Wednesday, August 01, 2007

How better AI can make racing games more fun

In some previous posts on this blog (e.g. this one, this one and this one) I've been discussing evolving neural networks to drive racing cars around a track. We did this research (published in several papers, e.g. this one and this one) for several reasons, the main motivation being to explore how games can be used as environments in which (artificial) evolution can create complex (artificial) intelligence. The related topics of which evolutionary algorithms and controller architectures (neural networks, expression trees etc.) learn best and fastest have also been investigated.

While the interest in this kind of research from the point of view of artificial/computational intelligence and machine learning is fairly obvious, one might wonder whether it might also have applications in computer games. This is less obvious. For example, most racing games would not benefit from having faster, better driving opponents; who would want to play a racing game where you always finish last? Apparently, minor "cheats" (such as allowing the computer-controlled drivers more complete information than is given to the human player) is enough for game designers to be able to manually create opponents that drive well enough.

Racing games are not alone in this respect: in most game genres (with the notable exception of strategy games like Civilization), game designers have no problems at all coming up with sufficiently (appropriately?) challenging opponents, without resorting to blatant cheats (again, remember that Civilization and its likes are exceptions to this rule). Instead, the challenge for designers is coming up with interesting enough opponents and environments, and doing it fast enough. In fact, this consumes huge amounts of money, and is a major expense post in the development of a new game.

So, the challenge we set ourselves was to use the technology we'd already developed to come up with something that could make racing games (and in the future other games) more fun and interesting.

What we came up with was this: modelling the driving style of a human player, and use our model of the driving style together with an evolutionary algorithm to create new racing tracks that are fun to drive for the modelled player. This combination of player modelling and online content generation has, as far as we know, never been attempted before.

The technical details of (different versions of) our proof-of-concept implementation of this was presented at an SAB Workshop last year, and at the IEEE CIG Symposium in April (read the paper online). A discussion of the experiments will also be included in a chapter in a forthcoming book from Springer. But the basic procedure of the most recent version of our software is as follows:

  • Let the human player drive on a test track, designed to contain different types of challenge (straights, narrow curves, alternating smooth bends). Record the driving speed and lateral displacement (distance from the center of the track) on a large number of points around the track.
  • Take a neural network-based controller, which has previous been evolved to be a competent driver on a large variety tracks, and put it back into the evolutionary algorithm. This time, however, the fitness function is not how well the controller drives the track, but how similar the its driving style is to the human's. Specifically, the more similar the speed and lateral displacement of the neural network-controlled car is to the recorded values of the human driver on the same track, the higher fitness it gets.
  • Next, a track is evolved. For this we need an evolvable representation of the track. We've experimented with a couple of different solutions here, but what currently seems to work best is representing the track as a b-spline, i.e. a sequence of Bezier curves.
  • We also need a fitness function for the track. Here, it should be remembered that we are not looking for a track that is as hard or as easy to drive as possible (that would be easy!), but rather the most fun track for the modelled player. To be able to measure how fun a track is, we looked at the theories of Thomas Malone and Raph Koster. The outcome of the rather long discussion in the paper, is that we try to maximize the difference between average and maximum speed, the maximum speed itself, and the variance in progress between different trials. But you really have to read the discussion in the paper to see the point of this, or possibly another blog post I'll write later.
  • Finally, we evolve the track, using this fitness function and track representation, by driving the controller modelled on the human player on each track and selecting for those tracks in which the controller has maximum speed, maximum difference between average and maximum speed, and maximum progress variance.

Below is a few evolved tracks:

This procedure works well enough in our proof-of-concept implementation, but how well it actually works in a full racing game remains to be tested. The most obvious candidate for testing this would be a racing game that comes with a track editor, such as TrackMania. On the horizon, we could have racing games with endless tracks, that just keeps coming up with the right types of track features as you drive, i.e. the ones which are neither to easy nor too hard, and thus keeps you challenged in the right way.

And of course we have been thinking a bit on how this general idea might be extended to other types of games, we just haven't had any time to do experiments yet...


Fredrik said...


Julian Togelius said...


Drama Mama said...

I just read your article, it's quite interesting! Having really good drivers would be really interesting in a racing game manager where you don't drive. You just adjust some characteristics for your car and pilot and watch the race. I know this might not sound fun but I am actually working in such game and it works!!! I just started working on it and I will be working on the IA among other things. So I started looking for interesting ideas to improve the racers IA...