This year's Game Developers Conference offered ten to twenty parallel tracks of keynotes, lectures, workshops and roundtables, complemented by up to six sponsored sessions, some of which covering a full day -- and all that following two days of full-day tutorials. Such an abundance of offerings forces attendees to cull and prioritize, and as our needs at Loki revolve around portability, I tended to have a distinct preference for OpenGL.

Unfortunately, registration delays left me late for the OpenGL tutorial that started at 10:30, but luckily the organizers of the tutorial offered complete and detailed course notes -- weighing in at a good 150 pages. (The same held true for several others tutorials at the show.) During the day, John Spitzer offered sane ways to maximize performance, and Brad Grantham closed the session with a brief description of OpenGL on Linux. The bulk of the material (and most of the presentation) was given by Mark Kilgard of Nvidia.

By the time I arrived, the room was already full, with quite a few attendees left without chairs as they listened to a description of shadow volume techniques. This part of Kilgard's presentation was kept brief, as it would later be used in his Friday afternoon presentation, "Advanced Hardware and Rendering Techniques." The tutorial subsequently covered translucency and the well-known problems of non-commutative blends, touched on the usefulness and pitfalls of screen space stiple, and moved on to a review of GL fog.

The OpenGL specification allows fog to use depth instead of Euclidean distance, which can lead to problems -- especially with wide fields of view and steep fog functions. As Kilgard pointed out, the GL specification contains several examples where the accurate solution is merely recommended, but alternative implementations that trade small artifacts for potentially huge performance gains are allowed. Another example is perspective-corrected color interpolators. In some cases, GL even specifies features that are inspired more by practicality then modeling; GL EXP2 fog is a good example of this. GL EXP2 fog does not implement a model of atmospheric attenuation, but it's well-suited for effectively hiding a far clipping plane (and as it avoids a square root, it is potentially cheaper to implement for correct distance calculations).

Propagating Euclidian distance through the hardware pipeline is an expensive addition. Kilgard proposed the GL Fog Coordinate extension as a possible solution, which puts the burden of calculating the proper fog distance on the application, but empowers it to implement layered (e.g., altitude dependant) fog. In general, GL extensions played a prominent role in the tutorial as well as GL-related lectures at the conference, culminating in an OpenGL Extension safari in the afternoon of the tutorial in which Kilgard performed an annotated walkthrough through thirty-six out of over two hundred published GL extensions, commenting on their possible applications and their availability.

The remaining time before the lunch break was dedicated to a few GLUT-based demos (the GLUT framework is still available from the SGI website). Kilgard used his low-poly dinosaur (which should be known as GLUTzilla, in my opinion) to demonstrate reflections and shadowing using stencils, and during his demonstrations he was careful to point out artifacts and shortcomings, such asvisible tessellation caused by projection from nearby point lightsources. He also showed a multipass stencil-based technique for rendering magic halos around an object, which I suspect to be of limited use. Much more striking was the demonstration of real-time rendering of six views into a cube texture, followed by environment mapping on a reflective sphere using the GeForce-supported texture cube map extension. A Direct3D version of this demo was presented in Ron Fosner's "All Aboard Hardware T&L" article in the April issue of Game Developer.

The fill rate and bandwidth requirements for rendering six views of the scene can be quite expensive, but image caching techniques and sparse updates can certainly be applied. In addition, reflective objects like mirrors are usually used as special gadgets -- they are placed to make certain parts of a level more memorable, or to underscore a moment of a game's plot. Hardware support for environment mapping has entered the market, and techniques that are comparatively expensive today might be common one or two product cycles later.