations were timed rather than given simultaneously. The same "detached" augmented visual feedback, "bouncing basketball" was used. The experiment was designed as a 2x5x2 factor within-subject. The three factors were (1) inclusion of the visual feedback (with or without), (2) intended locations of tactile illusion (five locations between the fingers labeled P1 ~ P5) and ( 3) direction of the stimulation (from right to left or vice versa). Five survey questions were answered in a 7 Likert scale asking of the various aspects of the phantom tactile experience (see Table 4).

Detailed Procedure

Twenty paid subjects (15 men and 5 women) participated in the experiment with the mean age of 25.2 (a different pool from Experiment I). The experimental procedure was mostly identical to the first. Thus, we only describe the way saltation (i.e. timed stimulation) was administered in the treatments. Figure 9 pictorially describes how the timed vibro-tactile stimulations were given to create saltation effects. A total of three consecutive stimulations were given, each labeled S1, S2 and S3. The first two stimulation were given at P1 and the third at P5, intending to create a phantom sensation somewhere between P1 and P5. Inter-stimulus intervals of S1-S2 and S2-S3 were given with the intention to create phantom sensations at the prescribed positions, namely, P1~P5 (800ms-50ms) respectively (and stimulation duration of 80ms) based on recommended values for best effects from prior research and our own previous experiments [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF].

Half of the saltation treatments were administered with right to left stimulations (at P5, then P1) and the other half, in the opposite directions (at P1, then P5). The visual effects were rendered at P1, P5 (where the actual stimulations were given) and at the intended location of sensation and stayed on for 80 milliseconds, same as the stimulation duration. As shown in Figure 10, the visual feedback was given, synchronized with the corresponding of the three tactile stimulations (S1~S3), at three locations (two at the finger tips where the actual stimulations are given and one in between at the intended location of sensation, one of P's). We stress again that the users were asked of the location and quality of the tactile feedback, not the visual. Each subject experienced, in a balanced order, a total of 72 positional feedbacks in all the conditions: 60 times with visual feedback (2 directions, 5 intended locations of sensation and 6 repetitions) + 12 times without visual feedback (2 directions and 6 repetitions). Each stimulation was followed by a 10 second inter-stimulus rest interval. As it was so in the first experiment, for each treatment condition, two exact same stimulation patterns were given, then subjects were asked to indicate the place of phantom sensations in terms of the five prescribed positions (L1~L5). The subjects were explicitly asked to report the place of tactile sensation (e.g. rather than visual) right after experiencing the stimulation. In addition, after all trials, they were asked to answer a short survey about their subject feelings (questions shown in Table 4). The fifth survey question asked of the perception regarding the directionality. Table 4. The five survey questions for Experiment II regarding the subjective feel for the phantom sensation answered in 7 Likert scale.

Q1

Were you able to perceive phantom sensation?

(1: Not at all ~ 7: Very well) Q2

When you perceive phantom sensation, did visual feedback affect you?

(1: Not at all ~ 7: Very much) Q3

How confidence are you about your answer to Q1? (1: Not confident at all ~ 7: Very confident) Q4

How long did it take you to perceive the phantom sensation if any?

(1: Instantly ~ 7: Few seconds) Q5

Were you able to recognize a particular direction?

(1: Not at all ~ 7: Very well)

