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Post-Play'em - Observations on Game AI

Posts Tagged ‘pathfinding’

Dragon Age: Origins — Final Thoughts

Wednesday, November 16th, 2011

Well, after messing around with it for over a year, I finally got around to finishing Dragon Age: Origins. I have to admit that the game grew on me after a while. Here are some final thoughts on it. (Note that you can read my earlier posts on the game.)

Custom AI Tactics

“I kind of liked dealing with the AI tactics.”

Once I finally dedicated a little bit of time to looking into it, I kind of liked dealing with the AI tactics. It’s a simple rule-based system, of course, but one that gives you a feeling of control. The reason that I specify “feeling” in this case, is that most of the rules are pretty obvious. For example, a rule for Morrigan might read “When there are 3 or more enemies clustered together, use Fireball.” I’m sorry, but that goes in the “duh” column. Really, that should be a part of the logic for the Fireball spell in the first place. The same goes for the “dual weapons sweep” logic that I used on my main character (for when I wasn’t controlling him). “If there are 2 or more enemies surrounding you, use dual weapon sweep.” (I also found that I used it with one enemy just because it looked cool and did great damage.) Regardless, many of the usage rules just simply made logical sense.

“The solution to this is to build the logic into the abilities themselves.”

The solution to this is to build the logic into the abilities themselves. If you are wounded, use a healing poultice (if you have one). If you are low on lyrium energy and have a potion, use it. If you are strong and have a shield, do a shield bash against your enemy. There really should be no need for the player to construct these manually. Of course, I don’t know what the chicken/egg order was on this mechanism and the “tactics” abilities that were built into the character customization. That is, did they create the AI tweaking mechanism and then decide to add the character abilities that allowed you to unlock more tactics slots or did they create the notion of “has better tactics” and have to come up with a way of expressing that?

“That character has that cool new ability but never uses it!

The unfortunate by-product of this is that I can see how it would be very frustrating for people who didn’t know that the tactics screen was there or didn’t understand how to use it. I’m sure there were more than a few people who would cry, “that character has that cool new ability but never uses it! This AI is stupid!” Of course, there is always the possibility that the reason this mechanism was in place was because they were struggling with how to have the NPCs use their skills intelligently. By putting the onus on the player, they absolve themselves of a little of the responsibility. I don’t buy this line of thinking because, by creating a customizable rule-based system in the first place, the battle is almost won.

Companion Pathfinding

“I usually was pretty pleased with their positioning.”

With Skyrim being out, one of the comments you hear about the companions is that they can do silly things like block doors or passages and be completely clueless about the fact that you want them to move. This is not something that I ever found to be a problem in DA:O. In fact, despite the fact that I usually had 3 other companions along with me, I usually was pretty pleased with their positioning. They followed at respectful distances, they didn’t get trapped behind geometry, they handled it fairly well when I would reverse direction on them, etc. Most of the time their “group” behavior was very believable.

There was only one time that I noticed anything really odd. I was in Dust Town in Orzammar and Zevran (easily my favorite NPC, by the way) managed to get up on a pedestal of some sort and was unable to get down. After some examination, I realized that it was an extension on either side of some stairs. He had somehow managed to enter it from the top of the stairs but couldn’t exit that way. Likely there was navmesh geometry on the pedestal (why, I don’t know) but no link to the rest of the mesh. For some reason, when my back was turned, he skipped a mesh boundary by being nudged off the main mesh. I’m just speculating here. Anyway, I had to actually exit the zone without him and come back.

Enemy Targeting

In an earlier missive on DA:O, I had complained about how the target selection seemed to be a bit nonsensical at times. Perhaps I got used to it or perhaps it wasn’t as bad as I initially thought… but this issue didn’t seem as bad as I had commented on earlier. There were times when I thought it was a bit odd, but it wasn’t altogether unreasonable. That, of course, has been one of my mantras for some time and, in fact, is something that I stress in my book: “Not always perfectly rational, but at least reasonable.” I don’t know if that was the goal here or if their targeting was simply sub-optimal.

Of course, with regard to my companions, some of it had to do with the tactics selection and aggressiveness settings. If I had them set wonky, they would act wonky. However, I can’t use that excuse with the enemies. On the fly, it is hard to determine what sort of logic they are using… e.g. “nearest visible” vs. “strongest”. This would be something I would have to record and slow down to decipher. However, since it doesn’t seem to be that weighty of an issue, simply “knowing” wouldn’t really change much.

Again, though, some of this is mitigated by the fact that you can control each character in turn. Is this a bandage, though? Are we at another chicken/egg moment? Did they allow you to control the character targeting because they felt people would be frustrated by the target selection? Probably not… it’s not that bad to force them to build a whole huge mechanism (character control) around it.

