Checkline

A checkline in STK is a virtual line preventing unintended shortcuts.

Explanation

Every track has to have a few checklines. They are set so that players have to stay on the road and cannot cut a part of the track. If checklines are set wrongly by track creators or even are missing, exploits are possible. In STK 0.9.3, a lot of tracks included unintended shortcuts because of this.

Technical details

Right now, a checkline is considered to be a rectangle of height 5, constructed by extruding a segment between two points on the same height in the upwards direction. This means a tilted checkline, or a checkline that has a bigger height, are only possible if several checklines are combined.

A lap line is a special checkline which doesn't require any coordinates, a track creator needs to make some other things to the driveline to make a lap line. A checkline can either be inactive (crossing it gives nothing) or waiting for the player (crossing the checkline makes it inactive and makes one or more other checklines waiting for the player).

Initially, the lap line is waiting for the player, and all other checklines are inactive. For each checkline, the track creator can specify which checklines begin to wait after this particular checkline is crossed. Checklines are usually organized into cycle (i.e. one checkline makes exactly one other checkline active), but if the track has several drivelines, checklines can be grouped, so that checklines in one group make the same set of checklines waiting, and usually these checklines in a group are located on different paths.

If a track has no checklines except the lap line (like on these tracks), a player can cross the lap line multiple times without completing a lap which leads to weird lap times. If there is only one or two extra checklines, there still could be some kind of unintended shortcut.

Some tracks explicitly mark checklines with objects like powerups to make intended cuts. Good examples of those tracks are USC74, USC70 and USC19.