The Last of Us: AI so good you’ll weep when you kill it

We speak with one of the Lead Gameplay Programmer for Naughty Dog’s The Last of Us about the next generation of AI, and how it could change the way you think.

In a dilapidated hotel room in what used to be known as the city of Pittsburgh, a man is on his knees begging for his life.

The events that led him to this point still resonate through the ruined hotel, a building now littered with the bodies of his fellow hunter scavengers. It all went wrong as soon as they spotted the grizzled middle-aged man escorting a spry 14-year old girl that, unlike the rest of them, seemed to belong in this shattered new world. It’s all she knows.

He would have sworn he heard the sound of a hammer falling on an empty pistol chamber. Otherwise, he and his fellow marauders would have adopted a different tactic. It would still have been risky to attack a man with a loaded weapon of course, but the lure of scavenging the real, working ammo was just too great. Even when a Molotov Cocktail forced him to abandon the frontal assault and try to silently flank the grizzled man, the odds were good. Then the gun sounded its empty cry and the attraction of ammo turned to the primitive rage of revenge.

He had the man too, dead to rights. He should never have backed up into the hallway to get a better angle. If he hadn’t, the girl wouldn’t have been able to hit him with a rock and disorient him. And now, staring up at the barrel of his own shotgun pointed at him by a stranger with a grim demeanor, the failure of his tactics echo through the world. Maybe the next group will have better luck. Maybe they will attack the grizzled man in a better way. He’ll never know though, as his pleas fall on deaf ears and the shotgun blast permanently ends the discussion.

The dead man is not a playable character in The Last of Us, but rather an incredibly smart enemy AI, that is programmed to react based on your actions.  This time he ended up as the loser, facing a shotgun as the penalty, but if the player had taken a left instead of a right, or the AI happened to see the gun earlier and taken cover, things may have played out in a significantly different manner, and the player might be the one staring at the shotgun. Maybe next time.  

As the AI enemy drops to the floor, dead, the man left holding the shotgun is Joel, the protagonist of the upcoming game from developer Naughty Dog, The Last of Us; the teenage girl is Ellie. Together the pair are fleeing west in a world that looks like a cracked reflection of our world. Twenty years have passed since a plague wiped out most of humanity, leaving billions of corpses along with mutated things in its wake. Cities once grand and dominant now sit abandoned, slowly being reclaimed by nature.

As you take control of Joel, you and Ellie embark on a memorable and compelling adventure through this scarred landscape. Naughty Dog, the developer behind the critically acclaimed Uncharted series, already knows how to craft a tale and make it cinematic (just look at all the awards the Uncharted series has won for proof of that). But this game will feature a new element. For The Last of Us, to match the all new setting, story, and characters, Naughty Dog has also completely reworked the computer AI for your companion Ellie, as well as your enemies. Not only will this make Ellie a more effective partner, it means that the foes you face will have a logic to them — they will react to you. And when you kill them, you will feel the weight of that decision.

“We can have characters that are not just pop up targets that die after five seconds of being on screen, and you can really get to understand them as real humans — to feel that impact, and maybe a little bit of guilt of having killed someone, realizing this is a real guy,” Jason Gregory, Lead Gameplay Programmer for The Last of Us told us. “He would have killed me if I hadn’t killed him, but it still kind of feels bad. That’s a fascinating emotion for me as a game player.”

It is no secret that Sony has had a huge advantage over the Xbox 360 when it comes to exclusive titles, and no third-party publisher better exemplifies that than Naughty Dog. Following the success of the Jak and Daxter games, it released what may be the best series on this generation of consoles, Uncharted.

Those games (at least beginning with Uncharted 2) redefined what an adventure game could be, and offered a cinematic flair that would leave most Hollywood directors gasping in jealousy. From just barely escaping the onslaught of an attack helicopter by jumping from rooftop to rooftop in a Himalayan city, to escaping a capsized cruise ship before it sinks, the series is renowned for its presentation. It is not, however, well known for its combat mechanics.

It isn’t a bad combat system per se, it’s just uninspired. You typically run in, find cover, and wait for enemies to pop their heads up so you can conveniently shoot them. Barring the odd grenade, you don’t even have to move too often. That is not the case in The Last of Us.

