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# Types of vection vection can be caused by any

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Types of vection Vection can be caused by any combination of angular and linear velocities of the viewpoint in the virtual world. To characterize the e ff ects of di ff erent kinds of motions e ff ectively, it is convenient to decompose the viewpoint velocities into the three linear components, v x , v y , and v z , and three angular components, ω x , ω y , and ω z . Therefore, we consider the optical flow for each of these six cases (see Figure 8.15): 1. Yaw vection: If the viewpoint is rotated counterclockwise about the y axis (positive ω y ), then all visual features move from right to left at the same velocity. Equivalently, the virtual world is rotating clockwise around the user; however, self motion in the opposite direction is perceived. This causes the user to feel as if she is riding a merry-go-round (recall Figure 8.2). 2. Pitch vection: By rotating the viewpoint counterclockwise about the x axis (positive ω x ), the features move downward at the same velocity. 3. Roll vection: Rotating the viewpoint counterclockwise about z , the optical axis (positive ω z ), causes the features to rotate clockwise around the center of the image. The velocity of each feature is tangent to the circle that contains it, and the speed is proportional to the distance from the feature to the image center. 4. Lateral vection: In this case, the viewpoint is translated to the right, corresponding to positive v x . As a result, the features move horizontally; however, there is an important distinction with respect to yaw vection: Fea- tures that correspond to closer objects move more quickly than those from distant objects. Figure 8.15(d) depicts the field by assuming vertical posi- tion of the feature corresonds to its depth (lower in the depth field is closer). This is a reappearance of parallax , which in this case gives the illusion of lateral motion and distinguishes it from yaw motion. 5. Vertical vection: The viewpoint is translated upward, corresponding to positive v x , and resulting in downward flow. Once again, parallax causes the speed of features to depend on the distance of the corresponding object. This enables vertical vection to be distinguished from pitch vection. 6. Forward/backward vection: If the viewpoint is translated along the op- tical axis away from the scene (positive v z ), then the features flow inward toward the image center, as shown in Figure 8.15(f). Their speed depends on both their distance from the image center and the distance of their cor- responding objects in the virtual world. The resulting illusion is backward motion. Translation in the negative z direction results in perceived forward

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8.4. MISMATCHED MOTION AND VECTION 233 (a) yaw (b) pitch (c) roll (d) lateral (e) vertical (f) forward/backward Figure 8.15: Six di ff erent types of optical flows, based on six degrees of freedom for motion of a rigid body. Each of these is a contributing component of vection.
234 S. M. LaValle: Virtual Reality motion (as in the case of the Millennium Falcon spaceship after its jump to hyperspace in the Star Wars movies).

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