Like a Rolling Stone: Effects of Space Deformation During Linear Acceleration on Slope Perception and Cybersickness

The decoupled relationship between the optical and inertial information in virtual reality is commonly acknowledged as a major factor contributing to cybersickness. Based on laws of physics, we noticed that a slope naturally affords acceleration, and the gravito-inertial force we experience when we are accelerating freely on a slope has the same relative direction and approximately the same magnitude as the gravity we experience when standing on the ground. This provides the opportunity to simulate a slope by manipulating the orientation of virtual objects accordingly with the accelerating optical flow. In this paper, we present a novel space deformation technique that deforms the virtual environment to replicate the structure of a slope when the user accelerates virtually. As a result, we can restore the physical relationship between the optical and inertial information available to the user. However, the changes to the geometry of the virtual environment during space deformation remain perceptible to users. Consequently, we created two different transition effects, pinch and tilt, which provide different visual experiences of ground bending. A human subject study (N=87) was conducted to evaluate the effects of space deformation on both slope perception and cyber-sickness. The results confirmed that the proposed technique created a strong feeling of traveling on a slope, but no significant differences were found on measures of discomfort and cybersickness.