Results

Table 5 shows that the sensed tactile locations after saltation were statistically different from the actual location of stimulation for both when visual effect was given and, surprisingly again, when not given at all. Note that, according to Miyazaki, phantom sensation from saltation was not observed without visual feedback nor mediating object [START_REF] Miyazaki | The "Cutaneous Rabbit" hopping out of the body[END_REF]. While our result is somewhat contrary, its extent was very small (see Figure 11(b)). Figure 11(a) and (b) each shows the locations of the tactile sensation (vertical axis), elicited by saltation, as reported by the users vs. the intended locations (horizontal axis) of sensation with visual effects and without. When visual feedback was given, there were as usual five intended location of sensation according to the five respective locations of the visual feedback. However, only four statistically different sensed locations for S2 were found (i.e. L1, L2, L3, L4=L5; p-values L1-L2: <0.001; L2-L3: <0.001; L3-L4: <0.001; L4-L5: 0.088). There were significant differences in the accuracy (or variance) for the intermediate locations, between L3~L5 and P3~P5. Higher variance and lower accuracy/match (even at L1/P1) was observed as compared to the case of funneling, getting worse at the place of the third stimulation. When there was no visual feedback, we were only looking for (if it existed) one location of phantom sensation somewhere between the fingers. As mentioned above, this nonvisual case did exhibit a phantom sensation at one location significantly different, though very small, from the place of actual stimulation, around 0.83cm away. Postbriefings with the subjects also reflected the observation this sensation was barely perceivable. Finally Figure 12 shows the responses to the five survey questions, which are consistent with the quantitative analysis. Subjects were conscious of the helpful effects of the visual feedback and confident of their phantom sensations. Subjects were also able to recognize the direction of the stimulation and the described the sensation to as soft bounce (15/20), hard contact (3/20) and moving vibration (2/20) in the postbriefing.

Discussion and Conclusion

Previous research has found the phantom tactile sensations for virtual objects external to one's body. However, it required a virtual imagery to be attached to the user for directly extending one's body. This paper has investigated in whether similar phantom tactile sensations exist when the virtual object is visually detached from the user's body. Our results have shown that in addition to the perception of the phantom sensations with the "detached" visual feedback, the interaction experience was significantly enriched (vs. when without explicit visual feedback). We also discovered for the first time that for funneling, phantom sensations can be elicited without any visual feedback at all. We can further conjecture with both mediating visual feedback and the actual dynamic visual content, the tactile experience has to be improved even more with even higher localization controllability.

While the two tested phantom sensation techniques generated similar qualitatively enhanced tactile experience (both quantitatively and qualitatively), the funneling technique produced higher overall accuracy than saltation. On the other hand, saltation due to its nature seems fitting as a mean to provide directional tactile experience. The post-briefing also revealed the same. While both subjects answered both techniques did produce phantom sensations for certain, they also felt the funneling to have produced more efficient and stronger sensation.

The findings can be applied to the tactile interaction design using minimal number of actuators on a variety of media platforms including the mobile, holography and augmented reality. From a more practical perspective, we would be much interested in comparing the relative effect between (1) the usual single vibrator scheme and (2) using perhaps 2 or 4 vibrators for 1D or 2D [START_REF] Kim | Extending out of the body saltation to 2d mobile tactile interaction[END_REF] localized phantom tactile effect. The single vibrator scheme simply cannot be used for the case of "Out of the Body" virtual objects with virtual medium (e.g. two handed interaction with a dynamic holographic object). However, for the case of "Out of the Body" virtual objects with physical medium (e.g. two handed interaction with moving virtual objects on a mobile device), it is unclear whether there is sufficient benefit-to-cost in the additional effort to employ funneling or saltation. Single vibrator scheme has been quite successful in eliciting pseudo-haptic effects in smart phones and game controllers [START_REF]Bouncing Ball on Nokia N[END_REF].

However the distinction must be made clear in terms of the role of the visual feedback. The single vibrator scheme can be explained to be a phantom or pseudo sensation directly caused by the visual feedback, while our paper has addressed the phantom sensation being strengthened reversely by the visual effect. Note that our performance measures were tactile experience, and not visual. This suggests, that although our experiment has only tested the case with virtual medium, it is plausible to expect that the tactile experience to be significantly richer than the case of single vibrator scheme, e.g. if applied to mobile devices. The plausibility is high also from the previous research indicating the weakened sensation when the "connecting" medium was virtual [START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF]. Either way, the combined effect can be explained by the Modality Competition Theory that the modal fusion will depend on the disparity among the stimulations in term of their consistency [START_REF] Ramachandran | Phantoms in the Brain[END_REF][START_REF] Wickens | Compatibility and resource competition between modalities of input, central processing, and output[END_REF]. For instance, the disparity between the location of the phantom sensation and the visual feedback seems less than that that in a single vibrator scheme.

