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Post-Play'em - Observations on Game AI


Left 4 Dead: Companion State Changes

Another interesting observation in Left 4 Dead. In this case, it is about the state machine that the companion AIs are using. First, the observations:

The first clip in the video below shows me getting ready to leave the safe house at the beginning of the level. My companions did their usual “grab some stuff” behaviors and then lapsed into “random wander” idle behaviors (I couldn’t hit the screen shot key fast enough to show Louis standing with his nose in the corner like a punished boy.) When I went down the stairs to the door, I was mildly perturbed that they didn’t follow.

I then opened the door and shot the zombie standing outside. They still had not moved to join me. A zombie rushed me, I leveled him, and my sidekicks were still admiring the walls upstairs. Only when I stepped across the threshold did they move to join me (with Francis doing a completely unnecessary walk on the railing… but that’s a future post). Something not shown in the video is something I have experienced before. Usually, when I step across the threshold of the safe house, they are in quite a hurry to leave the room, even to the point of pushing through me to do so. It probably would have happened in this case if we had not been under attack at the time.

In the second clip, we were at one of the intermediate buildings on a map. We had run inside, closed the door, stocked up on ammo and health packs, healed ourselves and whatnot. I opened the door and left the building. The video picks up as I look back inside realizing that my pals didn’t seem to want to leave. This was different than the first clip in that the trigger was not leaving the room.

There was a Boomer behind the building that I could hear. Even when the Boomer came around the corner and started waddling toward me, they didn’t move. Only when I fired my weapon did they decide it was time to rush out.

Now, for the explanation. It seems that Valve is using an HFSM (Hierarchical Finite State Machine) or another such tiered approach (a behavior tree can cause this as well). There is likely a high-level state that we will call “In Safe House”. When in this state, other lower-level states are things like “random wander,” and “random comment.” The only thing that seems to override it is if they see a zombie outside a door (many of the safe house doors have those barred windows). They will actually engage and kill zombies outside the safe house from inside the door. Therefore, there is an “engage/kill” state that is contained under “In Safe House”.

On the other hand, another high-level state is “In the World”. It is in this state that the AI spends most of its time. Apparently, “stick with the player” and “defend the player” are only included in this high-level state. That is why they would not follow me down the stairs while I was still in the safe house or defend me when I got attacked. However, once I crossed the threshold, a message was sent to them to change states to “In the World” at which point, they were free to analyze their usual parameters such as distance (move to the player) and threats (defend the player).

Note that this would not have been a big deal if I had simply stepped out the door. Alternately, in other safe house situations, the random wander location is in sight of the door. Therefore, when I got attacked, they would have likely seen the zombies and fought back. This particular arrangement did not allow for that.

Now, I don’t know what happened with the building in the second half of the clip. Because that isn’t an “beginning/end of level” safe house, I would suspect the above rules don’t apply. Why did they not leave, then? I have seen other behaviors where they don’t seem to follow me like I would expect, but I have usually found other explanations for that (another post on that later). In this case, it would have made sense for them to leave along with me just like they tend to stick close in other circumstances. That leads me to believe that there was an artificial state in play that led them to believe that they were supposed to be there (or rather had yet to convince them it was time to leave).

Regardless, in this case, the obvious trigger was me firing my weapon. This was not the case in the safe house example.

Neither of these issues is dreadfully wrong in a gameplay sense. They are only noticeable in certain circumstances. And certainly the companion AI in L4D is better than some we have seen. However, when issues like these happen, they do make us pause and ask “what are you guys thinking?” Therefore, while the logistics of the game may not be affected too much, the perception of the game is. It breaks that coveted suspension of disbelief by making us ask (in typical gamer parlance) “WTF?!?”

A simple solution would have been to pay more attention to what was under the HFSM state of “In Safe House”. Alternately, have more than one trigger to transition from “In Safe House” to “In the World” would have been better. For example, opening the door is an obvious trigger that it is time to go. Getting attacked certainly is urgent enough to warrant attention as well.

Additionally, abandoning the rigidity of a HFSM could be the answer as well. Much of the problem would be solved by using a system of free-floating priorities such as what I describe in my book, “Behavioral Mathematics for Game AI”. In that arrangement, you can generally dispense with the state/transition model in favor of one that always has all possible actions in play through a system of calculated utilities and priorities. Something similar to this is probably already in effect in L4D for things like target selection, action selection (fight, heal, reload, etc.) and other actions. Therefore, extending it to cover the situations covered above would not be terribly difficult, in my opinion.

Anyway, all in all the companion AI seems to be fairly decent. As we AI programmers know, companion AI is a beast simply because of how involved the companion is with the player. There’s more scrutiny, more options of what to do, and far more potential for the “WTF?” moment and the ensuing frustration. I think that companion AI is the next frontier of game AI that we are already in the middle of. L4D is in the vanguard of this movement and doing an admirable job of it.

Remember to click the tags below for more Post-Play’em observations on Left 4 Dead and other related subjects!

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