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Post-Play'em - Observations on Game AI


Posts Tagged ‘Half Life 2’

Fable 2: First Look

Sunday, March 8th, 2009

Continuing with my pre-GDC blitz of “must at least look at” games, I have just putzed around with Fable 2 for about 90 minutes. I had a look at some of this material back at the 2008 GDC when Peter Molyneux was going through his pre-release hype. (Isn’t he always?) Therefore, I knew some of the things to look for.

Most importantly, on the AI front, is the inclusion of the almost legendary “dog” AI. To hear Peter talk, you would have thought this game was a bigger and better “Nintendogs” game… that the entire purpose of buying it was to play with your doggie and bond with him. While that isn’t exactly the case, I have to admit that the dog is a good point of interest. The dog certainly is convincing in many ways.

My Dog Runs Rings around Me!

I think one of the things that was startling to me was how it moves in a general area around you. Most companion AI tends to lag behind you. For example, my recent mucking about with the companion AI of Left 4 Dead has me convinced that I am the only person in the entire world with any initiative. This is obviously a logistic issue because very rarely can the AI know where you want to go or when. This is sometimes mitigated in linear games where there is only one place to go. In my short experience with Gears of War 2 today, I saw that exact phenomenon. My companions were able to move ahead of me and take up positions specifically because there was no other way to go.

In Fable 2 this is done via the “breadcrumbs” trail. Despite being far more “open world” than other games, if you have selected (or the game has selected) a destination, by extension, the dog knows where you are headed. Of course, if you turn around and backtrack or go off to one side for some reason, the dog has to react and come back to you… but this is what happens in real life anyway.

The result of all this is that the dog will playfully romp or explore ahead, behind, of off to one side or the other in a general “halo” around you. If you stop traveling for a period of time, he will come back and see what’s up. If you stand still for very long, he may even lie down for a bit… all the while keeping an expectant eye on you as if to say “now what?” It’s all very convincing.

I really noticed one particular aspect of the dog’s animation after I put the controller down and switched off the 360. I went to go feed our own dogs and, as I was walking from the food bin to the bowl, one of our dogs was walking a few feet ahead of me. However, he had his head turned slightly back toward me, keeping an excited eye on me and the cup of food that I was carrying. He did that very dog-like routine of glancing forward and then back at me over and over all the way to his bowl. It struck me that this is exactly what the dog in Fable 2 was doing as I walked. As he would criss-cross in front of me, he would keep glancing back as if to make sure that I was coming along with him. It was this sort of attention to detail both on the behavior and the animation AI that really sold me on this dog. I’m looking forward to experimenting more with it soon.

Look at Me When I’m Talking to Ya!

Interestingly, there were times with the villager AI that I felt that the eye-contact issue was the problem rather than a feature. I’m not talking about how, while in conversation, people will glance away or past you. This was a problem with where they were facing in general. If they started their spiel while I was in one place, and then I moved to one side or the other, they would continue to face the spot where I was. Only after a moment (usually after finishing a line of dialog), would they turn to face me again. It was all rather disconcerting. Compare this to how well games like Half Life 2 continued to address you throughout a conversation… even to the point of turning and walking backwards if necessary.

Dialog Pauses

Another problem I had with the villager AI was how many of the scripted dialogs had very unnatural pauses between lines. There was an awkward silence between many of their sentences that left me wondering for a moment if they were finished or not. It was one thing when it was in response to something I had selected. However, when they paused in conversation with each other, it was odd. When they paused between their own lines, it was horrific and very distracting. I have to imagine that this is because each line of dialog needed to be loaded and accessed separately, but I can’t see why this would be a problem.

Both the facing issue and the dialog pause issue may be related to the same root cause, however. It may be a result of waiting for an associated animation to finish and report back that it is done. If a line of dialog (and the code to “face the player”) are associated with a particular animation, they would each have to wait for the other to finish before they both advanced to the next segment. If the animation finishes first, it isn’t as tragic because the animation can sit in idle for that extra second. However, if the voice finishes first, the next line must wait until the animation is done — causing the unnatural pause. This is just speculation on my part for the moment. I figure that if I analyze it a little further I can make the determination down the road.

