Post-Play'em - Observations on Game AI

Doom 3: Pretty Wind-up Toys

I dug out my Doom 3 again specifically for this blog. I hadn’t ever finished it when I started playing it about a year ago. It sure was purty, but it just didn’t engage me the way that I was hoping it would. Anyway, my thought as I returned to my saved progress was “pay attention to the AI so I can review it.” As I played, I became a little unsettled – not by my progress in the game, but by my progress in my quest for AI. I finally had to come to a realization that seemed almost blasphemous or sacrilegious.

Doom 3 doesn’t have AI.

Sure, I know that statement is a little over the top. Let’s face it, the AI in Doom 3is better than that of games from when I was a kid in the 80s. But that is also the heart of the problem. Doom 3′s AI isn’t much better than what was in the original Doom in 1993. Considering the related concept of Moore’s Law, you would think thought that AI would have increased at a geometric rate similar to that of anything else in computing. It is with that expectation that I claim that Doom 3 “doesn’t have AI”. Looking backwards down a Moore’s Law curve of AI, one could make the statement that the AIs in the past approach zero… therefore being zero for all intents and purposes – especially in a relative sense to where they should be today. So… my statement is now qualified somewhat. (Why do I feel like I’m going to end up having an uncomfortable beer with someone from id at the GDC in February?)

The comparison to the original Doom (or Quake, et al) is not far from the truth, however. The state machine has to look something along the lines of:

  1. Idle
  2. Agro
  3. If has ranged weapons – fire
  4. If no ranged weapons – approach target
  5. If health <= 0 then Die

That’s it. In the newer versions, you can splice in something towards the end that involves a side-step. The bad guy marines especially will do this. To me, the player, this makes me do the very involved strategic and tactical process of… uh… re-aiming my gun. Wow.

Admittedly, there are some baddies that give me more fits than others because they are not approaching me in a straight line. Lost Souls tend to zip around like gnats once they get close to you which is mildly annoying and Cacodemons will float around as you shoot them – but that means they have as much AI as a helium-filled piñata. There are other differences as well. The half baby/half fly Cherubs will back off a bit, prepare a moment, and then leap. Of course, so do the little spider things.

This is a major let down. I remember seeing some in-game clips at a game conference during an interview with Carmack. (I believe it was at E3.) The scene where the pink baddie tried to beat down the door, gave up and instead came crashing through the window was terrifying! “Wow! It gave up and used the window!” I told myself. When I played the game… and that exact same sequence happened, I realized that the whole thing was scripted. Oh. Bummer. Again, it wasn’t AI. Unlike F.E.A.R, where the agents do use their environment and re-plan when faced with obstacles such as the door above, the Doom solution was a movie crafted for my benefit.

I have been flanked by the computer, however. This I admit. Often! But that has nothing to do with the AI and everything to do with the placement of triggers. In true horror movie fashion, the computer does come up behind you often – but only because a designer placed a trigger on the floor that says “when player gets here, spawn evil dude behind him”. That isn’t AI. It isn’t even fake AI. In fact, it’s getting rather tiresome to know that every step I take is likely to generate an attack from a place I just cleared out. It makes for great anxiety and twitchyness – which is great for a horror movie. I admit that I have a good deal of tension when I play the game not knowing from which dark corner or what hidden panel I will receive the next assault. But it’s not AI. Really, it’s like making the “only average” fastball pitcher harder to hit by moving him closer to the plate or allowing him to throw from foul territory. He isn’t a better pitcher now… he just has been given an artificial advantage.

Really, the movement and attack logic for the enemy is not much more advanced than the little table-top wind-up toys. They will chatter away in my general direction, but there isn’t much purpose or reactivity to their actions. That gets very disappointing.

There does seem to be a bit of cheating going on, as well. The first time I fought a Mancubus, I was doing my best to flank him around walls. Despite having moved at least 60° off to the side, when I poked my nose around the wall to take another shot, I found he was already facing right at me. The explanation would have to be that there is omniscience on my position. Here, I had decided to do a simple, but what I thought clever, evasive maneuver only to have the game say “tough… I don’t care”. It’s deflating to have that happen.

There also may be some cheating going on for my benefit as well. This doesn’t fall exactly into AI, but it is relevant nonetheless. I swear that I take less damage per attack when I am almost dead. I haven’t done the math yet, but it seems that I can be in the teens or single-digit health for far longer than I spend in any other 10-number range. A hit that takes me down from 100 to 80, for example, may also only take me from 15 to 10. The result is that I spend a ton of time between 0 and about 40. Of course, if this is happening, this is a great device for creating tension in the game – and is a variant on rubber-banding. It’s rather artificial and arbitrary, though.

All in all, Doom 3 is a pretty game. It’s gorgeous. It’s use of dynamic lighting was cutting edge and very impressive for 2004. However, when you put it up against the interactivity and dynamicity of other shooters, there is something empty about it. It’s like trying to engage in a dialog with paintings in the Louvre. They are very impressive as art – but they aren’t going to talk back.

