Zerg Rush: A History of StarCraft AI Research
Real time strategy games are among the most challenging for artificial intelligence development and research. The need to manage resources and agent co-ordination in this genre still presents real challenges to even the most state of the art techniques in AI. My recent series on the AI of Total War highlights the continued efforts by series developers Creative Assembly to improve and expand the suite of AI systems required to craft the epic battles and nuanced diplomacy players comes to expect from that franchise.
Today I want to look at this same issue from a research perspective, with a particular focus on the franchise that is arguably most synonymous with the genre: Blizzard’s StarCraft. I want to take a look at the challenges this series presents to AI research and the significant efforts made in developing new AI techniques that adopts StarCraft as a test-bed. It’s important that this be re-iterated at time when mass media rhetoric suggests the recent interest by the likes of Google DeepMind is the first real exploration of the problem. AI research in this arena dates back to the turn of the century and has been a regular feature of the game AI academic community for over 15 years. So I’m going to take a look at the earliest RTS research, why and how it all started, the variety of competitions and benchmarks that are now coded into the original 1998 game and the future that awaits with the recent surge of activity behind StarCraft II.
StarCraft adopts many of the principle components of the real-time strategy genre with a focus on the control of territory and assets. Players assume command of one of three factions: the human Terrans, the insectoid Zerg or the advanced Protoss as they take control of land and resources within a defined area. By working through the occlusion that covers the map, referred to as the ‘Fog of War’, players lay waste to enemy forces and fortifications in order to establish resource locations, build structures and enhance their existing capabilities and by reinforcing and updating their existing assets using technology trees.
All of these mechanics and features ultimately influence the challenge that the game presents. In more professional play it impacts the strategies that are in effect during the beginning, middle and end of a match. In early game the real focus is establishing enemy locations and defending whilst construction is taking place, with the acquisition of materials and the correct build orders shifting based on the world state. This can result in more defensive behaviours or rush tactics to try and breakdown enemy structures of forces. All of this progresses through medium to end-game as players seek to construct the best army configurations for assaulting the opposing players, whilst ensuring they’re keeping the pressure on as they seek to advance their tech trees. It all gets pretty chaotic in the closing period of battle and any AI we seek to build to play the game has to be able to recognise numerous strategic elements at play and shift between strategies dynamically.
This is why there’s a lot of fuss about StarCraft for AI research given the overall complexity of the problem space: it’s a multi-agent control problem with imperfect information (given the fog of war) and a large state space given the number of possible actions and world configurations. Computer science research often seeks to quantify the difficulty of problems in order to establish whether they’re worth trying to solve. This computational complexity theory allows for researchers to formally define whether a given problem is ‘interesting’ or ‘challenging’ from a scientific perspective. This complexity notation models the amount of memory and resource as well as the time it will take for good solutions to be found for that problem. It’s been proven that classic NES and SNES-era video games such as Super Mario Bros., the Legend of Zelda and Donkey Kong Country can be formally classified as Non-Deterministic Polynomial Time Hard or NP-Hard for short. This means in essence that the games aren’t easy and carry some level of challenge before players can begin to control and master them in the long-term — not just for AI players but also humans as well. Meanwhile, RTS games are considered at minimum to be NP-Hard, but are predicted to be PSPACE complete or in the worst case EXPTIME — meaning that it’s anticipated that systems that can solve them effectively would take exponential time to validate their solution. In lay mans terms it just means they’re really really hard.
Early RTS Research
Back in 2003, a number of academics, most notably Michael Buro: a professor of Computer Science at the University of Alberta, were advocating that RTS games such as WarCraft, StarCraft and Age of Empires were the next ‘killer app’ for AI to be exploring, given that there are many facets of these games that make for really interesting decision problems for a system to try and solve. This included the need to manage resources, to make critical decisions in situations in which we are uncertain as to our current strength in relation to opposing forces, to conduct spatial and temporal reasoning of the in game world and foster collaboration with either AI or human players.
