In the last two years since Linden Lab released the source code for their client, a vibrant open source community has grown around the Second Life protocol. With projects such as the first version of realXtend, the possibility of moving beyond the limitations of that protocol are starting to be explored. Features such as inverse-kinematics-based avatar control and mesh-based prims extend the platform in ways that go beyond the capabilities of Second Life. There are also parallel open source virtual world efforts, such as Qwaq (based on Open Croquet), Sun’s Wonderland platform, and the Web3D/X3D set of formats and runtime tools.
One of my core interests in this space has been to find a way to bring some of my and other’s work in robotics, AI, and advanced simulation into this arena of shared virtual worlds. There are numerous reasons why this would be beneficial. Firstly, it would allow researchers to leverage a common set of tools, fostering collaboration and improving our ability to reproduce and critique each other’s work. Perhaps even more importantly, it could open the way for large numbers of people to interact with virtual entities (AI’s, if you prefer). This is a potentially major advance in AI research, as it represents a real-world, renewable source of training data, which many researchers think is critical for advancement in the field. It would also be ultra cool.
(Editor’s note: See also earlier article What do virtual worlds have to offer AI?)
Before I describe this work and the unique requirements it presents, let’s clarify some of the jargon. Linden Labs uses the term ’sim’, short for simulator, to signify a portion of virtual space supported by Linden’s servers. Opensim, the open-source, mostly Second Life-compatible server, inherits this usage. When I talk about simulation and simulators, I mean something quite different. In my work in robotics, I build software to simulate complex robots. This software uses some of the same underlying components as a Second Life or Opensim simulator – a physics engine, 3D representation of objects, and so on. However, the level of detail is quite different. The simulators I use in my robotics work are generally operating at a much higher degree of realism than a realXtend or SecondLife vehicle or avatar. They require proportionally greater resources in terms of CPU and memory, and of course the development time to get a simulation of this sort working is considerable.
While my own work in robotics requires a high degree of physical realism, there is work being done in this field that may have different requirements, such as robot swarms, where navigation and interaction are more important than physical realism. There is also a body of AI research into subjects such as language comprehension that may not need physical realism at all. However, my work is on physically realistic simulation, so I will be focused on that specific issue in this article.
There are several related issues that come up in the course of attempting to integrate ’serious’ simulation into the Second Life-derived model:
Basic Architecture
In Second Life (and related platforms), the user’s visual representation of the world consists of three major parts: the background (land, sky, sea); a set of objects (called ‘prims’ in Second Life); and avatars, representing the human (or possibly scripted) players. These three major elements have very different architectures, and present an almost mutually exclusive set of features to the system. Putting aside the background elements, let’s look at the prim and avatar systems.
Prims
Prims are physical objects that are described as a primitive shape (cube, cylinder, pyramid, sphere etc.), with various possible modifications such as cuts, tapers, skews, and hollows. Prims are controlled by the server software. Static prims, which generally don’t move unless they are being edited, make up architectural elements such as walls and other building elements. Other prims are allowed to move around the environment. They can be controlled by the physics engine (balls that roll), scripts (doors), or both (vehicles). They can have arbitrary textures, but do not have the ability to utilize deformable meshes, skeletons, or client-side animation.
Avatars
Avatars are always defined as skeletons with deformable mesh (skin and clothes). Their motion is usually controlled by the user’s input (walk, run, fly etc). They do react to the physics engine, but in a rudimentary way: avatars are represented at the physical level as ‘capsules’, egg or pill-shaped shells that provide basic collision with walls and other avatars. This is an important point: there is no physical representation of, say, your arm extending out and touching an object. Complex avatar motion is achieved through client-side animation, which in our case is based on the .bvh file format (see http://en.wikipedia.org/wiki/Biovision_Hierarchy). Again, it’s important to realize that the animation of an avatar is not reflected at the physics layer, at least not in the present implementation of Opensim.