I have always respected John Carmack for his vision – and the “visions” that his games have provided for me. I just can’t help wondering what would happen if they were to put a bit more focus on AI. I notice on the Wikipedia site that Johnathan Wright is listed as an “AI Programmer” but that in the credits for Doom 3, he is just listed as a “programmer”. I would love to sit with him at a GDC party and have that beer. “What’s up, dog? Is there anything else you could be doing?” I can’t blame him, necessarily. Given the depth of the graphics rendering, he may very well just be out of clock cycles. *shrug* I hope that he has a better chance in the future. I would very much like to see an id offering that does “speak to me.”
Until then… there’s more than enough to challenge me elsewhere.

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11 Responses to “Doom 3: Pretty Wind-up Toys”

  1. alexjc says:

    I guess I wasn’t bothered that much by the AI in Doom 3. I thought the game was very engrossing and the behaviors did the trick for what they were supposed to do.

    After all, they are monsters and zombies!


  2. Markus K says:

    Quoted from this (Carmack Interview):

    “Carmack is skeptical. AI is a very bleeding-edge science, and it can often be processor intensive, but when applied to games AI is usually a matter of scripting. What game designers want is a way to act as the ‘director,’ telling enemy and friendly characters where to stand and what to do. This doesn’t take a ton of processing power.”

    He goes on describing how the original Doom tricked Players into seeing behaviours which weren’t there but hey, time went on & I for one found Doom 3 somewhat lifeless and didn’t play through it. Even after playing through CoD 4 which IMHO is perfectly scripted, there’s this somewhat lifeless feeling and I know I won’t play through a 2nd Time since the whole experience will just be repeated 1:1 !

  3. Dave Mark says:

    Very nice comment from Carmack. My disagreement with him is how he stopped at “What game designers want…” and didn’t try to resolve that with “What players want…” As you have pointed out, the replayability gets kind of old with scripting. It’s kind of like watching a movie over and over. It doesn’t hold your attention and excitement compared to a brand new sporting event. Even if it is the same two teams, it is different every time.

    Thanks for stopping by!

  4. Anonymous says:

    The problem with game AI in FPS games is that if there was any decent AI AT ALL, it’ll become quickly un-winnable in single user campaign mode and no fun to play.

    Even the most basic “decent” AI will have the enemies either communicating and swarming you, or with even a basic self-preservation rule, they’ll be seeking cover and fighting hit-and-run until the levels extra ammo and health items are depleted and you die.

    So “good” AI would have to be developed, to only have it purposely sandbag itself make mistakes etc. to give the player a chance. So what would be the point?

    A few simple shoot-move-die rules are cheaper to develop, reduce the time-to-market and avoid any dreadded vaporware accusations, take up less system resources allowing a wider market of machine users to purchase your game.

    And the net results of “good” AI that has to purposely downgrade itself won’t be that much different than the “bad” AI FPS games have now.

  5. Dave Mark says:

    The premise that any sort of AI in a FPS game would make it unwinable is flawed. There are plenty of FPS games with very adequite, even challenging AI that are still winable. The Call of Duty series, for example, has fairly decent enemy AI. F.E.A.R is being heralded as groundbreaking throught its use of planning systems.

    However, your assertion of having to “dumb down” a perfect AI is correct. That is actually a common technique. It is a simple matter, for example, to make our AI bots aim perfectly. All the 3D geometry can be done with ease. Therefore, we have to “fuzzy it up a bit” by adding random fluctuations to the aim points of the bots – to make them more “human”.

    Aiming, however, is not the visible behavior that is what I am complaining about with regard to Doom 3. There are NO strategy or tactics being used at all in the Doom enemies. It’s not a matter of taking something perfect and dumbing it down… it’s a matter of simply trying to put some behavior in there at ALL.

  6. zwenkwiel says:

    well of course they don’t use tactics
    they’re monsters
    not paratroopers

  7. Dave Mark says:


    Even monsters shouldn’t get stuck behind the only pillar in a room. Small rodents can figure out how to get around an obstacle that’s only just as wide as their bodies – you would think the denizens of Hell could pull it off.

  8. Anonymous says:

    You completely misunderstood the December 22, 2007 11:17 AM post. He wasn’t talking about aiming. He was talking about the monsters using basic strategy and tactics.

    And ya, they’re supposed to be stupid. Try not to compare them with rats/monkeys/humans etc, okay? That’s not a fair comparison. They’re supposed to be big, lumbering, stupid brutes. And they are.

    Doom 3 is supposed to be a nice scary horror game, to scare you. Not a strategy game designed to make you think.

  9. Dave Mark says:

    Smart is a lot scarier than dumb, in my opinion.

  10. Gunnar says:

    “Doom 3 is supposed to be a nice scary horror game, to scare you. Not a strategy game designed to make you think.”

    Well, F.E.A.R is a nice scary horror game but still has some strategy that makes you think some of the times (and sometimes wet your pants).

    I agree that most of the Doom 3 creatures are “big, lumbering, stupid brutes”, but id could then inprint AI that made them act like that. There is a big difference in monster that is “big, lumbering, stupid brute” and a monster with (almost) no AI.

  11. [...] AI is necessarily What Our Players Want™, however. It certainly beats the old “Doom model” of “walk straight towards the [...]

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