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Post-Play'em - Observations on Game AI


Posts Tagged ‘Splinter Cell’

Splinter Cell: Conviction — Last Known Position

Thursday, July 21st, 2011

I’ve been playing quite a bit more of Splinter Cell: Conviction. Thankfully, I’ve been finding the game a lot easier than I did early on. Part of it has to do with getting used to the flow of things in a more stealth-based game.

I had noted in my “First Look” post that the enemies do a decent job focusing on the “last known position” mechanic that the game is proud of. To explain, when you are sighted and then go back into cover, the game draws a black and white line art model of your position when you were last seen. This is meant to be a marker of where the enemy thinks you are. The encouraged play is to separate yourself from that point so that you can either hide elsewhere or flank and surprise them. This is a helpful reminder and is only made possible by the fact that the AI has a mental model of your position other than the old fashioned “omniscient” method that games used to use.

Again, they do a good job of searching for you—whether at your actual location or your believed one. They will use cover, approach cautiously, and when the discover you aren’t really there, start to search in what looks like a meaningful way. This is where the problem occurs, however.

The enemies will begin to search the entire area even when they should know
that you are not in those places.

In some instances, when they find that you aren’t where they thought you were, there could be many directions or places to which you could have escaped. At other times, however, there are very few options—or even only one option—that you could have selected. However, the enemies will begin to search the entire area even when they should know that you are not in those places.

One example I saw that was glaring was when I had engaged enemies from a door to their room. I never actually entered the room but rather was firing from the doorway. Eventually, outgunned as I was, it was in my best interests to retreat into the room that I had come from. As I moved away, the line art silhouette marked me as still being crouched next to the door. I watched as they came up to the door to attack where they thought I was. Upon discovering me, however, they exclaimed something or other about losing me… and began searching their own room! They had been watching the doorway the whole time and should have known that I had never crossed to their side of it. The logical conclusion should have been that I was on my side of the door. The result was that I was able to slowly re-assault their position because they were no longer wary of me in the direction of my approach. Good for my survival; bad for my suspension of disbelief.

Good for my survival; bad for my
suspension of disbelief.

The solution to this problem is to leverage “occupancy maps”. The concept has been around for a while but very much came to the game dev forefront when Damián Isla gave an excellent presentation on it at the GDC AI Summit in 2009. The idea is based on the concept of influence maps where the grid space values represent the probability that a target is there. Obviously, if you can see a location, then the target is not there. You then simply spread the probabilities over the areas that he could have gone. By selecting the highest grid value at any one time, you are, by definition, going to the most likely place that the target could be. The result is a spectacular mimicry of how people (and animals for that matter) search for something that disappeared from site.

This method is slightly more memory and processor intensive than simply having a shared (x,y,z) location for the last known position of the player. I honestly don’t know if they had resources to burn in the game. However, I have to wonder if they even thought of trying the approach. I wish they had, though. An otherwise well-done stealth based game would have been that much better for their efforts.

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!

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