The PHANToM Used without Visual Guidance1 Gunnar Jansson and Katarina Billberger Department of Psychology, Uppsala University, Box 1225, SE-751 42 Uppsala, Sweden
[email protected],
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Abstract. The use of a PHANToM without visual guidance, as by blind people, means a heavier load on haptics. The aim of the paper was to compare accuracy and speed in identifying small virtual 3D objects explored with the PHANToM and analogous real objects explored naturally. The results for virtual objects were far from the results for real objects, which indicates that there is a large potential for improvements including training in the use of the specific exploration methods.
1
Introduction
A main criterion concerning the usefulness of a haptic display such as the PHANToM is to what extent it adds to the visual (and auditory) experience of the virtual scenes. For people lacking vision the usefulness of the device must be judged by partly different criteria, as haptics has to work without visual guidance.
2
Exploration Methods
Exploration is an important part of the perception process of all senses, but it is particular apparent concerning haptics. Naturally, all the fingers on both hands are often simultaneously involved, and there are many different exploration methods depending on kind of information sought for [1]. At each finger some extended skin area is in contact with the object which also provides information. Exploration of virtual objects with the aid of a haptic display, such as the PHANToM, is different. Information at each moment is restricted to one point of the surface of the virtual object. The observer contacts it with either a stylus or a thimble. The stylus is hold by several fingers, and information is obtained from different skin areas as well as from many joints, tendons and muscles that together get their information via the one point of contact between the virtual object and the end of the stylus. The information obtained over time is probably very important. When the thimble is used only one finger is involved. The spatially distributed pressure on the finger tip within the thimble does not carry any information about the form of the virtual object as the latter is represented at each time by only one point. 1
This study was made possible by grants from the Swedish Council for Research in Humanities and Social Sciences and from the Swedish Transport and Communications Research Board. The authors are indebted to Lars-Erik Larsson for technical assistance.
Even if information is obtained from (changing) muscles, tendons and joints the loss of cutaneous information may mean an important restriction [2] [3].
3
3D Form
In order to be useful, a method of rendering virtual objects must allow the observers to perceive the 3D form of objects accurately and swiftly. The aim of this experiment was to compare accuracy and speed of judging simple geometric forms explored with the two PHANToM options with judgements of real objects explored naturally. 3.1
Method
Participants. Twelve paid sighted university students (10 women and 2 men) with a mean age of 24.0 years (standard deviation = 3.6 years). All except one used their right hand for exploration. Virtual Objects. Four simple 3D forms (cube, sphere, cylinder, and cone) were rendered in three sizes (maximum 5, 6, or 7 mm in all dimensions). Each object was located in the middle of a cubic enclosure which was 2 mm larger in all dimensions than each object, thus a “floating” object with at least 1 mm between the object and its enclosure. The relatively small dimensions of the enclosure were motivated by pilot experiments indicating that participants had problems with finding small objects in large enclosures. Both object and enclosure were given zero damping and maximal stiffness. In order to facilitate the participant’s discrimination of object and enclosure the objects were given both static and dynamic friction but the enclosures were given no friction. The same software (ENCHANTER) as in earlier studies [4] was used. Real Objects. Real objects in the same dimensions as the virtual ones were constructed by a professional model maker from layers of thin paper sheets glued and pressed together to make them similar to wood. Design. All participants took part in all three conditions, on separate days within a week. In each condition the four 3D forms in three sizes were presented in random order in four blocks of replications, which gives a total of 48 objects per condition. The order of the three conditions presented to the participant was one of the six possible orders given to two participants each. Instructions. In the conditions with virtual objects the participants were first informed about the PHANToM and safety aspects. Then, they were shown pictures of the 3D forms and asked to confirm their names. After that they were blindfolded, and a standard head protective device common in industry was placed on their head. They explored then a few virtual objects. In the condition with real objects the session started as well with showing pictures, confirming the names and blindfolding the participants. They were then allowed to acquaint themselves with the real objects, optionally using one or both hands (most participants used one hand). In all conditions the participants were instructed to identify the form of the object as fast and
