AI and Games is a crowdfunded YouTube series that explores research and applications of artificial intelligence in video games.  You can support this work by visiting my Patreon page.

In collaboration with Ubisoft and Massive Entertainment, I present three blogs exploring the AI behind Tom Clancy's The Division 2, including excerpts from my interview with the Lead AI Programmer of the franchise, Philip Dunstan.

Part 2 of this series, which explores open-world and systemic design, can be found here.

2019's Tom Clancy's The Division 2 brings the war to Washington D.C. as the United States is still caught in the grip of the Green Poison outbreak six months after the fight to reclaim New York City.  Massive Entertainment returned with a larger and more ambitious title, with a variety of new gameplay features that reshaped how player interact with both friendly and enemy AI characters within the game.  

In this first of a three-part series in which I interview Division 2's Lead AI Programmer Philip Dunstan, I take a look at the lessons learned from shipping and maintaining the first Division.  What the teams goals were as they moved into production on the sequel and the changes laced throughout the games AI systems to build a fun and nuanced combat puzzle: keeping the player engaged from the very first encounter all the way to endgame.

The Lessons Learned

Tom Clancy's The Division - like many a live service game - was an exercise in growth as the project not only made it to store shelves in March of 2016, but also was maintained and updated for a period just shy of two years post-launch with the West Side Pier update 1.8 released in December 2017.  

The core AI behaviour of The Division is built through a proprietary behaviour tree system - a topic I've previously covered in blogs on Gamasutra  - with significant effort put into tools to help the design and programming teams build behaviours and address inconsistencies and bugs within them.

But while there's always a need for change, there's also a chance for growth and improvement.  A new title - even a sequel in a franchise - is an opportunity for systems to be refreshed and iterated upon.  Addressing bugs in the toolchain and re-engineering existing tools and systems to be more stable and reliable, with the navigation tools in particular an area that the team focussed upon.  But the biggest changes that players would notice was in the design and structure of the enemy archetypes and factions that players would face in DC.  This was a focus not just for the main campaign but more critically during endgame, where the majority of players would spend the bulk of their time.

To address the challenge in diversifying the enemy factions, the team focussed on three key areas:

This would help satisfy the teams goals of not just diversity, but ensuring challenging gameplay during endgame.  So let's walk through each of these in-turn and explore the big changes happening under the hood...

Improving Core Behaviour

One of the biggest tasks the AI team set themselves was to amplify the players impact on AI behaviour.  Naturally in a given like the Division you interact with enemy AI primarily by shooting them.  But players need to feel rewarded when their persistence and skill enable them to clear a room of enemies.  Communicating an AI's decisions is incredibly important for maintaining immersion for the player and its equally important in the heat of battle as it is during more quieter moment.

In the first Division, the enemies response to being shot was not as rewarding for the player as it could be.  Division 2 embellishes the stagger animations for when a character is under fire, which help give your attacks meaning, but more critically increase the rate of falter animations interrupt the AI's behaviour, given you an opportunity to re-evaluate the situation.  But having the AI character reacting to being shot is one thing, it's giving them a more natural reaction that enriches the experience.  Hence they focus more on dodging and ducking enemy fire, forcing players to keep track of their movements, not just shooting fish in a barrel, but constantly re-evaluating target prioritisation and maintaining and edge of uncertainty to the fight.

The last major change to the core systems, was the Time-to-Kill (TTK), the period of concentrated gunfire players need to maintain to take an enemy down.  Given the pseudo-realistic setting of The Division, there is a need to balance this alongside the design of each character.  The TTK of the Division 2 is - on average - much lower than the original, but the team still wanted to provide big meaty enemies that would require greater resilience and a bit of co-ordination to overcome.

But there were still many an additional change both aimed at improving the enemy immersion both within combat and idly in the world.  Weak points based on equipment enemies had on hand was pushed further, while a new barking system reinforces the intensity of combat as characters not only shout among one another, but also at the player.

Amplifying Factions

The Division 2 has three main enemy factions player face off against: the Hyenas, the Outcasts and the True Sons.   As players enter Washington D.C. the Hyenas, Outcasts and True Sons have taken up refuge in varying parts of the city and maintain numerous strongholds, forcing you to play against each faction throughout the campaign.  However, once the player reached level 30 and defeated the True Sons at the Capitol Building, the fourth and final faction emerges - the Black Tusk.  They become the games main end-game opposition, re-capturing strongholds and establishing their authority across the map, with many of the story mission locations now providing remixed variations that now pit you against them.

Ensuring each faction was not only distinct, but still proved valuable throughout endgame was a top priority for the development team.  In fact, much of this was driven by the lessons learned when building the post-launch content for the first Division:

So with this in mind, the AI design team designed the movement, positioning and combat parameters of faction against four traits: Aggression, Organisation, Training and Tech.  Each of these traits has numerous permutations and it allows for each faction to be unique in its overall design.  But still prove valuable for use in end-game.