So with this in mind, Buro and other academics began seeking to conduct research within the RTS space. But the problem was that video game companies were typically reluctant to provide open access to their game engines and API’s back then, as such conducting research in StarCraft itself was not possible lest significant effort was made to either replicate, mod or break the original game. This led to Buro leading the development of the Open Real Time Strategy or ORTS platform: a free and open source reduction of classic RTS games that was designed for researchers and hobbyists to experiment in building AI controllers for a variety of in-game activity such as combat or construction. This system slowly grew in scale, complexity and faithfulness to classic RTS games courtesy of around 30 undergraduate and graduate students who slowly contributed to the project over a period of around seven years.
This led to a small but steady body of research in building AI controllers for RTS games, such as the use of classical planning (an idea adopted in commercial games such as F.E.A.R. and Empire: Total War) to create intelligent build order systems, to developing improved pathfinding AI and even using Monte Carlo methods to evaluate the effectiveness of strategic plans: an idea explored in 2005, a year or two before the rise of Monte Carlo Tree Search and it’s eventual adoption in Total War: Rome II in 2013.
ORTS started running competitions back in 2006, encouraging developers to submit their own RTS controllers. Each tournament had bots compete in multiple game types, some of which a subset of the main game such as gathering resources or unit combat in flat terrain to the eventual complete RTS experience with economies, tech trees and fog of war in place. The fourth and final tournament for ORTS ran in 2009. The big reason for this was that the focus was moved towards building something within StarCraft itself, thanks to the release of the Adam Heinermann’s Brood War Application Programming Interface (BWAPI).
The BWAPI is an open source C++ framework that is designed to interact with the original StarCraft. It provides a full suite of tools that allows for programmers to build their own AI controllers within the game. What’s pretty interesting and vital to the BWAPI being useful in a research and competitive capacity is that it accurately reflects the available information that human players would have in a similar capacity. BWAPI provides information on the overall game state, the available unit types, technologies and weapons as well as provide full control of build behaviours as well as individual units. In addition, while a units position and properties can be made available to your custom AI player, it is only permissible if the enemy unit is not occluded by the fog of war and will be removed from the world model should it leave the players view again. This prevents AI bots from cheating and forces them to maintain their own representation of the perceived active units in the game at a given time. Despite this, a Brood War API bot could conceivably cheat given there is no limit to the number of actions they can issue to the game in a given frame. As a result, it was possible for strange behaviours such as walking ground units over walls and making buildings that slide around. Though the community of bot developers that have came to adopt the API heavily enforce a code of conduct for appropriate and legal moves that can be executed by a bot in a tournament context.
StarCraft AI Competitions
Fast forward to 2010, and with the Brood War API in place, academia swung away from ORTS to full blown StarCraft: with the original competition hosted at the 2010 Artificial Intelligence and Interactive Digital Entertainment conference (AIIDE). AIIDE is one of the largest game AI research conferences in the world and arguably the most prominent in the United States, so it’s a pretty fitting location to kickstart this new tournament. The StarCraft AI Competition was first co-ordinated by Ben Weber — a PhD graduate of UC Santa Cruz, who also collaborated with Johan Hagelback and Mike Preuss for a small follow-up competition later that year at the IEEE Computational Intelligence and Games conference. However, since 2011 it has been coordinated by Dave Churchill a PhD graduate of the University of Alberta and at the time of publishing this video an assistant professor of Memorial University of Newfoundland.
The original event was structured around four tournaments that — much like ORTS — are focused on delivering a variety of game types:
- Tournaments 1 and 2 focusing on unit management and combat on flat and uneven terrain respectively.
- Tournament 3 had players explore a tech-limited version of StarCraft without any fog of war and a requirement that they use the Protoss race without any advanced units permitted.
- Tournament 4 was the complete StarCraft experience: with fog of war enabled, all factions permitted and a double-elimination format for entrants, with each match comprised of the best of five games.
