Thresh's comments in BLACK

Curves Collision, from the Creator

Kenn: The actual surface of a curve is determined beforehand and rendered into the level at load time. The most likely ways to do this are by using invisible clip brushes or hard-rendering clipping around a specific tessellation level. The former would take much more time in terms of manual labor, but placing clip-brushes in is a necessity of complex level design.

The visual curves can be adjusted to whatever detail level the player requires, and are independent of what your player model can actually hit. This works well for the most part, but sometimes results in odd game behavior, as players clip through small bits of the curve before hitting the solid clip brush, or hit the clip before they come in contact with the visible surface.

Is this correct? Moving against some curves seem to show that the actual beziers are not used for collision detection, which would imply that it is precalculated from some tessellated form of the curve, or manually done with clip brushes.

John Carmack: Very close, except the approximation is done at load time, not at bsp time.

John Carmack: There just aren't any in the test, but the wavy surfaces are strictly visual -- no clipping changes.

Kenn
The benefits to this are mainly elegance and automation. Clip-brushes would require level makers to spend the time making the invisible clip brushes by hand, time they were supposed to save by using beziers. Actually taking into account the curve geometry allows for the "unhindered creativity" promised by curves.

From the long level load times, it seems that calculating curves collisions is not a trivial task. This would explain why true beziers or even lowest-level tessellation are not used for collision detection, even though they would be the most accurate representation of the true object, an approximation provides the best balance between load time and accuracy.