the first of this two-part report on the state of game AI, Steven Woodcock
shares what issues came up while moderating the AI roundtables at the
2000 Game Developers Conference. Next week, in Part Two, John E. Laird
will discuss how academics and developers can better share information
with each other, and Ensemble Studios' Dave Pottinger will peer into
the future of game AI.
was made clear in the aftermath of this year's Game Developers Conference:
game AI has finally "made it" in the minds of developers,
producers, and management. It is recognized as an important part of
the game design process. No longer is it relegated to the backwater
of the schedule, something to be done by a part-time intern over the
summer. For many people, crafting a game's AI has become every bit as
important as the features the game's graphics engine will sport. In
other words, game AI is now a "checklist" item, and the response
to both our AI roundtables at this year's GDC and various polls on my
game AI web site (www.gameai.com) bear witness to the fact that developers
are aggressively seeking new and better ways to make their AI stand
out from that of other games.
level and quality of the GDC AI roundtable discussions continues to
increase. More important, however, was that our "AI for Beginners"
session was packed. There seem to be a lot of developers, producers,
and artists that want to understand the basics of AI, whether it's so
they can go forth and write the next great game AI or just so they can
understand what their programmers are telling them.
done in years past, I'll use this article to touch on some of the insights
I gleaned from the roundtable discussions that Neil Kirby, Eric Dybsand,
and I conducted. These forums are invaluable for discovering the problems
developers face, what techniques they're using, and where they think
the industry is going. I'll also discuss some of the poll results taken
over the past year on my web site, some of which also provided interesting
grist for the roundtable discussions.
The Big Non-issue
AI: The State of the Industry) mentioned that AI developers were
(finally) becoming more involved in the game design process and using
their involvement to help craft better AI opponents. I also noted that
more projects were devoting more programmers to game AI, and AI programmers
were getting a bigger chunk of the overall CPU resources as well.
roundtables revealed that, for the most part, the resource battle is
over (Figure 1). Nearly 80 percent of the developers attending the roundtables
reported at least one person working full-time on AI on either a current
or previous project; roughly one-third of those reported that two or
more developers were working full-time on AI. This rapid increase in
programming resources has been evident over the last few years in the
overall increase in AI quality throughout the industry, and is probably
close to the maximum one could reasonably expect a team to devote to
AI given the realities of the industry and the marketplace.
1: AI poll results from the GDC 2000 roundtables
interesting was the amount of CPU resources that developers say they're
getting. On average, developers say they now get a whopping 25 percent
of the CPU's cycles, which is a 250 percent increase over the average
amount of CPU resources developers said they were getting at the 1999
roundtables. When you factor in the increase in CPU power year after
year, this trend becomes even more remarkable.
also reported that general attitudes toward game AI have shifted. In
prior years the mantra was "as long as it doesn't affect the frame
rate," but this year people reported that there is a growing recognition
by entire development teams that AI is as important as other aspects
of the game. Believe it or not, a few programmers actually reported
the incredible luxury of being able to say to their team, "New
graphics features are fine, so long as they don't slow down the AI."
If that isn't a sign of how seriously game AI is now being taken, I
don't know what is.
didn't feel pressured by resources, either. Some developers (mostly
those working on turn-based games) continued to gleefully remind everyone
that they devoted practically 100 percent of the computer's resources
for computer-opponent AI, but they also admitted that this generally
allowed deeper play, but not always better play. (It's interesting to
note that all of the turn-based developers at the roundtables were doing
strategy games of some kind -- more than other genres, that market has
remained the most resistant to the lure of real-time play.) Nearly every
developer was making heavy use of threads for their AIs in one fashion
or another, in part to better utilize the CPU but also often just to
help isolate AI processes from the rest of the game engine.
continued to credit 3D graphics chips for their increased use of CPU
resources. Graphics programmers simply don't need as much of the CPU
as they once did.
Since Last Year
of AI technologies noted at the 1998 and 1999 GDCs has continued to
grow and accelerate over the last year. The number of games released
in recent months that emphasize interesting AI -- and which actually
deliver on their promise -- is a testament to the rising level of expertise
in the industry. Here's a look at some trends.
life. Perhaps the most obvious trend since the 1999 GDC was
the wave of games using artificial life (A-Life) techniques of one kind
or another. From Maxis's The Sims to CogniToy's Mind Rover,
developers are finding that A-Life techniques provide them with flexible
ways to create realistic, lifelike behavior in their game characters.
smart rover navigates a maze in CogniToy's Mind Rover
of A-Life techniques stems from its roots in the study of real-world
living organisms. A-Life seeks to emulate that behavior through a variety
of methods that can use hard-coded rules, genetic algorithms, flocking
algorithms, and so on. Rather than try to code up a huge variety of
extremely complex behaviors (similar to cooking a big meal), developers
can break down the problem into smaller pieces (for example, open refrigerator,
grab a dinner, put it in the microwave). These behaviors are then linked
in some kind of decision-making hierarchy that the game characters use
(in conjunction with motivating emotions, if any) to determine what
actions they need to take to satisfy their needs. The interactions that
occur between the low-level, explicitly coded behaviors and the motivations/needs
of the characters causes higher-level, more "intelligent"
behaviors to emerge without any explicit, complex programming.
of this approach combined with the amazing resultant behaviors has proved
irresistible to a number of developers over the last year, and a number
of games have made use of the technique. The Sims is probably
the best known of these. That game makes use of a technique that Maxis
co-founder and Sims designer Will Wright has dubbed "smart terrain."