The first AIIDE tournament was a huge success, with 26 entrants to the competition, 17 of whom competed in tournament 5. Victory in tournament five was handed to the Zerg playing bot Overmind — built by a team of developers from the University of California. It’s success came in rushing towards building Mutalisk aerial units to maintain an active defence and attack where necessary. This was achieved courtesy of a refined path planning system and active memory of threat locations that could allow ground units to attack more effectively during early game to destabilise enemy construction followed by Overlords removing fog of war and identifying when resources needed to be diverted to building anti-air defences. Once Mutalisks were unlocked and trained, it could maintain defence of their base during any continued construction and expansion whilst also targeting the occasional enemy using Mutalisks. The Mutalisk’s adopt a method called artificial potential fields — a principal from robotics that creates a field of attractive and repulsive potential forces in an environment — with valid targets considered attractive and threats to the enemy considered negative. This leads to behaviours such as this on-screen now, where a hit-and-run strategy can be established by disabling any attractive forces in between attacks. The parameteres used to dictate field strengths was tested by repeatedly running trials in test maps.
But despite its success, could Overmind have any chance at competing against human players? Overmind was tested by playing against — and occasionally defeating — Berkley PhD candidate Oriol Vinyals, who was not only one of the developers behind the bot, but was formerly Spain’s national StarCraft champion. While Overmind is long behind him, Vinyals is still actively involved in StarCraft AI, given at the time of this article he’s working at Google DeepMind as part of the StarCraft IIresearch team.
The competition has subsequently continued with an increase in scale and organisation, with some changes made to format. As Churchill assumed responsibility for the competition in 2011, all bot source code had to be made public and tournaments one through three were removed from the competition due to low entry rates in the first year. In addition, all competition matches were now executed on a client/server framework rather than the previous attempts which were conducted on two laptops! Meanwhile 2012’s AIIDE competition allowed for persistent storage, meaning that bots could learn by watching replays of previous matches.
The subsequent years saw numerous entries, with three participants regularly competing for the top spot between 2011 and 2013 at both the AIIDE and CIG conferences, this is largely because Overmind didn’t participate again in subsequent years. However, this doesn’t diminish the fact that the winning bot in 2011 called SkyNet was developed by a single person: British developer Andrew Smith. SkyNet was a Protoss bot that adopted a multi-phase strategy reliant on an early-game defensive strategy whilst periodically attacking using a Zealot rush strategy: a tactic that of pushing your enemies off guard by attacking with large quantities of Zealot units.
Meanwhile the Aiur bot — another Protoss player that frequently scored in the top three — used similar strategies to SkyNet. This included a Photon Cannon rush strategy (referred to as ‘Cheese’) as well as heavy use of a Zealot and Dragoon army for mid-game. Aiur is once again a one man team, developed by Florian Richoux: then a graduate student of the Université de Nantes and at the time of this article an associate professor at the institution as part of the Laboratoire des Sciences du Numérique de Nantes. Aiur is an acronym for Artificial Intelligence Using Randomness, with the bot reliant on the idea of having a mood system that dictates gameplay decisions. Moods are selected against a probability distribution for a given opponent, with said distribution continually being improved as the system records how effective a given mood type is against that player. This keeps enemies on their toes given the opponent can’t say with certainty how AIUR will play against it in any two consecutive matches.
The final big contender is the UAlbertaBot, submitted by the competition organiser David Churchill and developed in conjunction with a number of students at the University of Alberta. This bot is interesting given the team switched out from playing as Zerg after the 2010 competitions to Protoss: thus cementing the dominance of Protoss forces in AI competitions. The reason for the switch — and indeed the dominance of Protoss — was that the strategies using that faction were easier to build. UAlbertaBot’s biggest innovations come in two distinct subsystems, the Build Order Search System (BOSS) build system and the SparCraft simulator. BOSS is a simulation system for planning build orders to ensure optimal execution. Meanwhile, SparCraft is a combat simulation module that would enable the bot to more accurately estimate the outcome of combat between two forces, thus helping the bot identify when best to push and continue an attack, or to retreat to base and consolidate its forces. SparCraft can be configured to use different search algorithms such as Alpha-Beta pruning and the Upper Confidence Bound in Tree or UCT algorithm.
