Interface - Designing Games - page 240

Let's take aim assist as an example. In console shooters, the player
aims by manipulating a tiny joystick with his thumb. This method is so
fiddly, inaccurate, and frustrating that for years, people thought that con-
sole shooters would always be inferior to mouse-controlled shooters on
the PC. Then, in 2001, Halo showed us all how to use aim assist to make
shooters work on the console. Since then, they've become one of the big-
gest genres of console games. Without aim assist, console shooters would
never have become what they are today. But how, exactly, does aim assist
work?
Aim assist helps players track targets. The trick is in helping play-
ers aim without them noticing that anything is happening. Just pulling
the crosshairs toward the target feels too obvious. The only way to make
nonobvious, effective aim assist is to create a number of interleaved sub-
systems that help players in different ways. One subsystem helps players
track moving targets by replacing part of their stick input with input that
perfectly matches the target's motion on-screen. This doesn't force the
crosshairs onto the target—it only helps compensate for a target's motion.
Another subsystem helps players stop moving their crosshair when it is
over a target. It detects when the stick is released, looks for nearby targets,
and quietly slides the crosshair onto one of them. This sounds like it would
be jarring, but the player can't feel it because it only happens during the
crosshair's deceleration. A third subsystem watches for players who are
about to just barely sweep the crosshair past a target, and slightly shifts the
direction of the crosshair motion so that it falls onto the target. Since we're
only slightly adjusting the direction of the crosshair's motion and not its
speed, the player can't feel the effect. The strength of each of these effects
changes depending on the target's distance to the player, its angular dis-
tance from the crosshair, the presence of other targets, the difficulty level,
the weapon being used, and various other factors. And these three are just
a sample—many others subsystems are used in various games.
The key to this working without players noticing is that it only in-
fluences things that players are naturally unable to detect. People notice
crosshairs moving of their own accord, but they don't notice them stop-
ping a few pixels early, or accelerating slightly faster, or adjusting their
direction, because these events are masked by the motion going on around
them. That's why aim assist works only in these nooks and crannies in the
input stream.