Visual: world-fixed vs. user-fixed

Now consider adding a visual display. You might not worry much about the perceived location of artists and instruments while listening to music, but you will quickly notice if their locations do not appear correct to your eyes. Our vision sense is much more powerful and complex than our sense of hearing. Figure 2.7(a) shows a CAVE system, which parallels the surround-sound system in many ways. The user again sits in the center while displays around the periphery present visual stimuli to his eyes. The speakers are replaced by video screens. Figure 2.7(b) shows a user wearing a VR headset, which parallels the headphones.

**Figure 2.7:** (a) A CAVE VR system developed at Teesside University, UK. (b) A 90-year-old woman (Rachel Mahassel) wearing the Oculus Rift DK1 headset in 2013.
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Suppose the screen in front of the user's eyes shows a fixed image in the headset. If the user rotates his head, then the image will be perceived as being attached to the head. This would occur, for example, if you rotate your head while using the Viewmaster (recall Figure 1.29(b)). If you would like to instead perceive the image as part of a fixed world around you, then the image inside the headset must change to compensate as you rotate your head. The surrounding virtual world should be counter-rotated, the meaning of which will be made more precise in Section 3.4. Once we agree that such transformations are necessary, it becomes a significant engineering challenge to estimate the amount of head and eye movement that has occurred and apply the appropriate transformation in a timely and accurate manner. If this is not handled well, then users could have poor or unconvincing experiences. Worse yet, they could fall prey to VR sickness. This is one of the main reasons why the popularity of VR headsets waned in the 1990s. The component technology was not good enough yet. Fortunately, the situation is much improved at present. For audio, few seemed to bother with this transformation, but for the visual counterpart, it is absolutely critical. One final note is that tracking and applying transformations also becomes necessary in CAVE systems if we want the images on the screens to be altered according to changes in the eye positions inside of the room.

Now that you have a high-level understanding of the common hardware arrangements, we will take a closer look at hardware components that are widely available for constructing VR systems. These are expected to change quickly, with costs decreasing and performance improving. We also expect many new devices to appear in the marketplace in the coming years. In spite of this, the fundamentals in this book remain unchanged. Knowledge of the current technology provides concrete examples to make the fundamental VR concepts clearer.