In fact, our next focus will be to verify such a proposition. In addition, we are interested in the neurological or cognitive explanation to this phenomenon. The typical explanation for funneling and saltation has been based on the continuity of the body map in the somato-sensory area in the brain [START_REF] Chen | Optical imaging of a tactile illusion in area 3b of the primary somatosensory cortex[END_REF]. However, with the discovery of the "Out of the Body" phenomenon, it seems there may be a cognitive element to it too (reconfiguring of the body map is also possible but it is usually regarded a very slow process as demonstrated in the phantom limbs [START_REF] Ramachandran | Phantoms in the Brain[END_REF]).

Fig. 1 .

 1 Fig. 1. The concept of "Out of the Body" tactile experience from a hand-held physical or virtual medium.A medium for extending and connecting the body parts is required, virtual or real. The phantom sensation is more evident with the physical medium (e.g. felt at 3cm from the left) than the virtual (felt at 1.3cm with the same stimulation)[START_REF] Lee | Funneling and Saltation Effects for Tactile Interaction with Virtual Objects[END_REF].

  (b)).

Fig. 2 .

 2 Fig. 2. Possible applications of the "Out of the Body" phantom tactile sensation: Two handed/fingered interaction and feeling tactile sensations as if coming from the middle of the (a) mobile device, (b) hovering holographic-virtual imagery, (c) indirectly from a virtual object in a monitor and (d) an augmented marker (e.g. seen through a head mounted display).

Fig. 3 .

 3 Fig. 3. The experimental set up for Experiments I and II. Vibratory stimulations were given to the two index fingers and the augmented visual feedback (e.g. bouncing basketball), detached from the fingers, seen through a large display monitor.

Fig. 4 .

 4 Fig. 4. The rendering method of stimulation originally proposed by Alles [1] for funneling to generate phantom sensations at five different positions (top). Examples of tactile stimulation: at P1 and P3 with visual feedback (bottom left and middle) and at P4 without visual feedback (bottom right). For example, to produce a phantom sensation at P2, a simultaneous stimulation of 4V at A and 2V at B are given as shown in the top left.

Fig. 5 .

 5 Fig. 5. Accuracy of reported locations of the phantom sensation with respect to the intended: (a) with associated visual effect and (b) without.

Fig. 7 .

 7 Fig. 7. Comparison of correct localization of visual and non-visual feedback based on stimulations across intended position.

Fig. 8 .

 8 Fig. 8. The collective responses to the general usability/experience survey.

Fig. 9 .

 9 Fig. 9. The rendering method for saltation stimulation. Three timed stimulations of S1 (at P1), S2 (at P1) and S3 (at P5).

Fig. 10 .

 10 Fig. 10. Administering for the saltation effect with three timed vibro-tactile stimulations with visual effects (above) and without (below).

Fig. 11 .

 11 Fig. 11. Accuracy of reported locations of the phantom sensation with respect to the intended for saltation: (a) with associated visual effect and (b) without. Star marks indicate those with statistically significant differences.

Fig. 12 .

 12 Fig. 12. The collective responses to the general usability/experience survey.

Table 2 .

 2 Statistical differences (p-values) in the perceived locations (L1 ~ L5).

		L1-L2	L2-L3	L3-L4	L4-L5
	With visual	< 0.001	< 0.001	< 0.001	< 0.001
	Without	< 0.001	< 0.001	< 0.001	< 0.001

Table 5 .

 5 Effects of saltation. Perceived location for S2 were in all cases different from S1 (both S1 and S2 stimluations were given at the same physical location) with statistical significance (p-values) indicating the existence of the saltation effect.

