Trisouping: 2-point versus 3-point clipped brush... 16bit colour problems

3 point clipping brush volumes

In the previous page we saw how to utilise trisouping in an effort to prevent distance related Z-fighting, this page deals with 3 (pt) Point Clipping.

Below is a 'rock' selected in GTK Radiant. It was made by cutting chunks from a solid block at differing angles in order to 'reveal' and interesting shape. Typically this type of brushwork object is simply placed 'on top' of other brushwork (as shown below) which often means that any long faces that have textures on them will be partially buried under another surface ('ground' in this instance); you can see in the shot below how it disappears under the terrain mesh.

In an ideal situation we'd like to be able to slice it along the axis where it meets the grass covered terrain, caulking the underside of the rock; if the terrain were a completely flat surface this could be done, but as it's not, it's better to leave the rock and ground brushwork intact - in situations like this using CSG Subtract is not a good idea due to the number of brushes already in the area; cutting them up more will lead to too many small brushes that usually cause problems when compiling the map.

3 point clipped brush buried into the ground surface

3 point clipped brush buried into the ground surface

The trouble with clipping ^

As was mentioned briefly on the previous page, 3pt. Clipping does create more interesting shapes than trisouping but it's here that we come to the reason for writing this tutorial - in game when you are in close proximity to brushwork like this it usually doesn't present any problems with Z-fighting and it does look quite visually interesting;

close proximity to the brushwork

Close proximity to the brushwork

It's not too noticeable in the shot below but as you get some distance away the effects of z-fighting (were in 16 bit remember..!) starts to occur because the game can't figure out which texture it should be drawing as the 'top' image - it's basically trying to draw both at the same time, and because it can't, it 'swaps' them in and out of render buffer creating the Z-fighting 'zigzag' error as a result; hence the term - 'Z' for the depth buffer used by the renderer, and 'fighting' for the apparent swapping or 'fighting' of the images.



The end result ^

Most games aren't run in 16 bit these days (enter the era of Doom 3, Quake 4 and next Gen), but you can still get z-fighting if you're not careful, even in 32bit mode. The thing to keep in mind is that it's difficult to produce interestingly shaped rocks like those shown below by using trisouping without using an excess of brushwork which then has a knock-on effect on the FPS under certain circumstance. Whichever method you use you be compromising on something but having read this tutorial you'll be aware of the immediate 'Pros' and 'Cons' of both 3 point clipping and trisouping in relation to terrain.

3pt clipped interestingly shaped rocks

3pt clipped interestingly shaped rocks

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