“Both Ellie and the enemy AI has been completely revamped for this game,” Gregory said. “We’re still using some of the lower-level systems — the animation systems are obviously leveraged from prior technology — but all of the enemy perception, the way that they analyze the play space, how they decide where they are going to go, is brand new technology.”

While not an open-world setting, The Last of Us will always give you options on how to proceed. There are a few situations where the story forces you into a combat situation, but in most areas you can make it through quietly by taking out one or two guards. You’ll certainly miss some items, which are important elements to your survival thanks to a new crafting system, but you’ll also avoid the intelligent AI that react to what the developers are calling the “Balance of Power.”

“Basically every AI character has a set of behaviors that are for when they are being aggressive, and another set of behaviors for when they are trying to hide from you, take cover, flank you, and so on,” Gregory explained. “And they make those judgments based on their perception of what you have shown to them. So it’s a bit like a poker game. If you reveal your hand, if you brandish a gun at them, they’re like ‘holy crap, this guy has a gun!’ and they start running. If you sneak around they may not know that you have a gun until you take that first shot.”

Since video games got big enough to need marketing that went beyond the odd page in a comic book, publishers and developers alike have claimed that their new AI system is revolutionary. You hear it again and again, how the AI is so responsive that it will blow your mind. “It is all most like it can think for itself!” we’re told, but again and again the result is generally underwhelming, and the promoted freedom of enemy behavior typically boils down to a few key behaviors programmed into an enemy that may have a random pattern of movement. It can still be challenging and immersive, but figuring out the enemy AI is generally easy. It isn’t often that the enemy AI can figure you out instead.

“It’s all based on their perception, and they’ll switch behavioral modes based on what’s going on,” Gregory said. “When they’re in attack mode, they’ll band together, they work as a team and they’ll be more aggressive. When you’re in the position of power, when the balance of power shifts, they’ll try to take cover more, they’ll try to flank you, they’ll try to sneak up behind you.”

It all comes down to that “Balance of Power,” which means that everything you do, every item you collect or pass up, matters. If you enter a situation where you are well armed, then the AI reacts accordingly. If you are spotted and weaponless, you will end up reacting to the enemy instead. It fundamentally changes how you will approach each situation.

Helping you maintain the position of power will be your partner Ellie. The nature of the relationship between the two characters is at the heart of the story and will unfold through the course of the game, but it is more complex than a traditional surrogate father-daughter pairing.

The game begins in a quarantine zone in Boston, 20 years after the plague. Through an as-yet-unrevealed set of circumstances, Joel and Ellie’s paths cross. Joel’s past is shrouded in secrecy, although it is somewhat implied that he was part of a hunter group and has a dark history. When a dying friend asks Joel to look after Ellie, he agrees, but that quickly proves to be far more difficult than first thought. The pair are soon forced to flee west as they are pursued into the brave new world.

Ellie is a child of this new world, and in many ways far more comfortable in it than Joel will ever be. She knows how to survive and adapt, which makes her a useful ally. Thankfully the game is not an escort mission, and you won’t spend your time having to protect Ellie. Instead, when not helping you through the new landscape, she can scavenge on her own and make herself scarce when necessary. The game takes place over the course of a year, and during that time Ellie’s capabilities will grow to match the maturation of the character.

This is a far cry from ally AI of just a few years ago. When Resident Evil 4 was released in 2005, it was (and still is) hailed as one of the greatest games of that generation. To this day you will still hear people rave about how it was moody and suspenseful, and the action was fun and balanced. You won’t, however, hear too many people say the same about the game’s AI-controlled ally, Ashley Graham, who more often than not became a liability that you had to protect. Games have come a long way since then, and The Last of Us is on the forefront of that technology.

“Ellie is very capable on her own,” Gregory said. “In a way, I kind of think of her as a power up. When you are separated from her, you really feel ‘Wow, I don’t have my ally with me and it’s harder to face the challenges. With she’s with me it’s very helpful.’”

The Last of Us won’t be released until the first part of next year, but it is already well positioned for 2013. As a developer, Naughty Dog has already earned the respect of gamers everywhere. But rather than simply pushing out another Uncharted (which we hope is still in the works as well), it decided to create an entirely new intellectual property, featuring a completely new world, and then decided to completely scrap existing AI programming and create something new.

So the next time you find yourself in a game preparing to hunt down and kill waves of enemies, you may not find it quite so easy. You may find them hunting you.