The competition continued on, with newcomers beginning to exert control in 2013, pushing SkyNet, Aiur and UAlbertaBot down the rankings, but it was at this time a second strand of competitions arose courtesy of the StarCraft Student AI Competition.
StarCraft Student AI Competition
2011 saw the release of the Student StarCraft AI Competition (SSCAIT): a separate tournament for those interesting in applying their work in StarCraft AI. The SSCAIT was founded by Slovakian PhD graduate Michal Certicky and operated under his supervision in his current capacity as senior researcher in the Games and Simluations AI Research Group at the Czech Technical University in Prague. The tournament is aimed at being a more open event than the main StarCraft AI competition, with competitors ranging from hobbyists to students and academic researchers, as well as live streaming of both tournament and practice matches live on Twitch.
To accommodate for this change in scope, there are some changes to the format and submission procedures. The tournament is reduced down to one match type: 1 vs 1 melee, with victory achieved should the opposing player lose all buildings, their AI code crashes or the decision making processes they’re reliant on result in some significant slowdown of in-game execution. Should you want to write a bot, programmers can be develop either in C++ using the standard Brood War API or in Java instead. The Java bots need to utilise one of two interface’s aimed at wrapping the core functionality of the C++ API: JNIBWAPI or BWMirror. Whilst bot source code is required as part of submission, the actual code isn’t made public and is only used to run plagiarism checks against other existing works.
The two competitions largely exist in harmony, with the likes of UAlbertaBot competing in both competitions.
Now with all these innovations in mind, how far have StarCraft AI players came to being able to compete against the best human players. Whilst the media has placed emphasis on the more recent competition, matches held against human players have cropped up once or twice at the AIIDE conference. 2015 saw the top three ranking bots of the StarCraft AI Competition — tscmoo by Vegard Mella, ZZZKBot by Chris Coxe, and Overkill from Sijia Xu — being put to task against Djem5: a pro StarCraft player from Russian regarded as one of the best non-Korean Protoss players in the world. All three bots were summarily destroyed by Djem5 with no matches won by AI bots.
Fast forward to late 2017 and another competition took place at Sejong University in Seoul, South Korea. Four AI competitors stepped up to the plate: the MJ Bot from Sejong University, ZZZKBot, tscmoo and lastly CherryPi developed at Facebook’s AI research lab. Their opponent? Song Byung-Gu (Stork): a high profile professional StarCraft player from South Korea considered one of the best in the world. It’s at this point that the gulf between human and AI play becomes more readily apparent, with all four AI opponents defeated within 27 minutes and the easiest victory achieved in four and a half minutes.
Google DeepMind & StarCraft 2
Now let us consider some more recent developments and most notably, Google DeepMind’s interest in StarCraft. Having successfully tackled the game of Go courtesy of their AlphaGo system and the well documented competition against expert player Lee Sedol, DeepMind have set their sights on StarCraft. The AI research space, the players involved and the collective interest in game AI research has changed quite drastically in the past 15 years. As such, what seemed like fantasy in the days of ORTS is now a reality, with Blizzard openly collaborating with Google to provide an official AI API for StarCraft II.
DeepMind and Blizzard launched the SC2LE or StarCraft 2 Learning Environment in August 2017 with fans getting a chance to try their hand at it with help from Blizzard themselves at the 2017 BlizzCon in November over in Anaheim, California. SC2LE is a collection of exciting tools for developers and researchers that effectively provides many of the same features as the Brood War API, only for StarCraft 2. But it also has some pretty cool new features as well, this includes:
- A complete API built for both machine learning and classic AI techniques that enables complete control of StarCraft II using the Python programming language. Not just controlling an AI within the game, developers can start a match, get observations of current state, conduct in-game actions through bot controllers and watch match replays
- The ability to run the game faster than regular speed, which is highly useful for training machine learning players.
