Immersive display systems, on the other hand, do not simulate cameras, but how (human) observers see the real world. As opposed to regular 3D graphics, which presents virtual photographs to users, immersive displays treat the display as a portal into a virtual world. To create convincing, distortion-free displays, the degrees of freedom available for virtual cameras must be tied to physical constraints. As an example, consider a typical desktop setup with a user sitting about two feet away from a 20" flat-panel monitor (see Figure 1). This results in an actual field-of-view of about 37°; much smaller than those typically used by virtual cameras in regular 3D graphics. As a result, the image on the screen will appear distorted: spheres close to the display's edges, for example, will appear as ellipsoids. This distortion is particularly obvious when rotating around the viewpoint, and can even lead to motion sickness. To properly match a 90° field-of-view, the user would have to move very close to the monitor. This, by the way, is the fundamental reason why immersive virtual reality environments typically use very large screens (CAVEs), or small screens very close to the viewer's eyes (HMDs): those are the only ways to create wide fields-of-view without distortion.
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Figure 1: Plan view of typical desktop system, where a user (left) is sitting 24" away from a 20" flat-panel monitor (right). The user's actual field-of-view is much smaller than the 90° typically used in regular 3D graphics applications, such as games. |
According to this model, Vrui treats every environment, even desktops, as immersive environments defined by a collection of screens and viewers. A desktop system typically has one fixed viewer and one fixed screen; a CAVE-like system has several fixed screens and a single head-tracked viewer; and an HMD-based system has a head-tracked viewer with two small screens at fixed positions relative to the viewer.