The terms virtual reality and cyberspace have become very popular outside the research community within the last two decades. Science fiction movies, such as Star Trek, have not only brought this concept to the public, but have also influenced the research community more than they are willing to admit. Most of us associate these terms with the technological possibility to dive into a completely synthetic, computer-generated world—sometimes referred to as a virtual environment. In a virtual environment our senses, such as vision, hearing, haptics, smell, etc., are controlled by a computer while our actions influence the produced stimuli. Star Trek's Holodeck is probably one of the most popular examples. Although some bits and pieces of the Holodeck have been realized today, most of it is still science fiction.
So what is augmented reality then? As is the case for virtual reality, several formal definitions and classifications for augmented reality exist (e.g., [109, 110]). Some define AR as a special case of VR; others argue that AR is a more general concept and see VR as a special case of AR. We do not want to make a formal definition here, but rather leave it to the reader to philosophize on their own. The fact is that in contrast to traditional VR, in AR the real environment is not completely suppressed; instead it plays a dominant role. Rather than immersing a person into a completely synthetic world, AR attempts to embed synthetic supplements into the real environment (or into a live video of the real environment). This leads to a fundamental problem: a real environment is much more difficult to control than a completely synthetic one. Figure 1.1 shows some examples of augmented reality applications.
Figure 1.1: Example of augmented reality applications. The glasses, mustache, dragons, and fighter figure are synthetic—(a) and (b) augmenting a video of the real environment; (c) and (d) augmenting the real environment optically. (Images—(a) courtesy of Vincent Lepetit, EPFL ; (b) courtesy of Simon Gibson , Advanced Interfaces Group © University of Manchester 2005; (c) and (d) prototypes implemented by the Barhaus-University Weimar.)
As stated previously, augmented reality means to integrate synthetic information into the real environment. With this statement in mind, would a TV screen playing a cartoon movie, or a radio playing music, then be an AR display? Most of us would say no—but why not? Obviously, there is more to it. The augmented information has to have a much stronger link to the real environment. This link is mostly a spatial relation between the augmentations and the real environment. We call this link registration. R2-D2's spatial projection of Princess Leia in Star Wars would be a popular science fiction example for augmented reality. Some technological approaches that mimic a holographic-like spatial projection, like the Holodeck, do exist today. But once again, the technical implementation as shown in Star Wars still remains a Hollywood illusion.
Some say that Ivan Sutherland established the theoretical foundations of virtual reality in 1965, describing what in his opinion would be the ultimate display :
The ultimate display would, of course, be a room within which the computer can control the existence of matter. A chair displayed in such a room would be good enough to sit in. Hand-cuffs displayed in such a room would be confining, and a bullet displayed in such a room would be fatal. With appropriate programming, such a display could literally be the Wonderland into which Alice walked.
However, technical virtual reality display solutions were proposed much earlier. In the late 1950s, for instance, a young cinematographer named Mort Heilig invented the Sensorama simulator, which was a one-person demo unit that combined 3D movies, stereo sound, mechanical vibrations, fan-blown air, and aromas. Stereoscopy even dates back to 1832 when Charles Wheatstone invented the stereoscopic viewer.
Then why did Sutherland's suggestions lay the foundation for virtual reality? In contrast to existing systems, he stressed that the user of such an ultimate display should be able to interact with the virtual environment. This led him to the development of the first functioning Head-Mounted Display (HMD) , which was also the birth of augmented reality. He used half-silvered mirrors as optical combiners that allowed the user to see both the computer-generated images reflected from cathode ray tubes (CRTs) and objects in the room, simultaneously. In addition, he used mechanical and ultrasonic head position sensors to measure the position of the user's head. This ensured a correct registration of the real environment and the graphical overlays.
The interested reader is referred to several surveys ,  and Web sites , 193] of augmented reality projects and achievements. Section 1.2 gives a brief overview of today's technical challenges for augmented reality. It is beyond the scope of this book to discuss these challenges in great detail.
Spatial Augmented Reality
by Oliver Bimber and Ramesh Ras
A K Peters © 2005 (384 pages)