Scripted “Random” Conversations

I found the seemingly random chit-chat between companions as one of the most amusing parts of the game. I have to admit that it was so endearing, that it made me want to replay the entire game with different companion combinations just to see what all these conversations were. (The combination of Zevran and Morrigan was a riot!)

“You begin to get used to the fact that riiiighhhtt here someone is going to start talking.”

The initial impression is that these asides simply take place “whenever”. Conveniently, they would also take place when you really weren’t doing much else. It didn’t take long, however, to realize that there were “hot spots” in the terrain that triggered these conversations. Not to the extent that Place A triggered Exchange A; they were randomly selected from the available list. They were obviously placed in areas where you needed to do a bit of walking and, therefore, wouldn’t be interrupted with much else. The long bridge to the Proving Ground in Orzammar has one, for example.

The problem is when you have to walk the stretch more than once — especially in succession. You begin to get used to the fact that riiiighhhtt here someone is going to start talking. It’s even worse when you have to reload because it doesn’t remove the conversation from the potential list… and you may get stuck with the same one over and over. While I like the conversation as a way of both fleshing out the characters and making the scenic walks a little less boring, I have to wonder if there is another way of doing it.

Facial vs. Body Animation

“The lip-syncing is worthy of being looked into by the pop-music industry.”

While this doesn’t necessarily seem to be AI-related, I figure that it probably is. The facial animation in DA:O is great. The lip-syncing is worthy of being looked into by the pop-music industry. With how good the lip-sync is, I found myself staring obsessively at the mouth only while people were talking. Maybe because I was drawn to the only realistic part in the whole face.

The rest of the facial animation is spotty, however. I would love to see more expression in the rest of the face, too. The eyes, eyebrows, and, in fact, the entire head, needed to be more animated. While the head wasn’t “Fallout” creepy, it needs to move more… look around a bit. Same with the eyes. Little tiny twitches are nice, but people do things like glancing at the floor, etc. You’re on the right track, but need to do more.

“People don’t just wave their hands randomly.”

The body animation during conversations was simply horrible, though. I like the whole “hand-wavey” thing that they tried to do. It’s better than standing perfectly still. On the other hand, people don’t just wave their hands randomly — they use them to augment what is being said while it is being said. They use it to punctuate words much the way I used italics in the this paragraph. If I had italicized different words that were obviously not meant to be stressed, I wouldn’t have communicated the same thing or, worse, it would have distracted or even confused the reader. That’s what I felt about the hand motions in DA:O. They seemed vaguely relevant but so imprecise and ill-timed that it made things worse rather than better.

The solution likely lies in a markup of the dialog to map to the animation selection. By tagging the stressed words with the appropriate gestures, you can match them dynamically. The trick, of course, is that you have to back-time the start of animation so that you can finish on-time. The shorter the target animation, the easier this is to pull off.  That would actually require two markers… e.g. “move to position” and “execute”.

For example, if I want to slap the table as I’m saying “now”, I would tag a point 2-3 words ahead of time with the “raise hand” animation and then the word “now” with the “slap table” animation. Imagine the impact (so to speak) of having a character be able to slap that table, point the figure, shrug the shoulders, incline a head, wave something off dismissively, etc. and have it happen right at the precise moment it needs to. Now that would be immersive! (Note that the word “that” would have been a great place for a couple of key animations, eh?) (Note that the word “eh?” in the prior sentence would have been a great place for a… OK… never mind.)

That said, the rest of the animation was spot-on.


“The companions didn’t cause me undue stress… which is a spectacular accomplishment.”

All in all, Dragon Age: Origins was a really well done game. I enjoyed a lot of it and never felt really “constrained” by a specific narrative. I know I left a lot of side-quests on the table, but that’s almost a requirement for a Bioware game these days. You aren’t going to do them all and still manage to be a productive adult.

The companions were, indeed, companionable and didn’t cause me undue stress… which, as far as companion AI goes, is a spectacular accomplishment. The enemies were reasonably intelligent and only some of the more obvious boss monsters were pattern-style scripted (which is a disappointment because pattern-style scripted AI looks less intelligent).

All things considered, I enjoyed the hell out of the game. People have told me that DA2 is well worth the time. I just wish I had more of it to dedicate! I have a feeling that it isn’t a game I can finish in a weekend!


Splinter Cell: Conviction – First Look

Sunday, July 10th, 2011

I’ve been putzing with Splinter Cell: Conviction on and off in recent weeks. As a side note, my use of it has mostly been “off” because I find it frustratingly difficult, even on “normal”. That being said, I’m impressed with the game as a whole.