	Intended position (with visual)	p-value for difference between S1 and S2 (Stim. Dir. LR)	p-value for difference between S1 and S2 (Stim. Dir. RL)
	P1	< 0.001	< 0.001
	P2	< 0.001	< 0.001
	P3	< 0.001	< 0.001
	P4	< 0.001	< 0.001
	P5	< 0.001	< 0.001
	Without visual	< 0.001	< 0.001

Acknowledgement

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No 2012-0009232) and this research was funded by the Forensic Research Program of the National Forensic Service (NFS), Ministry of Security and Public Administration, Korea.


Kasper Rodil

Matthias Rehm
email: matthias@create.aau.dk

Heike Winschiers-Theophilus

Homestead Creator: using card sorting in search for culture-aware categorizations of interface objects

Keywords: categorization, indigenous knowledge, 3D visualization, card sorting, HCI, interface design

Designing intuitive interfaces for rural African users requires us to understand the users' conceptual model. We acknowledge differences in categorization approaches based on cultural factors, among others. In the absence of comprehensive literature and theories, we explore card sorting as a means to derive a local categorization of interface objects for one of our prototypes. Results indicate a locational-relational categorization scheme among Herero elders in Namibia.

Introduction

Since late 2010 we have investigated 3D graphics as means of contextualizing indigenous knowledge in digital representations. Besides the numerous challenges of technical constraints and understudied implications of interactions, perception and recognition by African rural dwellers we have had a major breakthrough in terms of dialogical design communication. The graphical representations have triggered uncountable discussions, narrations and design suggestions in the community. Thus at this point we further explore visual communication as a form of design dialogue to overcome language and cultural barriers. Kostelnick describes visual communication design as a continuum between global and culture-focused design, where hybridity of the two ends can exist [START_REF] Kostelnick | Cultural adaptation and information design: two contrasting views[END_REF]. He argues that the former 'view' is a product of cognitive perception having universal empirically testable characteristics. The latter is an argument also carried in linguistics that culture is an influencing factor. That the characteristics of visual communication are experiential and socially constructed only fully transferred within their contexts of origin. It is the balancing act between sensitivity to locally negotiated methods and artifacts of visual communication and introduction to technology that currently occupy our research and must be understood from this viewpoint. The notion we wish to raise is not to argue one or the other, but argue for the sensitivity and hybridity of these two ends. That nothing can be taken for granted especially not appraisal of universal metaphors in interface design or knowledge management structures.

Since the conception of this project, elders from the pilot community in Namibia have been co-designing prototypes with external designers. The elders are wellrespected and knowledgeable males, who have acquired much confidence in the usage of novel technologies as well as the ability to critically evaluate and suggest design improvements.

2

The HomeStead Creator

In 2011 we developed a prototype termed the HomeSteadCreator (HSC), which was received with enthusiasm by the Herero community members [START_REF] Rodil | Homestead creator: a tool for indigenous designers[END_REF]. The HSC is running on an Android powered tablet. The village elders consider the tablet as being less intimidating and the touch interactions to be more intuitive than the usage of laptops and prototypes we have evaluated earlier.

HSC is a tool for rural Herero community members to re-create their own environment or any imaginary context with familiar 3D objects. These re-created scenarios are then combined with locally recorded IK videos and audio files to provide a digital context. Fig. 1 shows an example of the 3D re-construction of a homestead from two different cameras. When the HSC is launched, a textured terrain in a 3D environment seen from a 90degree tilt perspective (perpendicular to the terrain) is displayed. The user of the HSC is presented with a row of icons representing categories defined by us. When they are being activated by a single-touch they expand and show virtual 3D objects within the 'category' (visible on Fig. 1(b)).

