Most of the mechanics used in desktop experiences can also be used in VR. However, not all of them are best choices for VR experiences. Two of the most prominent examples are locomotion and interaction mechanics. In this section, we will briefly explore the different locomotion and interaction mechanics that are designed specifically for VR experience.
Locomotion can be defined as the ability to move from one place to another. There are many ways in which locomotion can be implemented in games and other virtual experiences. Depending on the employed camera perspective and movement mechanics, the users can move their viewpoint within the virtual space in different ways. Obviously, there are fundamental differences in locomotion possibilities when comparing 2D, 2.5D, and 3D experiences. Even within the category of 3D experiences, locomotion can take many different forms. To give you a very general comparison, consider the following:
Locomotion in 2D games is limited to the confines of the 2D space (X and Y axes). The camera used in 2D games employs an orthogonal projection. Therefore, the game space is seen as “flat”. In these experiences, users can move a character using mechanics such as “point and click” (as is the case in the most 2D real-time strategy games) or using keystrokes on a keyboard or other types of controllers. The movements of the orthogonal camera in these experiences is also limited to the confines of the 2D space. Consequently, the users will not experience the illusion of perceiving the game through the eyes of the character (e.g. Cuphead [1], Super Mario Bros, [2] etc.). Evidently, the same type of locomotion can be employed in the 3D space as well. For instance, in the City Builder game, we have seen in this lesson, the camera uses a perspective projection. However, the locomotion of the user is limited to the two axes of X and Z. The three-dimensional perspective of the camera (viewpoint of the user), however, creates an illusion of depth and a feeling of existing “in the sky”. In more sophisticated 3D games, such as First-person shooters (FPS [3]) where it is desired that the players experience the game through the eyes of their character, the feeling of movement is entirely different. We stress the word “feeling” since the logic behind translating (moving) an object within a 2D space compared to a 3D space from one point to another is not that different. However, the differences in the resulting feeling of camera movements are vast (unattached distant orthogonal vs. perspective projection through the eyes of the character). In many modern games (not necessarily shooters) with a first-person camera perspective, the players can be given six degrees of freedom for movement and rotation.
The FPS Controller we used in the previous lessons is an example of providing such freedom (except for rotation along the Z-axis). The mechanics for the movement in such games, however, are almost all the time through a smooth transition from one point to another. For instance, in the two different locomotion mechanics we used in this course (FPS Controller, and camera movement) you have seen that we gradually increase or decrease the Position and Rotation properties of the Transform component attached to GameObjects. This gradual change of values over time (for as long as we hold down a button for instance) creates the illusion of smoothly moving from one point to another.
As was previously mentioned, there are many ways in which locomotion can be realized in virtual environments, depending on the type and genre of the experience, and the projection of the used camera. Explaining all the different varieties would be outside the scope of this course. Therefore, we will focus on the one that is most applicable in VR.
The experience of Virtual Reality closely resembles a first-person perspective. This is the most effective way of using VR to create an immersive feeling of perceiving a virtual world from a viewpoint natural to us. It does not come as a surprise that in the early days of mainstream VR development, many employed the same locomotion techniques used in conventional first-person desktop experiences in VR. Although we can most definitely use locomotion mechanics such as “smooth transition” in VR, the resulting user-experiences will not be the same. As a matter of fact, doing so will cause a well-known negative effect associated with feelings such as disorientation, eyestrain, dizziness, and even nausea, generally referred to as simulator sickness.
According to Wienrich et al. “Motion sickness usually occurs when a person feels movement but does not necessarily see it. In contrast, simulator sickness can occur without any actual movement of the subject” [1]. One way to interpret this is that simulator sickness is a form of physical-psychological paradox that people experience when they see themselves move in a virtual environment (in this case through VR HMDs) but do not physically feel it. The most widely accepted theory as to why this happens is the “sensory conflict theory” [2]. There are, however, several other theories that try to model or predict simulator sickness (e.g. the poison theory [3], the model of negative reinforcement [4], [5], the eye movement theory [4-5], and the and the postural instability theory [6]). Simulator sickness in VR is more severe in cases where the users must locomote, particularly using smooth transition, over a long distance. As such, different approaches have been researched to reduce this negative experience. One approach suggested by [1] is to include a virtual nose in the experience so the users would have a “rest frame” (a static point that does not move) when they put on the HMD.
Other approaches such as dynamic field of view (FOV) reduction when moving or rotating have also shown to be an effective way to reduce simulator sickness.
In addition to these approaches, novel and tailored mechanics for implementing locomotion, specifically in VR, have also been proposed. Here we will list some of the most popular ones:
Other forms of teleportation include adding effects when moving the user’s perspective from one location to another (e.g. fading, sounds, seeing a project of the avatar move, etc.), or providing a preview of the destination point before actually teleporting to that location:
Another interesting and yet different example of teleportation is the “thrown object teleported”, where instead of pointing at a specific location, the user throws an object (using natural gestures for grabbing and throwing objects in VR as we will discuss in the next section) and then teleport to the location where the object rests.
There are many other locomotion mechanics for VR (e.g. mixing teleportation and smooth movement, run in-place locomotion, re-orientation of the world and teleportation together, etc.) that we did not cover in this section. However, the most popular and widely used ones were briefly mentioned.
[1] C. Wienrich, CK. Weidner, C. Schatto, D. Obremski, JH. Israel. A Virtual Nose as a Rest-Frame-The Impact on Simulator Sickness and Game Experience. 2018, pp. 1-8
[2] J. T. Reason, I. J. Brand, Motion sickness, London: Academic, 1975.
[3] M. Treisman, Motion Sickness: An Evolutionary Hypothesis” Science, vol. 197, pp. 493-495, 1977.
[4] B. Lewis-Evans, Simulation Sickness and VR-What is it and what can developers and players do to reduce it?
[5] J. J. La Viola, "A Discussion of Cybersickness in Virtual Environments", ACM SIGCHI Bulletin, vol. 32, no. 1, pp. 47-56, 2000.
[6] G. E. Riccio, T. A. Stoffregen, "An ecological theory of motion sickness and postural instability", Ecological Psychology, vol. 3, pp. 195-240, 1991.
Links
[1] http://www.cupheadgame.com/
[2] https://supermariobros.io/
[3] https://www.callofduty.com/
[4] https://upload.wikimedia.org/wikipedia/commons/f/fa/6DOF_en.jpg
[5] https://commons.wikimedia.org/wiki/File:6DOF_en.jpg
[6] https://www.youtube.com/channel/UCG08EqOAXJk_YXPDsAvReSg
[7] https://www.tomshardware.com/news/columbia-university-vr-sickness-research,32093.html
[8] https://www.youtube.com/watch?v=_p9oDSeUaws
[9] https://giphy.com/gifs/teleportation-KHiHRHPJ27SCgIBGW9?utm_source=media-link&utm_medium=landing&utm_campaign=Media%20Links&utm_term=
[10] https://www.youtube.com/channel/UCx78qBGQl-oCvGwX-MaVRgA
[11] https://giphy.com/gifs/teleportation-with-preview-hTICiMnGGKczkyoiYc?utm_source=media-link&utm_medium=landing&utm_campaign=Media%20Links&utm_term=
[12] https://www.youtube.com/channel/UCJLFizUO3gCQnyiYKMYvUgg
[13] https://www.youtube.com/channel/UCPhckk25N7P8OI580ucRidA
[14] https://www.youtube.com/channel/UCIeaxRCoSLFLHvrCInghqcw