Advanced 3-D Graphics: GNU Maverik—a VR Micro-Kernel
Is there any middle way possible? Can we construct some framework that supports the creation and management of VEs, without limiting what can be built to some lowest common denominator?
We believe there is, and the result of those efforts is GNU Maverik. A framework for interactive VEs has now been used successfully to implement a range of VE applications with markedly different demands and optimization strategies.
Maverik is a C toolkit that lives on top of a 3-D rendering library such as Mesa or OpenGL, providing facilities for managing a VE. It is best viewed as the next layer of functionality you would want above the raw 3-D graphics library. The novel aspect of Maverik is that it does not have its own internal data structures for the VE. Instead, it makes direct use of an application's own data structures, via a callback mechanism (rather like callbacks are used in window managers such as X11).
An application tells Maverik about the classes of objects it wishes to render, and supplies functions needed to do the rendering. Maverik supplies facilities for building flexible spatial management structures (SMSes) with which the registered objects are managed. As the user navigates the environment, Maverik tracks the SMSes and makes appropriate calls on the application to render its objects.
The application keeps control of its data, so there is only one copy to manage. More crucially, the application can bring all its knowledge of what the data means to bear when deciding how to render the objects; we find this is where the most telling optimizations come from. For example, if 3-D text is important to the application, then a good data structure would probably involve text strings and position information. However, exporting the 3-D text to a self-contained VE package will probably mean exporting the data in a common graphics format—usually polygons. The VE system will have a lot more data to process than we would like, which limits the size of environment we can construct.
With Maverik, the application might register the class “3-Dstring”. Later, when the user navigates around the environment, Maverik determines from the SMSes that certain objects are in view. For 3-Dstring objects, it calls the application supplied functions to render those, and the application then generates the polygons on the fly. The application can use whatever optimizations are appropriate when generating the polygons, which makes techniques such as dynamic level of detail, and coping with objects that change their shape, straightforward. In this way, Maverik can be a general-purpose framework, and yet benefit from optimal data representations and highly application-specific optimizations.
The 3-D text example is part of a real application—the “Legible City II”, a multimedia artwork by Jeffrey Shaw (ZKM Germany). It uses the above technique to render three large cities (Amsterdam, Manhattan and Karlsruhe) of words (see Figure 3). To render a more game-like environment, the application callbacks will use the more traditional games techniques to render their objects (see Figure 4).
For the Oil Rig, the CAD application's native data structures (pipes, valves, walkways and so forth) are used in a similar way. When the application is called to render a tower, it can decide what would be an appropriate rendering for this frame—for example, it may decide to always render the tower, because it is an important landmark for navigation. Thus, the application can change representations dynamically to suit conditions. Similarly, applications with special lighting algorithms (see Figure 5), or atypical behaviours such as abstract information visualization (see Figure 6) use their own bespoke structures and algorithms to maintain real time performance.
Examples of work done with Maverik are given in the Maverik gallery on the web page. The important feature is that the video sequences (which are quite large files) are all captured directly from a 450MHz PIII Linux PC using a Voodoo2 3-D accelerator, and give a good impression of what performance can be attained with a cost-effective hardware setup.
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