The objects are selected by single-touch and subsequently they are instantiated in the center of the terrain. The user can then position and rotate the object by common finger gestures (two-finger drag, two-finger circular motion etc.). The prototype was refined in 2012 based on the co-designers suggestions. From the introduction of the HSC, the tool has enhanced the design dialogue substantially while promoting crosscultural learning. E.g. the elders have shown us the correct position of the holy fire and the elder's house or 'the courtesy route around the homestead to greet the man of the house' by visualizing it on the HSC. Thus it has shown to be able to facilitate dialogue on specific issues of interaction design and as an ethnographical tool using visualization as a communication tool. In subsequent sessions the HSC was even expanded to be a storytelling device between elders and youths.

Current Categorization Attempts

One of the focal points in the 2012 revision was the organization of the interface with the categorization of virtual objects. This grew out of the continuous demand for new graphical objects by the elders. The 7-10 inch tablet screen can only display so many objects at a time. The current prototype has six categories with up to ten objects (see figure 2). Upon suggestions by the elders, we did a number of rearrangements of objects. In 2011, we had pre-grouped the objects according to our own categorization scheme. During the usability evaluation session of the prototype, we also explored the appropriateness of the categorization as well as the completeness of objects represented. For example the 'fence' which was previously grouped under 'objects' was requested to be moved to the 'cattle' category. The cattle were previously grouped with the other animals yet in the Herero tradition, cattle are attributed a different status than other animals thus the request for a separate category. Furthermore, the 'fire place' was requested to be part of the 'homestead items' together with the pots and washing line. See Table 1 for the updated categories.

Table 1. The figure shows the six groups with corresponding icons and objects within each group.

These seemingly minor re-arrangements reminded us of the underlying differences of conceptualization and categorization between the co-designers in the village and us. We acknowledge that with the increasing number of objects, neither scrollable lists nor our pre-ordered categorization system is adequate. Instead of continuously reiterating the categorization of newly added objects we attempt to conceptualize the underlying structuring of objects.

Situated Concepts in a Local Ontology

From a practical technology design point of view, we need a guiding ontology that captures the concepts and relations between them for a re-contextualization of the Herero homestead. Defining an ontological representation will allow for sharing and re-using the captured knowledge [START_REF] Gruber | Toward principles for the design of ontologies used for knowledge sharing[END_REF]. Assuming a culture-specific categorization of the domain, the ontology will not only be useful for the obvious task of structuring menu items in the HSC. It will also capture parts of the intangible knowledge of the user group [START_REF]Unesco: The UNESCO Convention for the Safeguarding of the Intangible Cultural Heritage[END_REF], thus allowing to create knowledge sharing applications that can partially construct relevant scenarios automatically based on the encoded knowledge.

Rehm [START_REF] Rehm | Being seaward-handed: a computational model of the acquisition of language-specific spatial references[END_REF] has focused on a situated acquisition of concepts and has shown the usefulness of combining nature and nurture views as described by Rosser [START_REF] Rosser | Cognitive development: Psychological and biological perspectives[END_REF]. On the one hand he has taken into account information processing and learning routines that have developed over time and are shared by all humans (nature perspective). On the other hand, based on these processing abilities, categorizations and concepts are developed by individual experiences, thus shaping an individual conceptual system that is in accordance with one's experiential history, taking into account environmental as well as social factors (nurture perspective). For the sake of the task we are interested in defining the relevant categories and their relations in the domain of the HSC. The 'fences and cows' example highlights the fact that there is no 'universal' conceptualization. Instead, we have taken into account Lakoff's ideas of situated concepts that cannot be viewed independent of the context of their use [START_REF] Lakoff | Cognitive models and prototype theory[END_REF]. And as Ingold explains: "…to individuals who belong to different intentional worlds, the same objects in the same physical surroundings may mean quite different things. And when people act towards these objects, or with them in mind, their actions respond to the ways they are already appropriated, categorized or valorised in terms of a particular, pre-existent design." [START_REF] Ingold | The Perception of Environment[END_REF].