accurately as possible (with equal emphasis on both aspects) and to report their judgement verbally. They were told that maximally 60 sec was allowed per object. Procedure. At the experiment proper, the real or virtual objects were presented one by one and exploration time and verbal answer were recorded. Time measurement started as soon as the exploring point was inside the enclosure of the virtual objects) or the real object was placed in the participant’s hand. It stopped when the participant began to respond. After half the number of virtual objects a short pause was introduced, but there was no need of such a pause in the real object sessions. 3.2 Results. It is apparent from Fig. 1 that the two Phantom exploration methods of virtual objects did not reach the level of the natural exploration method of real objects, neither in accuracy (100 % in all conditions) or speed (means close to 2 sec). Both four-way ANOVAs had all p-values far below .001. That these high statistical significances depend on the differences between the two Phantom and the natural exploration methods was demonstrated by post-hoc Scheffé tests, the p-values again being much lower than .001. According to the same tests there was no significant difference in the dependent variables between the two PHANToM methods (p > .05). Also the size and 3D form variables had the same high significance levels according to the four-way ANOVAs. As Fig. 1 demonstrates exploration times decreased and proportions correct judgements increased with size. According to the
Proportion Correct
Exploration Time (sec)
1,00
40 30
0,90 0,80 0,70 0,60 0,50 5
20
7
9
Size (mm) 10
Stylus 0
Thimble 5
7 Size (mm)
9
Natural
Fig. 1. Exploration time and proportion correct judgements as a function of object size for each of the exploration methods.
post-hoc Scheffé test the results for the four 3D forms were significantly different (p < .01 in all cases), the spheres having the highest proportion correct judgements and the lowest exploration times.
4
Discussion
The results indicate that there is a large potential for improvements of both accuracy and speed in judgement of small simple virtual geometric 3D forms. The proportions of correct judgements are about 70 % of those obtained with natural objects, and the exploration times about 30 sec compared with 2 sec for natural exploration. A comparison of the present results with an earlier study [4] indicates that the size of the objects is a most critical parameter. The objects in the present study were 5, 7 or 9 mm, in the earlier study 10, 50 or 100 mm. The accuracy for the objects in the latter study was higher (for all sizes of spheres even 100 %) and the speed better (exploration time 10 – 20 sec for the 100-mm objects). The two studies combined indicate that the accuracy obtained by new PHANToM users increases from about 70 % correct at the size of 5 mm to 90 – 100 % at the size of 100 mm and the exploration time decreases from about 40 sec to 10 – 20 sec. As the PHANToM exploration methods are new for the observers, training can be expected to improve their performance. An ongoing experiment is testing this prediction. The dissimilarity between judgements of virtual and real 3D form in the present experiment can be compared with the result in an experiment where judgements of virtual and real textures were contrasted [4]. The judgements of the coarseness of both kinds of sandpaper were very similar. It is probable that the reason for the difference in results concerning object properties is that judging 3D form is a more difficult task than judging texture. When exploring textures movements in one dimension may be sufficient, but much more complicated exploration patterns are needed when 3D forms are judged. This suggests again that training may be useful. It should be noted that the two PHANToM options, stylus and thimble, gave similar results both in accuracy and speed, in spite of the differences between them in many respects. This indicates, maybe, that their common feature, the restriction to one point of contact at a time between virtual object and exploratory device, is the most important aspect.
References 1. Lederman, S., Klatzky, R.L.: Hand Movements: A Window into Haptic Object Recognition. Cognitive Psychology 19 (1987) 342-368 2. Jansson, G.: Haptic Perception of Outline 2D Shape: The Contributions of Information via the Skin, the Joints and the Muscles. In: Bril, B., Ledebt, A., Dietrich, G., Roby-Brami, A. (eds.): Advances in Perception-Action Coupling. Éditions EDK, Paris, (1998) 25-30. 3. Lederman, S.J., Klatzky, R.L.: Sensing and Displaying Spatially Distributed Fingertip Forces in Haptic Interfaces for Teleoperator and Virtual Environment Systems. Presence 8 (1999) 86-104 4. Jansson, G., Petrie, P., Colwell, C., Kornbrot, D., Fänger, J., König, H., Billberger,, K., Hardwick, A., Furner, S.: Haptic Virtual Environments for Blind People: Exploratory Experiments With Two Devices. International Journal of Virtual Reality (in press)