- Means to build and deploy custom maps within StarCraft II for the game
- Seven mini-games developed by DeepMind to test and experiment with specific AI tasks and objectives.
- A collection of data that represented in-game playthroughs by human players that could be used for machine learning training purposes.
The API is broken up into two distinct collections: the ‘raw’ API which is more akin to the Brood War API that allows programmers to access specific information on a given frame and the main API that is largely for purposes of machine learning. This API takes all information from the game and analyses it to provide feature layers that are more accessible for a machine learning algorithm. These feature layers, such as height maps, unit density and selected units are scraped from a separate orthographic camera based on the players viewport. So whilst the game is rendered in 3D, the API presents a series of 2D images that are reflective of the feature layers in the current state. These feature extractions can prove more useful for machine learning algorithms to isolate and focus on key elements they wish to control and improve. This is elaborated upon in the research paper published by DeepMind, as they show how these feature layer are adopted in two convolutional neural network solutions they’ve tested to-date.
The agents implemented by DeepMind to-date run at around 180 actions per minute. When testing in the main game itself on the Abyssal Reef map, it’s clear there is still a long way to go given they are yet to win games against the built-in StarCraft II AI. This is largely to be expected, given the difficulty of the challenge faced and only really beginning to look at how machine learning to crack these problems.
However, the test games I mentioned earlier look a lot more promising, with some of the strategies formulating in these instances performing reasonably well. They still can’t compete at human level yet, but give them time and you might be surprised by what comes next.
StarCraft continues to be a relevant and exciting problem domain for AI research (and it sure seems to still be popular with players themselves). We can be sure to see some more innovation and success for AI players in StarCraft in the coming years. Though how long it will take for AI to successfully challenge if not defeat the best human players is difficult to ascertain. Hopefully having watched this video you now recognise the significant challenges that need to be overcome for AI to reach a level playing field. Nonetheless if and when these innovations come to light, you can be sure I’ll do a follow up to bring you up to speed.
This article is also aimed at highlighting the dangers found in recent work by major corporations as they become increasingly involved in game AI research. In 2001, John Laird and Michael VanLent’s publication “Human-Level AI’s Killer Application: Interactive Computer Games” sought to legitimise and advocate the use of video games as means through which to challenge the state of the art in AI, at a time where it was dismissed and considered a pointless use of our time. We sit here less than 20 years later, as the biggest tech giants in the world — be it Microsoft, Google, Facebook and others — are taking this very seriously and investing tremendous resources behind it. We’re in a new age of AI sensationalism in our media and not only do the big guys latch onto this, their involvement will re-frame the narrative on a given topic and obscure all existing research in this field — sometimes through no fault of their own. We saw this as DeepMind took a crack at Go, and it’s happening again with StarCraft. In some cases, it’s failing to acknowledge preceding research — a point that DeepMind got right in their papers on the SC2LE — other times it’s someone vying for a cool headline: a point that will resurface in my upcoming piece on the recent surge of research both academic and corporate in MOBA games such as DOTA2 and League of Legends.
Bibliography and Recommended Links
- M. Buro, Real-Time Strategy Games: A New AI Research Challenge, Proceedings of the International Joint Conference on AI, 2003.
- S. Ontanon, G. Synnaeve, A. Uriarte, F. Richoux, D. Churchill, and M. Preuss. A Survey of Real-Time Strategy Game AI Research and Competition in StarCraft, IEEE Transactions in Games, 2013.
- Google DeepMind, StarCraft II: A New Challenge for Reinforcement Learning, 2017.
- The StarCraft AI Competition:
- The Student StarCraft AI Competition
- The StarCraft 2 Learning Environment (SC2LE)
- The Brood War API
- ORTS — Open Real Time Strategy
- StarCraftAI.com, which gives an overview and tutorials on many of the topics behind bot construction