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Q3Test Review - Part 1
May 09, 1999   Dennis Thresh Fong > [View My Other Articles]
Kenn Hwang > [View My Other Articles]
Product Info | User Reviews | Article Images(40) | Image Gallery | Comments | Forum Thread

Thresh's comments in BLACK

Kenn's comments in BLUE

Curves Collision, from the Creator

Kenn

I wrote an e-mail to John Carmack inquiring into precalculated curves, consisting of what was written previously and the questions following. The short transcript follows:

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.

Kenn: Also, there don't seem to be any "moving" curves anymore - is that a byproduct of player clipping, or just not included with the levels in the test?

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

Thresh
Most q3testers know that the Loading: Collision Map line at level load usually takes up 50% or so of the wait period, and I think it's safe to assume that these are curve maps. From this, we can tell that standard "BSP" clip brushses aren't used for curves collision.

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.

   
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 Quick Fact
Player clipping and projectile clipping are handled separately in the Quake III engine. Collision detection between curved surfaces and projectiles has been in the engine for a while, while player clipping has just recently been implemented.


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