Not being members of the Herero culture we lack crucial information about the use of objects rendering the categorizations that we create from our own experiential history useless in the given cultural context. Studies have investigated these cultural differences. For instance Nisbett et al. found a difference between being either holistic (East-Asians) and analytical (Westerners) [START_REF] Nisbett | Culture and systems of thought: holistic versus analytic cognition[END_REF]. Hunn describes how the Tzeltal peoples (a Mayan ethnic group) classify butterflies and moths [START_REF] Hunn | The utilitarian factor in folk biological classification[END_REF]. They distinguish butterfly larvae into sixteen terminal groups due to characteristics of being edible, dangerous to crops etc. The adult butterflies do not have these characteristics (although being visually distinguishable) thus they are (locally) not important for categorization. Similarly, do members of the Herero community distinguish cattle from other animals and require a larger variety of the same type. Local trees are by the elders also not necessarily classified by species, but by characteristics that make them appropriate for the homestead or the kraal. To determine a local categorization scheme of virtual objects we have turned to card sorting as a means to find a local ontology for further developing the object structures in the HSC [START_REF] Ross | Comparing user research methods for information architecture @ONLINE[END_REF].

Card Sorting as a Method For Establishing The Local Classification Scheme

Card sorting is a traditional HCI method to organize information in web pages into meaningful categories and for intuitive retrieval of information.

The method is usually either an 'open' (generative type) or 'closed' (evaluative). One of the benefits with the 'open' version is that participants can categorize objects without pre-defined sorting, themes or adhering to an overlying taxonomy. Conversely, the 'closed' version is suitable for establishing at what level participants agree with the categorization and terms used for pre-sorted groups.

Card sorting is valuable for gaining insights into the participants' mental models by eliciting how they sort, group and organize items. Petrie el al. showed that card sorting can illuminate cultural differences in the mental models behind information architectures [START_REF] Petrie | Using card sorts for understanding website information architectures: Technological, methodological and cultural issues[END_REF]. The product of the card sorting method is a snapshot of a subjective categorization and developers might be fixed on the final sort to implement information architecture from. However we argue that the method itself (especially the open version) is also effective in creating a dialogue around the user's standpoint. Thus in the process of sorting the researcher gains an insight on the users' world views.

Method

In total we conducted the study at three different Herero sites in Namibia. With a total number of participants being 5 females and 9 males. The presentation of detailed results is beyond the scope of this paper, therefore we will only present results from the 5 elders from our long-term collaboration village.

In preparation for the card sorting sessions a set of 47 cards was printed. The set consists of laminated cards with images of the 3D objects from the HSC, which are rendered with a white background. The cards are printed as they appear as icons in the six interface categories in the HSC.

The village elders who are the future users of the HSC and have been co-designers on the project from its conception were recruited for the sessions. The sessions were decided to be facilitated individually to investigate each elder's viewpoint. The elders were prompted to say what they saw on the cards and to talk aloud while sorting, this was to confirm recognition of the represented 3D objects and to understand the rationale for the sorting decisions.

The agenda was explained before the sessions began and we stressed the openness of the studies by emphasizing that nothing was regarded wrong or right and their help would aid in improving the prototype. An open card sorting was chosen for not to impose any overlaying structure potentially overriding the local way of categorization. Thus the elders were instructed to group the cards as they preferred.

The sessions were documented with video recordings, still photographs, observation and interview notes. In the case of a participant not being English speaking the interviews were facilitated by a local Herero co-researcher. An independent Herero translator translated the videos after the field trips to minimize translation bias.

Results

We were able to recruit 5 elders in the village. Fig. 2 displays the 5 final card sorts from the study conducted in the village.

The first participant (Vehiha) began by looking at all the cards he was given. After about five minutes, he began placing cards into five, laid out, distinguishable groups with all the cards facing up. He said that the two groups representing the homestead could be piled together since they represent the same. The cards were completely mixed when relating the now four card groups (see Fig. 2 For instance, one card group contained cards depicting the community water pump (where they bring their cattle for water), a young boy, a bull, a dog, a cow, a solar panel and a water tank. He said that there would always be a young man taking care of the cattle and being responsible for herding them between the kraal (animal enclosure) and the water pump. Thus, the young man belonged to that water pump group.