Aggressive factions such as the Hyenas and Outcasts are quick to reposition and advance towards the player, they prioritise circling the player at close range, but also have longer uninterrupted shooting patterns.  Meanwhile the True Sons and Black Tusk are defensive factions: they prefer to stay at an optimal distance, use short controlled bursts of gunfire and relocate less frequently.  This results in more chaotic and scrappy fights with aggressive factions, while defensive factions force a more drawn out gun battle and spread themselves out to stay at range of the player.

Next up, the True Sons and Black Tusk are organised factions - meaning combat barks between AI reflect the hierarchy and order of the unit - plus their animations show a lot of non-verbal communication.  Meanwhile the Hyenas and Outcasts are unorganised - they don't communicate as effectively or really consider what other characters are doing.

The third faction design trait reflects the level of tactical and combat training a the group has been given.  Training reflects the overall combat performance of a given faction, including their use of cover fire, flanking maneuvers, time taken for winding up attacks such as lobbing grenades, weapon accuracy and the animation style.  The animations are really important give it communicates how quickly enemies can transition from one action to another, but the animation itself reinforces their training and effectiveness in achieving a given action.  The training of a given faction is broken into three groups.

The last trait of a given faction is their use of tech, the use of melee weapons, standard guns, improvised explosives and gadgets all the way to robotic tanks.  This is the one trait where the Hyenas aren't on the lowest setting.  Outcasts run on the lowest tech trait: given they use melee weapons and homemade explosives - which largely fits their tribalistic nature.  Meanwhile the True Sons are medium tech: given their focus on traditional weaponry.  The Hyenas are high-tech: they carry shields, use remote controlled bombs and even carry grenade launchers.  Lastly the Black Tusk has a 'very high' tech trait, with the use of the drones and mini-tanks.

So with all of these traits in place, each faction provides a different experience.  The focus isn't to make them clearly easier or harder to fight against - though it's clear that the likes of the Outcasts or Hyenas might be a little easier to get on top of versus the Black Tusk. But the traits provide a range of unique characteristics to each faction, providing diversity in enemy strategy without effectively making them too weak to be interesting.

Now the more seasoned Division 2 players might have listened to that list part about Faction traits and noticed an inconsistency: at the start of the game, the Hyenas attacking the White House don't fit the faction traits I just described.  They fight more defensively, seem a little more organised and have a modicum of training.  You're right, they don't.  It also isn't the only time that a faction doesn't necessarily line-up with their selected traits.

That's because they're one of numerous 'hidden factions': special versions of a given faction that are used in very specific situations.  Hidden factions are custom built by designers to provide a hidden tutorial in the games combat. Tutorial factions are designed to teach you core gameplay mechanics.  Meanwhile the hidden versions of each faction are used not only the first couple of times you meet them in the open world, but also in several of the opening missions in the game.  Once the opening hours of the game are completed, these hidden factions are removed from the game and you play against the true versions of each faction.  

Archetype Revamp

In the my earlier blog posts on The Division, I talked about how each faction is comprised of numerous archetypes: specific enemy designs that have their own preferences or behaviours.  These force the player to constantly evaluate target prioritisation and minimising their exposure to attack as different enemy types change the flow of combat.

Now the archetypes have largely stayed consistent between games, but with some new additions based on new tech:

But for Division 2 an effort was made to try and make each version of the same archetype play and behave slightly differently in each faction: providing both unique weaponry and more distinct AI behaviours.  If you want to consider the Rusher: which runs directly at the player and aims to attack, the same archetype is quite distinct for both the Hyenas and Outcasts: with the former using riot shields to block incoming fire, while the latter is a suicide bomber whose gas tank can be shot to eliminate them if you're fast enough.  Meanwhile the sniper behaves differently for the Outcasts compared to the True Sons.  The True Sons sniper is a more traditional version of that archetype: it uses a marksman rifle, has a long wind up but deals heavy impact damage.  However the Outcast sniper fires explosive arrows from a bow with a more pronounced wind up time that also has an area of effect on impact.  As players we recognise the archetype, but on a faction-by-faction basis we are now adapting our strategies accordingly.

Closing

Achieving diversity and novelty in the AI for games such as the Division can be a real challenge, there needs to be a level of consistency such that players can improve their skills.  However, your enemy AI needs to keep up with the player as their mastery of the game increases, providing new challenges, forcing you into tight spots to fight your way out of and helping maintain that balance of tension and enjoyment.  The Division 2 is an impressive feat in that the programming and designs teams took the lessons learned from the first game and went back to the drawing board to design new enemy AI characters such that they would avoid making the same mistakes twice.  

Plus with the release of the Warlords for New York expansion - which has players return to the island of Manhattan - we can now see these principles applied to some of the original factions, with the Rikers and Cleaners making their return to the franchise.  

But building the enemy factions is but one facet of how AI is applied in The Division 2. Looking beyond the need to shoot and kill, the world of Washington D.C. is one of rebirth: a painful process as the city rebuilds itself after months of conflict.  How do you create a more dynamic city full of patrolling civilians, of convoys moving between checkpoints and balancing it all without it turning into a chaotic mess?  I tell you all about that next time.

Special thanks to Ubisoft for the opportunity to work with them on this project.  And of course to my patrons who crowdfund the AI and Games series.