Individual Individuals

One thing that jumped out at me during the initial city quests was how the group of kids all reacted slightly different to my various gestures, poses, and flatulation. I would get more “love points” (or whatever) from some than I would from others in the group. Upon a little examination, I found that each person had their own likes and dislikes — that is, things that they were more impressed by or didn’t like. It’s one thing to have different “respect ratings” from different people. It’s quite another to have different models of what causes respect on an individual basis.

I found this rather amusing since I am doing a co-lecture on this very subject at GDC in a few weeks. (“Breaking the Cookie Cutter: Modeling Individual Personality, Mood, and Emotion in Characters“) It’s something I would like to see more of in games in the future — although maybe not in quite as transparent a fashion as what appeared on-screen in Fable 2.

Will You Treat Me Different When I’m Famous?

I think it will remain to be seen what happens when I’m more renowned than I am so early in the game. In the original Fable, it got rather annoying that everyone, everywhere wouldn’t shut up about you. The comments that they heaped upon you were not as many in number as there needed to be. (You can only hear “chicken chaser” so many times before you want to crack someone’s head.) With a nod to my friend and colleague, Adam Russell, who did the villager AI for Fable, that isn’t an AI problem as much as it is a content problem. (This is another bit that I will bring up at the AI Summit panel, “Characters Welcome: Next Steps Towards Human AI“… it doesn’t matter how many decisions our AI can generate if we don’t have enough art and sound assets to express it.)

Anyway, this is a bit long for a first look. I will tag on some more as I experience more of the game. On the other hand, I will be shipping this back to GameFly in the next day or so. I need to check out a couple more titles before GDC in a couple of weeks.

If you are just jumping into this one article, make sure you click the tag(s) below to see if I have written anything else about Fable 2.

Half Life 2: Paying Attention to Me

Tuesday, February 10th, 2009

When I posted my first observation on Half Life 2, “Movement on Rails” the other day my colleague, William van der Sterren commented that one of the cool things that they did in HL2 was making the NPCs acknowledge the player’s position during conversations. While I certainly agreed at the time, I thought this was limited to eye contact and head position. That was just the beginning. I had not experienced the full depth of what they had done in the game.

Last night as I was playing some more, I had some of the extended conversations at Black Mesa East. As Dr. Mossman was walking me through the lab, I was surprised to not only see her turning her head, but even walking backwards (convincingly well) at times so that she could continue to face me. I’m tempted to go through that section again just to mess with her. If I keep moving, does she keep changing?


Anyone who has coached kids playing baseball has probably taught the drill of how to watch the ball as you are running backwards. Part of that drill is to change sides without turning your head. That is, looking over your left shoulder, then turning your lower body to run with the other shoulder leading so that you are now looking over your right shoulder. I think it would be interesting to watch the NPCs in HL2 react to that sort of situation.

All in all, I agree with William that it was a new point of immersion for a game NPC. More specifically, it worked to not break the suspension of disbelief simply because it looked natural.

Monlogue Breaks

On the other hand, there were awkward pauses in the monologues of the various characters. Sometimes, I figured that they were waiting for me to cross an invisible trigger threshold. That is, as they entered a room, they waited for me to cross through the doorway that they were then going to close behind me. However, the next line of monologue was put into the same queue as the actions such as closing the door. Therefore, they could not proceed with their spiel until such time as I allowed them to close the door. If I had not moved, they would never have continued.

A solution for this would be to have a separate queue for the speaking and the actions. However, the speaking could be superseded by ad hoc lines such as, “Come on through the door so I can close it behind us.” and then continuing on with the monologue. This shouldn’t be too difficult to do with a priority-based system. The insertion of the vocal line “Come on through…” would be a higher priority than the other lines. As each normal line finishes, it triggers the insertion of the subsequent line into the queue with the appropriate priority. If there is nothing else more pressing, it would chain right on the end as per usual.

