The Drivatar model of racing line 

As we suggested previously, an AI car which exhibits no, or minimal, variety in the way it drives looks indisputably unrealistic. It appears too "artificial". The Drivatar model aspires to greater realism by first encoding driving style in terms of multiple racing lines.

The key to making this approach practical on a per-car basis is that every set of lines, such as illustrated above, is generated automatically via a combination of advanced numerical geometric path optimisation and probabilistic Gaussian process sampling. In other words, its possible for every car in a race in Forza to have its own individual identity in terms of the racing lines it desires to follow, without creating an unsustainable burden on the game's design, development and test resources.

This model clearly offers one possible way to modulate the difficulty of the races:

• “Hard” AI drivers follow smooth, consistent, closely-clustered lines
• “Easy” AI opponents drive less smooth lines with significant dispersion and variation

In addition to this multi-line paradigm, Drivatar models provide an additional layer of variety on-the-fly. At race time, the actual driven lines are generated dynamically via a probabilistic sampling and interpolation process. This implies that very few lines are actually required in the base model described above in order for AI cars to exhibit a comprehensive range of entirely credible variations in driving behaviour.

Furthermore, no two laps driven by an AI car in a lifetime of playing Forza will ever be the same!

The Drivatar model for speed 

In the quest for realism, it is also important that AI competitor cars drive at "appropriate" speeds. Its frustrating when an AI competitor in an identical car powers past the player as it comes out of the same corner that the player has just navigated to absolute perfection. Equally, it can be unsatisfactory to overtake an AI car when it slows down unnecessarily ahead at a mild kink in the track. This is an issue that appears to afflict many racing games, where AI speeds often appear contrived, even when aided by simplified car physics.

Despite the fact that Forza has an exceptionally detailed vehicle physics model, the Drivatar AI system ensures that the cars all drive at physically realistic speeds. These are calculated via an efficient and innovative solution to the physical simulation equations governing the cars dynamics. This solution is of course approximate (those equations simply cannot be solved analytically!), but takes into account all the more relevant variables — those pertaining to track geometry, vehicle dynamics and driver characteristics. Importantly, this can be evaluated in real-time.

Amongst other advantages, the Drivatar speed model allow the implementation of realistic AI driver attributes such as “late braking” and “tentative cornering” within the speed calculation, computationally "for free".