Too often in games can we do stuff that is completely ridiculous and people don’t even react at all.

One of the first thing I noticed was how “by-standers” react to me. If I’m crawling around on a wall or doing something else “odd”, they will turn and look or making some snarky comment. I like that in a game. Too often in games can we do stuff that is completely ridiculous and people don’t even react at all. In order to facilitate this, I assume that actions and/or locations were simply flagged as “odd” and broadcast to the people in the area.

Similarly, I like how the “Thief-like” environmental triggers can arouse the attention of the bad guys. The voice acting in this case really sells it. They are often very subtle about it, saying things like, “hey… I’m gonna go check something over there.” It could have been done in a more heavy-handed way (e.g. “I heard something over there.”)… but the way they did it almost projects how the AI is actually questioning itself. The minor difference makes you (the player) think more “I need to be more careful” rather than jumping straight to “Oh shit!”

Anyway, that isn’t what I was wanting to write about…

No Rails Here!

Obviously, being the type of game that it is, there are more than a few opportunities to lie in way for people. Early on, there was a point where I was waiting for a guy who was unaware of me. I found myself falling back on old gaming mentalities and trying to detect his “patrol route”. That has long been a staple of games—to the point where it has been ingrained in me.

I kinda just had to mentally will him in my direction.

Needless to say, the baddie didn’t follow a pattern of any sort. I kinda just had to wait him out and mentally will him in my direction. Plenty of that time was spent watching him not move at all. I actually wondered for a while if he was a static set-piece rather than an ambulatory one. He wasn’t… he was just alternating between hanging out and wandering around. It really did a good job of selling the idea that this guard was that reasonable mixture of responsible and bored to tears.

On the other side of the coin, when the guys are actually searching for me, the game does a good job of making them look reasonable in their search for me, but also looking somewhat unpredictable. They don’t do the standard “run right toward where the player should be” gameplan once they lose track of me. That would be too silly. Once they have “lost me”, however, they don’t just go searching around willy-nilly; they keep to a reasonable area. That makes for a lot of tense moments when you are holed up in a corner!

While they are searching, the agents do a good job of looking around with that slightly spooked feeling. They will swing their flashlights around—sometimes in radical changes of direction like they are either surprised by a noise or trying to catch me by surprise. I actually found myself just watching the AI search for me to see what all they would do. (Well… the fact that I was on the outside of a windowsill with nowhere to go was a factor.)

Down in Front!

The combat AI seems solid. Their reactions are decent and they usually take cover fairly well. I don’t believe they use cover quite as well as what I have been seeing in Gears of War 2 (which I am finally almost done with). Sometimes they seem to be on the wrong side of it. Not on my side, mind you… just on perhaps the 90° side of things rather than opposite me.

A guy ran up and hid along the side of the couch that I was crouching behind.

On the other hand, the enemies don’t seem to do as much pointless running around as they did in Gears. They might want to consider more than simply cover points, though. There was one moment where a guy ran up and hid along the side of the couch that I was crouching behind. I was so startled that he would run up to get within point-blank range of me that I was very taken aback.

They don’t seem to use their environment in as dynamic a way as F.E.A.R. did, but it doesn’t set them back. In fact, if they were flipping couches and knocking over bookshelves, it might have looked a little contrived.

You Really Get Around!

The pathfinding is decent (thanks, I’m sure, to the excellent navmesh work of my buddy Martin Walsh). I believe there was one moment early on where I saw someone backtrack 2 steps before doing a turn the other way as if he was on a grid. That may have been something else, however. I just remember noticing it because I knew ahead of time that the game was on a navmesh.

While the local avoidance on the streets seems OK, I think that the “personal space” buffer could have been increased slightly. People coming the other direction weren’t steering to go around me soon enough. That’s minor, though.

When I make Sam stop and reverse direction quickly while running, the animation reminds me a lot of the same action in Assassin’s Creed. I have to wonder if that is a coincidence or not.

All in all, I’m quite pleased with it so far. I do need to pay a bit more attention to the AI in the combat situations. On the other hand, the fact that I’m so distracted with staying alive means that the enemies are certainly doing their job well!

Dragon Age: Origins – Don’t Crowd!

Tuesday, May 25th, 2010

OK, so it has been a little longer than “later this week” for getting to Dragon Age: Origins commentary. I can’t believe I wrote my first look post back in late February. Of course what that really means is that I’m burning some serious GameFly time on this one title… and it’s not like I’m playing it much. My son has managed to get all the way through it, however.

