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 1 of this series, where we discuss enemy AI design can be found here.

Tom Clancy's The Division 2 introduces players to a war torn Washington D.C. A battleground not just in the streets, but the hearts and minds of its civilians as they fight to regain a semblance of order that is true to the values this city - and its country - are built upon.

It's been seven months since the Dollar Flu destroyed New York City and humanity is clawing back piece by piece, presenting a new game design challenge for developers Massive Entertainment: building an open world game that is dynamic, emergent and immersive.  Keeping players on their toes and giving a sense of life to the world.  

This leads to enemy factions wandering the open world looking for trouble, friendly survivors patrolling near their captured settlements, supply convoys that either need to be defended from enemies or raided for extra goodies, or supply caches air dropping into DC, resulting in all sorts of trouble.

Continuing my discussion with Division 2's Lead AI programmer Philip Dunstan, we discussed the process of building a system that allows for all these dynamic events to occur, the problems that arise when balancing these elements of systemic game design and the challenges faced as the established tools and systems migrated from the bitter cold of New York City, to the sunny streets of Washington DC.

Immersive Non-Player Characters

In the first Division New York City is designated a quarantine zone.  It's become a tomb, a monument to the devastating chemical attack just weeks prior and hence the number of non-player characters (NPCs) is relatively minimal, with pockets of scavenging friendlies and gangs of enemies found throughout the open world.  But as the franchise moves from the chilling winter winds of the Hudson to the warmer summer breeze of the Potumac, the team at Massive wanted to increase the impact and spread of both friendly and enemy non-player characters in the game map. 

To achieve this, the development team put together a Living World Activity system into the Division 2, giving specific roles to NPCs as they enter the world and ensuring that they perform a given task or duty.  So let's walk through how it all works and then some of the weird and interesting problems that emerged as the existing AI systems were being adapted to work within the Washington DC map.

The Living World Activity System

The Activity system within the division two allows for a variety of different sequences to be executed each with their own objective.  Throughout the execution of the sequence a NPC will assumes a role or duty within the world and as assigned to a post within a set space or space to operate within.  What that means when you see a groups of NPCs travelling through the open, be it as a patrol team near a stronghold, a convoy moving between outposts or as a unit gathering resrources from an area with food or water, each NPC has a defined purpose within the activity to complete.

As mentioned each NPC has a given role or duty to complete for their assigned objective.  These are split into two types: the special role which is given to one or more NPCs at the beginning of the action being executed, or the unscripted role which is assigned in an adhoc fashion depending on what's happening in the world at that time.  A special role may be the NPCs that are gathering the resources, or carrying the boxes of supplies in a convoy.  While the unscripted roles are the guards providing overwatch, either standing in a fixed position, wandering a patrol path or taking control of a mounted weapon.

But in order for NPCs to have a role, they need to know not just what roles are available in a particular activity, but where they should be doing it.  Hence every activity has a set of posts attached to it.  Each post is set by designers to identify what duties can be executed at that post, the locations they should execute the duty from and the minimum number of NPCs that are required to satisfy it, given you might need two or three NPCs to maintain overwatch or patrol a region.  These are different types of locations that can be attached to a post: volumes for when they're gathering resources or guarding a region of the map, waypoint paths that are for patrol duties and fixed points such as a mounted weapon.  

Whenever the active objective changes, the server attempts to assign NPCs that already have an active duty to a new one in context of the current task.  But also priotises what posts should be assigned, so mandatory posts that really help sell what these characters are doing are filled first, with other unfulfilled posts being populated depending on the makeup of the remaining NPCs.

This all does a great job of reinforcing the performance theatre of the game: characters know where in the world they should be standing and what their job is at that point in time.  But what happens when an opposing factions appear - or the player in fact - and things start to kick off?  Well each NPC post does actually dictate whether or not it should be maintained in combat and if so they'll continue to operate the behaviour once the fighting starts.  After all, there's no point standing around on that mounted machine gun if you're not going to use it.  Plus in the event an AI decides to execute a specific behaviour, the behaviour can dictate whether or not to release itself from a given post and get into the fight and provides a nice continuity between passive and active activity.

Unexpected Challenges

So with this fairly extensive overhaul of the systems to facilitate the introduction of more dynamic and systemic AI throughout the open world.  There were still issues the team had to address, and much of that comes largely from the change of scenery:

This resulted in a number of changes to how existing behaviours within the Division AI is executed.  The first major change was that behaviours for archetypes were retooled such that they were more competent when fighting the player at range, with archetypes having additional tweaks to their behaviour to compensate for the increased range.  This leads to some novel cases such as the Black Tusks Rusher which has an assault drone, so if the player is too far away it won't expose itself and instead rely on the drone to attack you.  The second major change to the systems was the NPCs would actively move around the open world.  In the original game, NPCs could only move in very small groups and even this was kept within fairly tight ranges.  Now convoys and control point attack units are moving through at a larger scale but they also had a direction and goal to complete.  This helps contextualise their position in the world and either results in you sneaking up on them and catching them off guard, or accidentally bumping into them in the open world and creating a chaotic combat sequence.

This all has a knock-on effect on managing the AI behaviour server-side given - as mentioned back in my blogs on the first Division game - AI decision making happens server side and the client visualises these decisions on your PC or console.  This resulted in the game's servers running low level simulations of where groups of NPCs are in the world and their objectives.  Spawning them into the game when necessary and de-spawning them should they complete a given task.  But not only is the number of active NPCs increasing and the tasks their being assigned more complex, there's also the issue of having them spread across a larger map.  Division 2's map is pretty much a one-to-one reproduction of downtown Washington D.C., stretching from Roosevelt Island and Georgetown in the west with the National Mall leading up to the Capitol Building in the east.  Totalling at around 4 square miles of map, it's an area that is around 30% larger than the original map of the Division, which was of course a scaled reproduction of midtown manhattan.  This resulted in a more aggressive level of detailing or LOD system compared to the first game.  The LOD ensures that AI behaviour processing server-side is minimised if players are not near them, as well as client-side rendering and animation operating at lower levels of fidelity if they're clearly farther away.

So this addresses the issues of map size and increasing the density of AI activity in the map, but this didn't resolve all the problems that emerged during development.  There were still legacy problems emerging from working on the first Division.  Namely that Manhattan has rather strict rules on how verticality is introduced to gameplay:

But with all these improvements, it led to another issue.  The increased sight lines meant that AI could more readily engage the player as they walk down an open boulevards, but this leaves it open to a potential flaw, in that enemy AI factions are much more likely to spot each other.

Closing

Building open world environments in games that feel lived in, that feel dynamic is an increasingly demanding challenge as the scope of games being built continues to grow.  The complexity of these characters and their ability to handle emergent gameplay as it happens in-game has no clear solution.  But here we're seeing a natural iteration of the tools and systems from Tom Clancy's The Division, alongside many of the interesting challenges that they've faced along the way.  

But there is still more to talk about on the AI of Tom Clancy's The Division 2, I'm going to take a look at one of the most powerful applications of AI within the game that players never get a chance to see: the testing bots whose job it is to play the Division during development that identify bugs or faults within the game.  And I'll be telling 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.