One of the effects you can pull off with this method is reaction-based stuff. For example, I was dying to fire my pistol into the ceiling during some of the speeches and conversations. Would they have reacted? I don’t know about HL2, but in other games, probably not. However, a priority-based system would override whatever sequence is being dictated with “What the hell are you doing?”, a few follow-ups about shooting weapons in the house (“You’ll put your eye out, kid!”), and the revert back to the regularly scheduled monologue.

Another effect would be acknowledging random people walking by. For example, during the hallway monologue, Dr. Judy could have received an event that a colleague was approaching coming the opposite direction. She could have inserted a slightly higher priority event to say “Hi” to the person in between lines of her monologue. She would say “Hi” at a break between lines and then go back to what she was talking about. Sure, there are caveats that you have to look for, but the inclusion of simple mechanisms like this work in a similar fashion to the head-turning one. They look natural and therefore feel natural to us.

(Remember to check the other Post-Play’em observations on Half Life 2.)

Half Life 2: Oblivious to Death

Tuesday, February 10th, 2009

One thing was kind of startling to me just now (I actually quit playing so I could write this). When I got killed, there were 2 bad guys right in front of me. I had only just hit the floor when they both immediately turned and began their “random patrol” state. I laid there and watched through my red-tinted death goggles as they paced back and forth in front of me. Despite being bombarded with radio traffic about my existence, and just enduring a firefight with me, neither of them was at all affected by my demise. Even a non-AI-savvy person would be jolted by this obvious lack of attention to my body. It just screams “state change”. Kind of a reverse aggro, I suppose.

In Halo 3, on the other hand, it was a very effective touch that, when I died, I could hear the Brutes and Grunts celebrating and taunting me. It was a reminder to me that I had let down my people. Also, it was a touch of “humanity” (loosely used) for the enemy AI. The one simple inclusion of an “enemy down” state would made a lot of difference.

Think about all the ends of firefights you see in the movies. There’s the “is he dead?” phase, there’s caution, relief, reporting in, etc. When I died in Half Life 2, I was wanting them to come over and prod my body or something. Really an extra couple of voice lines and a couple extra states to transition into and the extra immersion would have been very welcome. After all, dying (in a game) is an emotional moment for the player. Make him remember it.

(Remember to click on the tags below to see more about my observations of Half Life 2.)

Half Life 2: Movement on Rails

Monday, February 9th, 2009

OK, I admit that I had not gotten around to playing Half Life 2 until now. (It’s part of my post-book writing, pre-GDC ramp up.) I am not terribly far into it. I am alternating between riding my airboat (kinda fun!) and having to stop and do the various side-tasks.

So far the enemy AI is decent, although bland. (I’m playing on Normal, if that matters.) I’ve noticed a little ADD in the soldiers, however. They can sometimes forget where I am (or just was). If they are supposed to be looking for me, it isn’t working.

Another annoyance was the rails that the background people are walking on. During some of the opening scenes, when I was in the middle of a large room, people would walk straight through me. At one point, I didn’t notice what had happened right away other than that I was jostled out of the way. However, only about 15 seconds later, while I was standing in the same spot in the middle of the room, another person walked down that exact same line. I looked around a little to see if there was an obvious path that I was on but I didn’t see one. It looked like they were simply going from one point of interest to another and didn’t care that I was in the way.

Possible solutions involve the addition of a steering behavior on top of the standard pathfind. Even just the addition of a vocal cue of “excuse me” or “look out” would have softened the blow somewhat. Instead, the citizens came off as completely mindless.

On a positive note. The manhacks are convincing in their movements, though. They remind me of how difficult it is to control those little remote-control helicopters… which I suspect is the effect they were after.

(Remember to click on the tags below to see more about my observations of Half Life 2.)

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