Building AI to Fight on the Beat of BPM: Bullets Per Minute

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Bullets Per Minute is a brutal and fast-paced shooter that has players blasting through hellish dungeons in sync with some thumping rock opera. Building a rhythm game where players need to fight on the beat but are still able to lay wastes to hordes of monsters presents an interesting problem not just in developing player mechanics, but how the enemy AI design and behaviour fits into the core gameplay systems. In an interview with developers Awe Interactive, we find out about the games inner workings. So start banging your head to the beat as we find out not only how the game works, but also how AI and Games inadvertently helped give birth to Bullets Per Minute.

BPM: Bullets-Per-Minute

BPM: Bullets Per Minute is a fast-paced first-person shooter that is also a roguelike. Each instance of the game is procedurally generated with unique map configurations that keep players on their toes with each playthrough as they fight through monstrous hordes inspired by Norse mythology. But, the core mechanic of the game is that players play to the rhythm: firing weapons, dashing and reloading all occur in sync with the music, with bigger rewards and higher scores accrued for clearing rooms while every action lines up to the beat. Announced in April of 2020, the game reaches PC in early September, with console versions planned for 2021. During the closing weeks of development, we sat down with Dave Jones - BPM's game director and programmer - and Josh Sullivan - who handles level and sound design - to find out more about the origins of the game, how the AI works for the enemy characters as well as how the level generation processes used to keep each and every run of BPM unique

The concept of BPM pulls from many existing games, but melds them together in a way that players have never experienced before, but is immediately obvious when you see it for the first time.

Fighting on the Beat

The enemies in BPM - like any other shooter - need to provide an interesting and diverse challenge for players to fight against. But what is most interesting, is that each character is constrained to only think and act on the beat. The game is built around the principle of a beat, a basic unit of time in the music. Each action by the player or an enemy - with exception of movement - is coordinated to synchronise with the beat. Specific actions are designed to only execute on the whole note - which are the larger chevrons on the crosshair, or in some cases on the half-notes, which are the smaller chevrons and even the quarter notes in between. This changes with each action based on their cooldown timer.

In addition, some of the larger and more aggressive enemies - including the bosses - force specific behaviours on the beat. Such as dodging a move that takes up half the screen or jumping off the ground as they apply an area-of-effect attack. When this is combined with the fairly large range of enemies, from spiders to bats, flies, slugs, scorpions, ground thumper people, spider-people and more - all of which synced up to and coordinated around the whole and half notes of the music, the two burning questions on my mind, are how does it work? And how do you coordinate that so it doesn't turn into a chaotic mess?

BPM is built in Unreal Engine 4 and while the engine provides a behaviour tree system for building character behaviours, it isn't used in this game. BPM does something a little different; the game relies on a custom architecture where each enemy is given the opportunity to process information at different time intervals.

Typically an AI would process their behaviour on each valid frame of execution, given in a normal UE4 behaviour tree you process on the tick()- and with a bit of know-how you can slow down how often the Behaviour Tree ticks an update. But in this case, it's hardcoded to only allow for specific updates on the whole, half, quarter and even smaller note intervals. By limiting the AI to only acting on the note timings, it synchronises the enemies to be in line with the player. This helps manage the expectations the player would have that - given they are constrained to the beat-mechanic, it would be unfair if the AI was not.

Coordinated Chaos

Given each enemy in BPM is acting on the whole or half note, it would actually require even more coordination to ensure that it doesn't become overwhelming. Unlike other shooters where enemies have free-reign of when in a given second they can act, everything in BPM is constrained to the note system. Hence you might have a whole stack of enemies trying to attack you on the exact same beat. Thankfully, BPM adopts the same principles as DOOM (2016), by having a system that not only influences how many attacks can occur on a given note but also limits specific attack types and the number of a given unit that can attack at once. Plus it imposes constraints that prevent attacks that would force the player to do otherwise impossible or complicated actions - such as forcing a dash and jump or dashing two different directions at the same time

But even with these systems in place, there is still a need to consider whether the player can understand how and why they are being punished. Should an enemy get a shot off that hurts the player, ideally you knew that was coming your way, or at least you had the opportunity to recognise what was happening, hence spatial coordination of the enemies and preventing them from bunching up is really important. The end-result can still feel overwhelming if you're not careful, but instead of it being as a result of unbalanced play, it emerges from the challenges a player would face in any other shooter: prioritising targets based on the threat they possess and figuring out how to take them out - only this time you're kinda headbanging while you do it.

Building the Dungeons

In amongst all of this is the level generation process: given each run through the realms of the Norse gods is designed to maintain novelty with each iteration. And while the level generation works effectively, the emphasis was less on creating complex and vast dungeon environments, but building something that fits the ethos of the game and its inspirations. In fact, once you know how the level generation works, you can see how clearly the principles of Binding of Isaac's level generation have been adapted to fit the purposes of BPM.

The game operates on a 32x32 grid, where each room is ultimately a square to fill that space. At each decision point, the game decides which of the room types it will generate from, with limits and probabilities imposed on specific templates. Hence while you'll typically bump into the two merchants, Huginn and Muninn, they only appear once per level and Huginn appears a lot more frequently than his slightly more metal compatriot. But while the structure and design of the in-game world is fixed to this grid notation, it still leaves a lot of room for variation in what occurs in each room. The room is but one phase of the configuration, after that it is also the specific enemies that are spawned into the space as well.

The player's current performance is fed through the level generation, influencing the statues that appear in the world that you tithe coins to for skill buffs, as well as influences odds of the Armory, Library and Bank appearing and what items are found within them. Meanwhile, mutators can appear on a given level with varying probability, with the difficulty and current level of the game influencing the odds of what you come up against. This can result in reduced gravity, icy floors, brittle enemies and darker environments. All of these systems help maintain novelty but also provide opportunities for the player to excel in a given situation.

Closing

BPM has at its core some interesting and well-crafted pillars from which the rest of the game can establish itself. The level generation while relatively simple is sufficiently layered to provide depth in multiple playthroughs, but the focus is really all about smashing bullets to the beat into the hordes of the underworld. The enemy movement patterns and actions are synced up to the music, but are balanced and designed to maintain diversity while keeping players on their toes. While it is early days, these elements combined with the simple yet effective core mechanic leaves plenty of scope for potential expansions or additions. But the proof will be in the pudding when players make up their own minds as they start bouncing to the beat of Bullets Per Minute.