T w e n t y S i d e d

While playing Thief 3 some more I had a rather amusing AI mishap: A couple of NPCs went absoloutely sideways and started killing everyone.

All that glitters is not gold. Some of it is money and gems.

The setup was this: I’m in some guy’s jewelery shop, minding my own business and robbing the place. Then the owner gets home, and a couple of big goons are with him. From the sounds of it, he was involved with some underhanded deal with these guys and he tried to cheat them. He was supposed to hang on to some gems for them, but he was “skimming” the gems by (I think) cutting them. I don’t know. The goons are quite upset. The jeweler is unarmed and in a bad way. He’s trying to talk his way out of things and he’s not doing very well.

The conversation seems to hint that I might want to intervene. One of the goons says, “Sorry, but now we gotta kill ya. So unless you gots any friends hidin’ around here, this is it for you.” Odd thing to say. I can only assume this is the game’s way of suggesting I might want to help this guy, but I’m not going for it. If you make a deal with dangerous criminals, cheat them, and then get caught, then you deserve whatever fate you get. From my hiding place in the shadows I could drop one of the goons with an arrow, but then the other one would come after me. That is just too much trouble to save the life of a moron.

As the goons kill the jewler, he starts screaming for help. I decide to clear out. As I’m backing out of the room I bump something off a shelf and make some noise. The goons hear it and start hunting for me. An open doorway leading outside is on the opposite side of the room. I can’t make it there without the goons seeing me. However, I do have a noisemaker arrow, which is sort of a string of firecrackers on the end of an arrow. It draws the attention of people nearby. The goons are about to discover my hiding spot, so I fire the noisemaker out of the room. They hear the loud noise and go charging out into the streets…

…where they proceed to go completely nuts. They charge into the intersection outside and start slaughtering peasants in the street. The game auto-generates street traffic, so as they kill someone a new citizen is created nearby. (This happens off-screen, I never see them appear) This provides a steady stream of victims for the two killers.

My character is great at stealth kills, but nearly helpless in a stand-up fight. One town guard is usually more than I can handle. But these two guys are big, strong, and carry big swords. Whenever a member of the city watch shows up to help he just ends up on the growing pile of bodies. These guys are unstoppable.

I didn’t think to get a picture while this was going on, so the screenshot doesn’t really give the full effect. Whenever I look away the game does a bit of cleanup and removes some of the older bodies, so the pile is never bigger than five or six people. However, the real death toll must have been a few dozen. These guys were at it for a long time.

What a mess.

I’ve been thinking about this, and I’m still not entirely sure what made these guys go on the killing spree. Obviously it could have been a bug or a fluke or sloppy scripting, but I’m more inclined to think that the peasants that brought this on themselves, as the result of a chain of various behaviors. I obviously don’t have the source code, but I think that peasants become fearful when they see a character who has recently killed someone. (This is usually the player.) When they become scared they say things like, “I’m going to go find a town guard!” Then they run off to tell a guard about the actor that scared them. The guard will come and attempt to kill that actor.

Perhaps the noisemaker brought some peasants and the goons together, the peasants got spooked and decided to fetch the guards, which obliged the goons to kill them. Once this got started it would have fed on itself, since new peasants would get spooked by the pile of corpses in the street and forced the goons to kill them as well. If this is what happened then it’s an amusing case of unwanted emergent behavior.

Shamus Young is a programmer, an author, and nearly a composer. He works on this site full time. If you'd like to support him, you can do so via Patreon or PayPal.

A while ago I talked about emergent gameplay and how its superior to older, more scripted games. Of course, even newer AI can have some pretty amusing screwups. In fact, the “smarter” the AI, the funnier it is when the whole thing flies apart. Over the weekend I was playing Thief 3, and ran into a situation that made me think about how truly tough it is to make even passable AI.

The head of your average citizen has terrible acoustics, but the gold in their pockets makes a fun jingling sound when you pick it up.

In the game, you can sneak up on anyone (servants, nobility, or guards) and whack them on the back of the head to knock them out. (You can also stab someone in the back, but that’s noisy and bloody, and why kill them when you can just knock them out?) Once they are knocked out, you usually need to hide them. If you leave them laying in the middle of the room or hallway, someone else is likely to come along and discover your work. When this happens, they always assume the victim is dead. Then they start with the running and the screaming and everyone searching for you.

This can be amusing. I was in a large manor, working my way through a sleeping area for servants. One of them was already asleep in the bunk beds, but a few others were still wandering around. I zonked one of them and placed the sleeping victim into one of the beds. I thought I was being clever. Another servant came in, saw their compatriot in the bed and exclaimed, “Dead!? But who could have killed him? I’ll go tell the guards!” Then he ran off.

Blast it all.

When you knock somebody out, you need to stow them someplace dark and out-of-the-way. This spot is definitely sub-optimal.

I should have known better. The AI was just looking for knocked-out people. It didn’t care where the body was. I was so into the game I stopped the metagame thinking about AI and started thinking about what I’d do in the given situation. In that situation, placing a zonked person in a bed made a lot more sense than dumping them in a corner. However, to the AI it was just a poorly hidden body. Sigh.

But fixing this problem would be tricky, and would involve a lot of extra work. The level designers would have to designate certain areas or objects as “beds”, and the programmers would need to make it so that bodies laying on beds rouse less suspicion than bodies found elsewhere. Then they would need to add some new dialog and behavior: If an NPC sees someone “sleeping” on a bed (most likely not in their own bed) while fully clothed and while they should be working, he shouldn’t ignore them, but he also shouldn’t run away screaming about murders and dead bodies. You need some new behavior along the lines of “try to wake someone up and then discover they have been knocked out”.

The city that never sleeps. At least, not until I run around and give everone their good-night donk on the head.

But even with that extra effort, you can still have some amusing failure modes. Placing a servant girl on a bed in the priest’s quarters or the barracks should raise some eyebrows. Likewise, stacking two or more people in the same bed should tip off guards and servants that something is amiss. It wouldn’t make sense for them to just assume they all decided to take a nap together.

It would be annoying to code in such a way that it works right. The programmer would probably need to take the victim’s position into account as well. If I just toss somebody on the bed so that their upper body hangs over the side and their head is resting on the floor, it’s going to look pretty stupid if someone comes along and assumes they’re asleep.

Also, it seems like the length of time since the NPC’s last saw each other should be taken into account as well. If you greet your fellow housekeeper, walk out of the room, and come back a few seconds later to find them motionless in someone else’s bed, you are not going to think they are sleeping.

I’m the sneaking, robbing, creeping, crawling, thump-you-on-the-head so you can get a good rest medicine.

The game can be funny if you think about it too much. There is one house in particular where I stop by each night and give the occupant his good-night blow to the head. I’ve already robbed him blind, so he doesn’t have anything left worth taking. I just like to stop by after dark, sneak in, creep up behind him, and blast him on the back of the head. What must life be like for this guy, waking up each morning on his floor, wondering what happened and why his head hurts? Does he blame booze? Narcolepsy? His ex-wife?

Shamus Young is a programmer, an author, and nearly a composer. He works on this site full time. If you'd like to support him, you can do so via Patreon or PayPal.