The virtual world could be moved while the user remains stationary. For example, once drawing on the first virtual easel is complete, the virtual world could translate and rotate until the second virtual easel is aligned with the real one. However, this sort of manipulation might be expected to lead to disconcerting self-motion perception. A large discrepancy between a user’s vestibular sense (“I am stationary”) and visual sense (“the world is moving around me or I am moving through the world”) often results in discomfort and even simulator sickness [Pausch et al., 1992]. Ideally, the virtual world would move without these adverse effects.
Razzaque developed Redirected Walking, a technique for enabling users to explore VEs that are larger than the available tracked space [Razzaque, 2005]. It dynamically maps different areas
of the VE onto a smaller real space by injecting a discrepancy between a user’s real-world head rotation and virtual head rotation. During high-frequency head turns, the vestibular system is more sensitive than the visual system [Duh et al., 2004], and the injected discrepant visual rotation can be quite large and go undetected. Even when a user is not turning her head, small amounts of virtual-head rotation can be injected without her noticing. The injected rotational discrepancies cause users to walk along real-world paths that are different from their virtual paths, enabling navigation in larger-than-tracked-space VEs. One of the goals of Redirected Walking is to enable large-VE exploration without increasing the likelihood of simulator sickness. It seemed a reasonable choice for moving the virtual world. Nevertheless, Redirected Walking presents some difficulties in the context of passive-haptic feedback.
Passive haptics require positions in the VE to remain coupled with positions in the real world. Redirected Walking, by its nature, breaks this coupling by rotating the virtual world with respect to the real world. Suppose a real object and a virtual object are aligned. When Redirected Walking rotates the virtual world about the user’s head, the two objects become misaligned (Figure 2.3).
Virtual
Real
Physical prop
Figure 2.3: When the virtual world is rotated, real and virtual objects become misaligned.
Suppose the five virtual easels and the single real easel in Figure 2.1 are arranged such that they lie on a circular arc and that a user is standing at the center of the circle (Figure 2.4a). Assume the user remains stationary except for turning her head back and forth. As she turns her head, the virtual world is rotated such that one of the virtual easels aligns with the real easel (Figures 2.4b–c). The user can then approach the aligned easel and draw on it (Figure 2.4d).
Now suppose the user wanted to draw on the next virtual easel. If she remains in front of the current easel and turns her head, no amount of rotation about her head will align the real easel with the next virtual easel (Figures 2.4e–f). She would first need to return to the original circle center and then turn her head.
user virtual
easels
real easel
(a) A user stands in the center of a circle on which a real easel and five virtual easels lie.
virtual easels
real easel
(b) The user turns her head back and forth and the system rotates the virtual world about her head.
virtual easels
real and virtual easels aligned
(c) One of the virtual easels is now aligned with the real easel.
virtual easels
real and virtual easels aligned
(d) The user walks to the aligned easel to draw on it.
virtual easels
real and virtual easels aligned
(e) The user turns her head back and forth again and the virtual world rotates about her head.
virtual easels
real and virtual easels misaligned
(f) The incorrect rotation origin leads to real-virtual misalignment.
Figure 2.4: Rotating the virtual world around the user’s head
Thus far, the easels have all been oriented towards the center of the original circle. Suppose instead that the easels all faced in the same direction (Figure 2.5a). Aligning objects via virtual-world rotation is now much more difficult. Even though the easels still lie on the same circle, rotation about the circle center can achieve only positional alignment, and not rotational alignment (Figure 2.5b).
virtual easels
real easel
(a) All easels face the same direc- tion.
virtual easels
positional but not rotational alignment
(b) Virtual-world rotation about the user’s head leads to rotational mis- alignment.
All of this discussion has ignored the fact that users do not typically remain perfectly stationary, and that there may be other real and virtual objects in the environment, dramatically increasing the number of constraints imposed. It is theoretically possible to find a sequence of rotations about different points in the environment that would eventually align another virtual object onto the same real object. The problem can be cast as a motion-planning problem:
Given a user’s start pose, desired end pose near a virtual object, and sets of real and virtual objects, determine a real- and virtual-collision-free path between the start and end poses that aligns the desired virtual object with a real object; the virtual world can be rotated about the user’s head as she walks along the path.
Assume such a path is found for a given environment. If the path were to be followed by a robot, one could command the robot to follow the path. A human user, however, is less likely to follow commands exactly. Users typically want to explore more freely; even if a user is somehow enticed to follow the predetermined path, she is unlikely to do so perfectly. It is not clear under what circumstances all constraints can be satisfied. To test the basic concept, I simplified the problem.