Results and Discussion

Judging the actual grasping, the experts stated that they were able to select objects very intu- itively and robustly with the help of the grasping heuristics. Problems occurred with smaller objects due to occlusion. Interestingly, it turned out that grasping the objects with a tip pinch worked best for the users, considering it provides least occlusion and the most precise way of interaction.

Concerning the release of an object, the experts considered it mandatory to carefully balance the stop condition parameters. Sometimes objects appeared to be sticky and sometimes objects tended to follow the finger motion even though the users already opened their hand with the

Chapter 5. Evaluation

intention to end the grasp. The threshold of the first stop condition (see Section 3.1.3) was responsible for both of these effects. Given a generous threshold, the grasping heuristics are not able to rapidly detect the users’ grasping intentions. However, a small threshold can result in unstable grasp continuity due to jitter and imprecise finger motion. In both cases unacceptable releasing behavior is induced having the potential to quickly annoy the users. This trade-off could be solved by a calibration of this parameter or a careful adjustment per object. However, experts became quickly proficient with the technique and precisely released objects by dropping them carefully. Furthermore, the second stop condition was appreciated as a definite interaction stop in cases where objects appeared to be sticky.

The motion of the objects as a reaction to finger collisions was judged to be very plausible and realistic. In particular, the precise response to very fine grained motion of the finger tips fascinated the experts and was highly recommended. Multi-hand interaction was successfully tested by passing the bottle from one hand to the other. The same would be possible with our method for two or more users in a multi-user stereo setup.

The push interaction was also judged to be very intuitive since object reaction immediately follows a collision. However, the experts complained about the missing mass and inertia of the objects. At first sight it was unrealistic for them that objects did not move according to their size and mass when touched by the user. Also the missing mass and inertia of virtual objects that are grasped can appear to be unrealistic. However, unless appropriate haptic feedback de- vices are available, it is not possible to display forces coming from the virtual environment to the user. Without this haptic rendering, pseudophysical feedback of the objects taking their dynamic properties into account would appear to be unrealistic due to the direct character of the interac- tion metaphor. However, it could be interesting to think about pseudophysical force rendering as it was shown for indirect manipulation by L´ecuyer et al. [LCR+00].

With direct finger-based interaction, the experts were able to judge the accessibility of objects. In contrast to the indirect method they were used to — involving an input device such as the Flystick — they were able to select the objects directly with their hands. Moreover, they found the direct method to be more intuitive, since no button assignment needed to be learned. Due to the direct interaction involving the hands and arms of the user, the experts identified a potential range of new applications for these techniques. This range included assembly simulations which need to consider the clearance of the human hand — especially concerning car maintenance — to the assessment of ergonomics issues in the car interior. Furthermore, the experts stated that this interaction would enrich immersive design and concept reviews due to the increased interactivity and immersion, enabling a more realistic experience of the virtual model. However, the direct object selection and release method was stated to be less precise than pressing and releasing the button of an input device.

An interesting question raised was, if the experts would prefer to use a virtual representation of their own hands during interaction. Despite being occluded by the real hands, the virtual hands can be seen by the users. The reason for this is that a simple hand model was used, having average finger diameters and a solid palm. This visual feedback was preferred by half of the experts, giving them the opportunity to judge their virtual fingers’ relation to the objects. The other half preferred to interact without a hand representation. They appreciated that they were

Chapter 5. Evaluation

able to concentrate on the virtual object without being distracted by the virtual hand representa- tion. They further noticed that because they were not tempted by the virtual hand model to look at their real hands, they had fewer problems with the focus and convergence mismatch. Since the expert review did not reveal a clear preference, a further investigation of the influence of the virtual hand representation on grasping robustness became necessary.

Finally, the experts remarked that for the assessment of object clearance in very complex maintenance and assembly scenarios, object-object interactions through collisions are required. These object-object interactions cannot be realized with the pseudophysical approach presented here. These complex interaction processes cannot be realized without simulating the physical interplay between the users and the objects.

5.1.3 Conclusion

The expert review was performed for evaluating the usability of direct finger-based interaction within a test application. It proved that the experts were able to reliably and intuitively interact with the virtual objects. The high realism and intuitiveness of this method were explicitly em- phasized by the subjects. The applicability for virtual functional assessments of the car interior were explicitly emphasized by the users as well as the potential to enrich assembly simulations and concept reviews. Difficulties occurred with reliably grasping small shapes. These objects, such as the light switch, turned out to be challenging for direct interaction, as was expected. These challenges were the target of the hardware enhancements which will be evaluated in the next chapter. An interesting issue was that the experts were ambivalent concerning the benefit of a virtual hand representation for interaction. This issue will also be further evaluated in the next chapter.

Chapter 5. Evaluation