The virtuality continuum is the range of technologies integrating digital content into the user’s physical reality. The natural environment lies at one end of the continuum and a fully immersive virtual reality (sometimes referred to as a virtuality) at the other.
Augmented reality (AR), at the low end of the continuum, involves digital content superimposed over the user’s environment, typically a live stream of the physical environment on a mobile device although AR content may be also be viewed through a screen such as a windshield or visor.
Holography, a little further along the continuum, makes it possible to place 3D virtual entities and objects in physical space. A digital twin of a physical object can provide a real-time view to the current state of that object, enabling, for example, monitoring of an engine in an operating vehicle.
The CAVEman, another example of holography, was the first 4D human atlas: an interactive, object-oriented model of a human body consisting of over 3,000 anatomically correct, catalogued and computerized body parts. Viewers could walk around the model and view it from all angles. 4D includes the three spatial dimensions plus time, which allowed researchers to simulate the progression of a disease or the effects of a treatment over a period of time.
Mixed reality (MR) is an integration of digital content with the physical world in a way that enables interaction between elements in them. Depending on the particular application, someone playing an MR video game could pick up a real-world object to hit a game-based monster and see the monster react to the hit.
Virtual reality presents a total virtuality, replacing the user environment completely. Immersive virtual reality represents the end point of the continuum. A fully immersive VR environment would have to enable unconstrained movement and provide effective stimuli for all the human senses: Not just vision and sound but also touch, motion, proprioception, smell and taste. Like the holodeck from Star Trek, that level of virtuality would involve environments that were indistinguishable from physical reality, but they could also include elements that were not physically possible.
Although completely immersive VR might seem an end goal, it is unlikely that level of technology could be accomplished with current software and hardware. Furthermore, holodeck-level VR may not be desirable, because even if users enter the environments knowingly, such convincing presentations of simulated realities could have negative and unforeseeable psychological consequences.
Paul Milgram introduced the concept of the continuum in his 1994 paper, "Augmented Reality: A class of displays on the reality-virtuality continuum.”