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Certain things are shared in common among all of the many inheritors of MUD1, and these things make up the core of what a mud is—and therefore, what a virtual world is today. These are:
1. A spatial representation of the virtual world
2. Avatar representation within the space
3. A sandbox to play in that offers persistence for some amount of the data represented within the virtual world
Anything that meets these criteria can profitably be called a mud for the sake of examining it and comparing features in the design to things that aren’t ashamed to use the term. Those who are ashamed to use the term have seized on terms like “persistent world” (useful in that it informs the novice as to the nature of the beast) and the truly hideous “MMORPG” acronym, standing for “massively multiplayer online role-playing game.” I suspect that we will find this term falling away over the next few years as we begin to develop persistent worlds that are not explicitly role-playing games. MMORPG in particular is a game industry term which undoubtedly arose from a couple of factors—the desire to avoid the term “mud” (though one of the first major commercial graphical endeavors, Meridian 59, in fact advertised itself as “the first graphical commercial MUD”—it wasn’t, since that honor probably belongs to the original Neverwinter Nights on America Online) and the need to explain the difference between the multiplayer games—including role-playing ones such as Diablo--that supported 32 players or less, and the mud-like games that supported a minimum of 150.
One is tempted to use the term “virtual reality” except for two key facts: one, that the VR field has developed definitions of its field of endeavor that do not accord with the topic of building multi-user virtual spaces; and two, that the VR field has made little appreciable progress in tackling the issues underlying shared virtual spaces. This is not to say that virtual reality research has been useless—rather, that it does not have direct applicability to the problems that by and large will be discussed in this book.
The name “mud” itself is subject to a fair amount of debate. Its original use was as an acronym for Multi User Dungeon and later, Dimension or Domain. But by now, it’s best used as a word, in lower-case, in order to avoid the religious wars that come with the issues of whether something is a MUD, a MUSH, a dungeon game, another type of space, etc. There’s a bewildering amount of these acronyms (and religious wars) and by and large they are only useful to know about for identifying certain broad types of mud designs. Throughout this book, I’ll use “mud” interchangeably with “virtual spaces” and other such terms.
In general, muds are client-server applications, with the client merely being some manner to display the culled and collated output of the server to a particular user connected via an IP protocol. The fact that muds generally fit this sort of pattern, however, does not mean a mud has to be that way. Rather, it is that way because it is the best practice for achieving the design goals of a mud without sacrificing the security of the world’s data.
Muds do not have to visibly reflect any of the heritage of the past (particularly text muds, which originated the genre) in terms of their interface and appearance. As basic definition for what a mud is, let’s take the following statement:
A mud is a spatially based depiction of a somewhat persistent virtual environment, which can be experienced by numerous participants at once, who are represented within the space by avatars.
This is different from much of virtual reality research, which often fails to tackle multiple users, often skips avatars, and is generally lacking in persistence. Prior to Jonathan Steuer’s radical redefinition of the term “virtual reality” in his paper “Defining Virtual Reality: Dimensions Determining Telepresence,” the definitions were, as he notes, centered around technological niceties such as VR goggles. Steuer instead defines virtual reality in terms of telepresence:
Telepresence is defined as the experience of presence in an environment by means of a communication medium.
In other words, "presence" refers to the natural perception of an environment, and "telepresence" refers to the mediated perception of an environment. This environment can be either a temporally or spatially distant "real" environment (for instance, a distant space viewed through a video camera), or an animated but non- existent virtual world synthesized by a computer (for instance, the animated "world" created in a video game).
This leads him to his final definition:
A "virtual reality" is defined as a real or simulated environment in which a perceiver experiences telepresence.
In order to better refine this definition, however, Steuer has to make use of terms such as “vividness” and “interactivity.” Roughly, these refer to the manner of presentation of the virtual environment depicted, and the degree to which a user of the space can affect the content of the space. In this scheme, muds place midway up the vividness scale and all the way over on the interactivity scale. Of course, we know full well that muds have not yet reached anywhere near their full potential in interactivity. Steuer also rates graphical muds higher in vividness, and graphical muds with sound higher yet. Interestingly, the highest rated elements that fulfill the convergence of vividness and interactivity are fictional: Gibson’s cyberspace, Bradbury’s children’s playroom from the science fiction short story “The Veldt,” and of course, Star Trek’s Holodeck. Janet Murray, the hypertext and interactive storytelling theoretician at MIT’s Media Lab entitled her classic book on the subject Hamlet on the Holodeck.
For my purpose, I am going to immediately discard vividness as a factor in the definition of a virtual world. There are far too many systems in which a user can experience a sense of telepresence with a highly vivid system (meaning, one with a reasonably high degree of fidelity in representing all types of sensory inputs—visual being the key one, of course). As Steuer himself notes, users have reported the sensation of telepresence with chat rooms and webpages. Frankly, all of the most interesting challenges in designing virtual environments have nothing at all to do with vividness or client-side representation. And of course, for a true virtual reality, to my mind, you have to have other people participating in the space at the same time, which Steuer’s definition does not require. Lastly, of course, the idea of a virtual reality being a “spatially distant ‘real’ environment... viewed through a video camera” is simply not helpful and is clearly outside the field.
It’s worth noting that there are games such as Diablo and Quake which offer virtual spaces with multiple participants and persistence of data (indeed, Quake and other first-person shooter games offer a higher degree of vividness than any other sort of virtual space available today). One can point at the hard cap of 32 maximum participants, at the fact that the space does not persist, but rather is recreated every game, at the session-based nature of the gaming experience, and other such things that do not meet up with the expectation of a mud—but there’s only so much value to be had from the exercise. The definition offered is not intended to completely pin down what a persistent world, MMORPG, or mud is going to be. There are gray areas and dragons lurk in the less-explored fringes of the definition.
 A protocol such as Telnet, raw TCP/IP, or UDP. Culling in this context means that the server has a great deal information about the world, and needs to decide which bits it will send to an individual user. The key here is deciding what not to send. Collating means that this information generally needs to be gathered from a number of different subsystems within the server (such as ongoing updates, movement, the activity of other players, etc) and must be organized into coherent output of some sort.
 http://www.cyborganic.com/People/jonathan/Academia/Papers/Web/defining-vr1.html has an electronic copy of the paper. It was published in the Journal of Communication, 42(4) (Autumn, 1992), 73- 93.