In the game, all characters have various motivations and needs, and
the terrain offers various ways to satisfy those needs. Each piece of
terrain broadcasts to nearby characters what it has to offer. For example,
when a hungry character walks near a refrigerator, the refrigerator's
"I have food" broadcast allows the character to decide to
get some food from it. The food itself broadcasts that it needs cooking,
and the microwave broadcasts that it can cook food. Thus the character
is guided from action to action realistically, driven only by simple,
3: The Sims made ample use of A-Life technology
were definitely taken with the possibilities of this approach, and there
was much discussion about it at the roundtables. The idea has obvious
possibilities for other game genres as well. Imagine a first-person
shooter, for example, in which a given room that has seen lots of frags
"broadcasts" this fact to the NPCs assisting your player's
character. The NPC could then get nervous and anxious, and have a "bad
feeling" about the room -- all of which would serve to heighten
the playing experience and make it more realistic and entertaining.
Several developers took copious notes on this technique, so we'll probably
be seeing even more A-Life in games in the future.
In a remarkable departure from the roundtables of previous years, developers
really didn't have much to ask or say about pathfinding at this year's
GDC roundtables. The A* algorithm (for more details, see Bryan Stout's
excellent article Smart
Moves: Intelligent Path-Finding) continues to reign as the preferred
pathfinding algorithm, although everybody has their own variations and
adaptations for their particular project. Every developer present who
had needed pathfinding in their game had used some form of the A* algorithm.
Most had also used influence maps, attractor-repulsor systems, and flocking
to one degree or another. Generally speaking, the game community has
this problem well in hand and is now focusing on particular implementations
for specific games (such as pathfinding in 3D space, doing real-time
path-granularity adjustments, efficiently recognizing when paths were
blocked, and so on).
4: Ensemble Studios revamped the pathfinding in Age
of Empires II: The Age of Kings by including terrain analysis
become more comfortable with their pathfinding tools, we are beginning
to see complex pathfinding coupled with terrain analysis. Terrain analysis
is a much tougher problem than simple pathfinding in that the AI must
study the terrain and look for various natural features -- choke-points,
ambush locations, and the like. Good terrain analysis can provide a
game's AI with multiple "resolutions" of information about
the game map that are well tuned for solving complex pathfinding problems.
Terrain analysis also helps make the AI's knowledge of the map more
location-based, which (as we've seen in the example of The Sims)
can simplify many of the AI's tasks. Unfortunately, terrain analysis
is made somewhat harder when randomly generated maps are used, a feature
which is popular in today's games. Randomly generating terrain precludes
developers from "pre-analyzing" maps by hand and loading the
results directly into the game's AI.
games released in the past year have made attempts at terrain analysis.
For example, Ensemble Studios completely revamped the pathfinding approach
used in Age of Empires for its successor, Age of Kings, which
uses some fairly sophisticated terrain-analysis capabilities. Influence
maps were used to identify important locations such as gold mines and
ideal locations for building placement relative to them. They're also
used to identify staging areas and routes for attacks: the AI plots
out all the influences of known enemy buildings so that it can find
a route into an enemy's domain that avoids any possible early warning.
game that makes interesting use of terrain analysis is Red Storm's Force
21. The developers used a visibility graph (see "Visibility
Graphs" sidebar) to break down the game's terrain into distinct
but interconnected areas; the AI can then use these larger areas for
higher-level pathfinding and vehicle direction. By cleanly dividing
maps into "areas I can go" and "areas I can't get to,"
the AI is able to issue higher-level movement orders to its units and
leave the implementation issues (such as not running into things, deciding
whether to go over the bridge or through the stream, and so on) to the
units themselves. This in turn has an additional benefit: the units
can make use of the A* algorithm to solve smaller, local problems, thus
leaving more of the CPU for other AI activity.
Closely related to the subject of pathfinding in general is that of
unit formations -- techniques used by developers to make groups of military
units behave realistically. While only a few developers present at this
year's roundtables had actually needed to use formations in their games,
the topic sparked quite a bit of interest (probably due to the recent
spate of games with this feature). Most of those who had implemented
formations had used some form of flocking with a strict overlying rules-based
system to ensure that units stayed where they were supposed to. One
developer, who was working on a sports game, said he was investigating
using a "playbook" approach (similar to that used by a football
coach) to tell his units where to go.
machines and hierarchical AIs. The simple rules-based finite- and fuzzy-state
machines (FSMs and FuSMs) continue to be the tools of choice for developers,
overshadowing more "academic" technologies such as neural
networks and genetic algorithms. Developers find that their simplicity
makes these approaches far easier to understand and debug, and they
work well in combination with the types of encapsulation seen in games
using A-Life techniques.
are looking for new ways to use these tools. For many of the same reasons
A-Life techniques are being used to break down and simplify complex
AI decisions into a series of small, easily defined steps, developers
are taking more of a layered, hierarchical approach to AI design. Interplay's
Starfleet Command and Red Storm's Force 21 take such an
approach, using higher-level strategic "admirals" or "generals"
to issue general movement and attack orders to tactical groups of units
under their command. In Force 21 these units are organized at
a tactical level into platoons; each platoon has a "tactician"
who interprets the orders the platoon has received and turns them into
specific movement and attack orders for individual vehicles.
at the roundtables who were working on strategy games reported that
they were either planning to implement or already had used this type
of layered approach to their AI engines. Not only was it a more realistic
representation, but it made debugging simpler. Most of those who used
this design also liked the way it allowed them to add hooks at the strategic
level to allow for user customization of AIs, building strategies, and
so on, while isolating the lower-level "get the job done"
AI from anything untoward that the user might accidentally do to it.
This is another trend we're seeing in strategy games that players find
quite enjoyable -- witness the various "empire mods" for games
such as Stars, Empire of the Fading Suns and Alpha Centauri.