There is one more subtle but critical point to keep in mind about avatars vs. prims: avatars, by definition, are associated with a user account. Users have abilities that go outside the physical and graphic simulation layer – they can buy and sell things; they can chat with each other, join groups, get banned for TOS violations, and so forth. They represent an entity within the social network of Second Life (or one of the Opensim grids, or the emerging hypergrid). There are complexities – avatars can have ‘prim’-based attachments – but fundamentally, if you want to be part of the Second Life society, you need an avatar.
Users
Users, through their avatars, can interact with each other (chat, give/receive items…). They can interact with prims, by creating and editing them (if they have the permissions), or through menus and scripts. It may seem obvious, but it’s worth pointing out the heirarchy: Prims cannot create or edit avatars (!); they cannot buy, sell, give, or receive assets on their own behalf. They are, by definition, owned and controlled by at least one user, which is by definition represented by a single avatar.
The problems this heirarchy and separation of capabilities poses to those of us working in AI is hopefully becoming evident. If I design a robot using prims, scripts, and physics, there is no obvious way to give that entity the same rights and privileges of a “real” user. I also can’t take advantage of graphics abilities such as skeletons, deformable skin, or animations. On the other hand, if I design a robot in the typical ‘bot’ fashion, by pretending to be a user (using libsecondlife for instance), right at the start I must accept a number of constraints that may not be desirable: I need to look roughly human (because of skeleton constraints); I must have skin; I can’t interact physically at a fine-grained level with the rest of the system; and my behavior must be animated – I cannot generate low-level behavior on-the-fly.
Distributed simulation and synchronization
Second Life started as a proprietary system run by Linden on a local server farm. They made the choice to divide up the simulation by geography; in essence, each 256×256 meter region of land is assigned its own processing stack (set of CPU threads), and operates in parallel with all the other regions. Interactions between region modules are achieved through network communication.
There are obvious objections to this model, the main one being that popular sims get overloaded, and sparsely visited sims waste resources. Some of this can be addressed with load balancing and clever thread management, but in my opinion it represents a fundamental flaw in the Second Life architecture. Putting the resource issue aside for the moment, there is a deeper flaw in this state of affairs. To build a useful and reliable simulated world for AI research, it is critical that you not introduce major defects (artifacts) into the simulation. Learning algorithms are notorious for finding devious ways to “cheat” researcher’s goals. If something in the simulation is non-physical, you can pretty much bet that your genetic algorithm or neural network will find that defect and use it to its advantage, avoiding the more difficult, “honest” approach you are trying to develop.
Boundaries between regions in Second Life, especially in the case where the regions are separated by network lag, are places where the physics of the simulation break down in nasty and difficult to predict ways. One reason for this is that events in Second Life have no concept of operative time. An event is sent over the wire, and the receiving node assumes that this event is happening ‘now’, in relation to that node’s local concept of time. This creates some issues on the client side (perhaps I’ll post about that later), but more fundamentally, it’s a problem at the server level. Region modules in Opensim pass messages back and forth between each other, for instance posting the position, rotation, linear and angular velocity of objects, so that you can visualize what is happening across region boundaries. Since these messages are subject to server lag, each sim in effect has its own version of reality. In my sim, your objects are delayed by the average network lag time between our sims; in your sim, my objects are similarly delayed. Clearly this is not acceptible if our goal is to have a coherent, realistic, detailed simulation of the environment.
Open Croquet, an alternative virtual world platform, has developed a concept of distributed, synchronized simulation that sounds pretty good to me (I haven’t actually deployed it, so I’m taking some of this on faith. Decide for yourself: http://www.opencroquet.org/index.php/TeaTime_Architecture).
In summary, I hope I’ve managed to outline some of the features and requirements that need to be considered for the development of sophisticated, high-fidelity simulated worlds in the context of a massively distributed, shared environment. In the future, I hope to post some ideas on what we can do to support these concepts in future releases of Opensim and realXtend.
Artificial Intelligence, OpenSim, realXtend
agi, avatar, bot, hypergrid, lag, network, opencroquet, OpenSim, physics, qwaq, realXtend, robotics, Second Life, strong ai, vehicles, wonderland
Top left dialog is the server telling that someone is connecting. 
