I've been following the thread on semantics with great interest, but it's gone long enough and in enough different directions I'm going to start a new thread for my thoughts.
It's important to note that my background is in agent-based simulation (and, to a lesser extent, search engines), so incorporating semantics into the structure of a scene has always been an explicit goal for VOS. I think the kernel of Kao's argument is that if you have a choice of different dimensions to organize your data, whether based on semantics, rendering efficiency, download time or whatever, that you should choose a semantic layout. Having thought about this a bit, this makes a lot of sense: optimal organization for rendering or downloading can be determined mechanically by software, whereas the semantic information by definition requires intellegence to organize. My experience has been that piling metadata on top of a non-semantic structure tends to lead to a compromise design. Certain parts of the graph need to be flattened and other parts stretched out to fit the semantic structure, leading to a layout that is neither optimal for rendering nor ideal in expressing semantics. Braden's idea of a "CSS for 3D" is an interesting one, however I'm not sure how well the analogy really works. In particular, CSS is supposed to separate content from layout, since most semantic interpretations of the text don't care about where exactly on the page or screen the text appears. Conversely, the 3D environment is explicitly spatial, and semantic information more often than not includes information like paths through the environment, waypoints, sensitive zones, that the object at position X,Y,Z is a tree, etc all of which are based explicitly on a some level of geometric interpretation of the space. To address Kao's question: VOS fits in here at a couple of levels. The first is the basic services it provides. VOS as a distributed programming system tries to split the difference between declarative and imparative programming, by providing a data model in conjunction with a message passing model. The message passing model defines how the object behaves. The data model allows attaching arbitrary objects to other objects, so you can inject metadata at any point in the graph. The data type model also allows for interface aggregation (effectively multiple inheritance) so objects can have a number of different aspects which can be added on to at runtime. Because no distinction is made between metadata and "real" data, the synchronization protocols are the same. The higher level is the structure of the actual 3D scene. This uses the facilities provided by VOS, but as I discussed above, different uses of the scene suggest different layouts. Since the Abstract 3D Layer (A3DL) is the model for 3D data in VOS, we want to ensure that it supports a design that is friendly towards semantic structures. In that vein I'm not sure what the "right" structures are, and is probably somewhat application-specific anyway (although I hope that over time people can get together and agree upon standard datatypes for, say, path networks). The advantage to VOS is that it supports that flexibility, so people can try stuff out without having to turn everything else upside down in the process. One last thing: it just occurred to me is that most other developers of 3D VR applications end up writing their own renderer on top of OpenGL, so there is a natural bias towards designing their data structures to fit their particular rendering loop. VOS from the beginning was intended to be implemented using an existing 3D engine, so we don't care so much about rendering specifics because someone else has already done the work of taking a scene and optimizing it on the fly... -- [ Peter Amstutz ][ [EMAIL PROTECTED] ][ [EMAIL PROTECTED] ] [Lead Programmer][Interreality Project][Virtual Reality for the Internet] [ VOS: Next Generation Internet Communication][ http://interreality.org ] [ http://interreality.org/~tetron ][ pgpkey: pgpkeys.mit.edu 18C21DF7 ]
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