Still, there are some things I noticed as I was going through (which, admittedly, is not all that far yet). One was an odd happening in the Ostagar camp. I was cruising around looking at things and talking to people. As I was standing there talking to Duncan (and a couple of others), a camp soldier of some sort came and stood near us. From the looks of it, he was supposed to be dealing with another person there, if I recall correctly. This didn’t really catch my notice until a 2nd soldier came up and attempted to stand in the same spot and talk to the same person. They jostled each other for a bit until the first one walked away – apparently on a random patrol route.

…a 2nd soldier came up and attempted to stand in the same spot…

I believe that it was the random patrol routes that caused this issue in the first place. My guess is that the guard logic simply selects a new destination and heads out in that direction. If, for some reason, there is someone already there, it doesn’t detect this or care… it simply continues to try to get to the node.

There are a few ways to either avoid that or fix it. First, the goal nodes could be reserved and released as they are selected and then left. The problem with this solution is that a goal node would be “out of commission” from the moment someone reserved it to the time they left it. If they were coming from all the way across the map, that node would be off the market for quite some time without actually being used. Additionally, it would prevent someone from heading in that direction until the original occupant actually left.

A lot of work to make the camp “feel alive” went down the drain…

A more realistic solution would be to simply be aware of other people. If there is someone on your “spot”, don’t actually try to move onto it. This could be done by simply stopping short of your intended goal (e.g. stand next to him) or by using some local avoidance. The later is probably better because collision avoidance should be part of the movement scheme anyway for other reasons.

Anyway, the result of this was that it was a jarring cancellation of my suspension of disbelief. Because it was right next to me and in view, I couldn’t help but notice that these two dolts were climbing all over each other. A lot of work to make the camp “feel alive” went down the drain right there.

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.)

Company of Heroes: My First Look

Saturday, February 7th, 2009

Well, now that I’m finished writing my book, I am grabbing a few demos of games that I really need to take a look at. I pulled down the free demo of Company of Heroes off of Steam the other day and have had a chance to putz with it a bit.

One of the reasons that I wanted to see CoH was the articles by Chris Jurney in AI Game Programming Wisdom 4. He wrote up a pair of articles on the pathfinding algorithms that they used–one for constructing realistic turns for vehicles and one for the ‘leapfrogging’ of squads. Both are good reading.

Anyway, I haven’t touched an RTS since Empire Earth (which chewed up way too much of my time in its day). It took me a bit to get back in the RTS groove. I was immediately impressed by the way the squads moved, but perhaps that is because I was looking for it. Also, they seem to react well to the unexpected. One concern that I had was how they didn’t seem to always go for cover when I would have expected them to. Despite having plenty available, sometimes they would hang out in the open. This is more alarming when 4 of the 6 squad members are in cover nearby but the other 2 don’t go. It makes me briefly wonder if the value being tracked is the amount of cover per squad rather than per soldier. I will have to investigate further on that (or just ask Chris when I see him next month).

The vehicles, as well, move admirably. Calculating vehicle movement on the fly (in a destructible environment!) is, indeed, a pain. Again, I will observe more, but they seem to do fairly well. The tanks have the advantage of being able to turn in place. However, the jeeps and trucks do not have this luxury and need to plan ahead lest they find themselves unable to make a turn.

On the 2nd campaign map (with the 3 bridges), I noticed some serious influence map work going on. As I got my bridge defenses shelled (and failed to rebuild quickly), the offensive seemed to shift to that bridge. As I reinforced it at the expense of another bridge, they seemed to shift to the one I had borrowed from. Now, with the fog of war in place, I don’t know for certain that is what happened. They may have been hanging out over there and just were able to push forward when I moved away. I will have to see if there is a replay mode that shows the enemy.

Sometimes I wish that the squads were a little more autonomous. There are times when I wish that they would take some more initiative with their orders. When I give them orders, everything is cool. I can tell a squad to attack/move all the way across the map and they will engage and move repeatedly until they get there. However, if I only give a unit partial orders, they will wait patiently until I get back to them… even if that means standing in the middle of the street.

One example of this is when I had an anti-tank artillery unit pointing one direction with no target and a tank started firing at them from behind, they didn’t turn around. While sticking to my orders is cool and all, some intelligent reactive behavior would have been warranted there. I’m not sure if this aspect is more of a design decision than a failing of the AI, however.

Again, I need to play with the game a bit more to get a feel for it. Right now I am spending more time noticing what I am doing rather than what the AI is doing.

(If you jumped into this article, you may want to click the “Company of Heroes” tag below to see if there are more observations on it.)

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