Then Vehiha added a boy to the cattle group (3) and said (trans.): "while the big man is busy at the homestead the others are at the kraal." So the 'boy' card belongs there too.

We asked where we should place a certain tree. He said that specific trees in the bush do not belong in the groups he sorted, but there should be a tree for the boy to rest under at the water pump group, which he then added. The other elders followed the same categorization scheme, and where consistent each in making a group for the homesteads (1) and the water pump (4). Participant 2 (Job) emphasized the layout of the homestead and the order of objects relational to each other situated there. For instance by showing that the man of the homestead per tradition sits to the right of the entrance between the house and the holy fire (see Fig. 2 (b)). This explanation was consistent with similar explanations given by the elders, thus the card sorting method was also a trigger for sharing local customs.

All cards were placed according to location of where the real objects/people represented as cards are functioning or located in relation to each other. Unsurprisingly none adhered to our categorization scheme. It must be stated that all findings across the three sites were showing the same categorization scheme as the results as from the five elders reported here. And for instance when we prompted on where to place the snake (this was a hypothetical card mentioned in the discussion), the participants stated that the snake would not belong in any group, since snakes have no purpose within the village.

Discussion

The open card sorting proved very useful since it was possible to uncover an unexpected categorization scheme. The exploratory nature allowed external researchers to ask follow-up questions ensuring that the rationale behind the categorization was transferred from the participants. It also accommodated through dialogue, that the categorization would need duplicates of some objects per group. This finding is highly interesting since card sorting is often used to categorize an item only one place based on participants' suggestions. Here it was established -contrary to our categorization scheme-that some objects should be multiple places based on participants' suggestions. The set of cards did not have duplicates for all objects, which actually shows how our method was influencing the study with a preconception of dichotomous thinking. For instance if there were more 'fence' cards the participants would have been able to separate the groups more visibly as the places are separated by fences in the village. Conversely, the elders were able to place more than one 'boy' card since we made three 3D models of a boy (thus more cards). Although we gained the feedback in the sessions on this matter, further studies should have more cards of the same type to accommodate the possibility to place many of the same type.

Conclusion

All findings from the categorization tasks point towards a shared local way of grouping the cards. We observed a 'place' based pattern, where objects meaningful to a location are grouped together, such as the 'boy' that looks after the 'cattle' at the 'kraal'. The open card sorting method supported the discovery of a new categorization scheme which we call location-relational. The pattern confirms prior research on Herero's prioritization of place as a meaningful location with activities and protocols attached to [START_REF] Bidwell | Pushing personhood into place: Situating media in rural knowledge in africa[END_REF]. The findings dramatically changes our previous design ideas and future retrieval of interface objects will follow this new categorization scheme for ensuring intuitiveness and preservation of tacit knowledge in the interface.

The full data set is sufficient for the purpose of informing our local design since it is in the hands of this study's elders the HSC will be used. We proceed to refine the study for robustness, investigate the many details left out for this contribution and conduct follow-up studies. We strongly encourage researchers to pursue the data collection and analysis for a sound theory building in this under-researched area.

Fig. 1 .

 1 Fig. 1. A re-created homestead seen from (a) 90-degree tilt or (b) free-roam camera perspective. (a) shows the six categories as default. (b) shows the cattle category expanded.

  (a)) to the six interface categories we implemented in the HSC. It puzzled us that he had actually made a map of the village. The four groups were each representing a place within the village. Marked on Fig. 2 as: 1: [homestead(s)], 2: [group of goats], 3:[cattle kraal] 4: [community water pump area].

Fig. 2 .

 2 Fig. 2. The pictures show the final card sorts by the 5 village elders. Annotations are added to visualize the categorization of cards based on a location-relational categorization scheme.