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Post-Play'em - Observations on Game AI


Posts Tagged ‘XBox 360’

Call of Duty 2: "I saw that grenade coming!"

Friday, November 30th, 2007

Another quick observation from my perusal of “Call of Duty 2″ for Xbox 360. I’ve noticed rather often that, when I toss a grenade, the intended “recipients” seem to know that it is coming as soon as it leaves my hand. They will yell some German variant of “Holy crap, there is a grenade! Run, dude!” This is all well and good if they are seeing the grenade coming and would like to amscray prior to its arrival. However, the quirky behavior comes in when I am doing something like bouncing it off a wall around a corner. I will hear them shout prior to the offending pineapple even making it to the corner that I am trying to circumnavigate. In other words, they either saw the grenade through the wall, or there is some sort of mirror-like sheen on the wall that allows them to see it coming.

The programmers seem to be using a messaging architecture wherein an event triggers a reaction in all agents within range. For example, if the grenade were to explode in front of them, the explosion itself would send a message out to nearby agents saying “if in range, die violently”. Messaging architectures are opposed to polling architectures where the agent is constantly looking through his environment to see if there is something to react to. Using the same example, the agent would constantly have to scan the nearby world to see if a grenade has exploded. Since grenade explosions are relatively rare, this would be a lot of wasted CPU. As you can see, messaging architectures are more efficient for event based situations.

In this case, the landing area of the grenade is roughly calculated. If the projected landing spot is near an agent, they react appropriately. They yell out warnings and move away if necessary.

However, the programmers would have faced a quandary as to when the grenade itself should send a message. If you wait until it lands or bounces off of something, it would discount the very possible scenario that they could have seen it flying through the air in the first place. If it is triggered when it is thrown, it creates the issue I observed above – that they know it is there despite the fact that they should not be able to see it at all.

One solution (albeit not a very efficient one) would be line of sight checks along the path of the grenade. Once a grenade launch has happened – and a message dispatched to the agent – the agent would now have to do a line of sight (LOS) check. If the grenade is visible, all is well – react appropriately. If it is not visible, the agent would have to change techniques to a polling architecture specifically doing periodic LOS checks at the grenade. When you consider that there may be 10 or 20 agents (friend and foe) in the area, multiple grenades in the area, and each agent-grenade LOS check would need to be done many times per second, the computational overhead adds up very quickly.

Another potential workaround would be to create a plane that is the intersection point of the path of the grenade and the visible area of the agent. At that point, the “see grenade” event can be triggered once the grenade passes that point. That seems to be not much of an improvement since there would be as many planes as there are agents and the grenade would now have to poll the planes many times per second. The overhead would be similar to the first example.

There is another way of handling that, however. Once the plane is generated, the distance from the source to the intersection of the plane can be calculated. Since the velocity of the grenade is near constant, the time delay until the projectile reaches the plane can be established. The message can be sent with a delay of x number of frames (or any other way that the game loop timer is built) so that the message is delivered and activated at the point that the grenade would have come into sight. No polling is necessary at this point. Just like before, a single event message is sent. The only additional overhead would be creating the planes representing the lines of sight for the agents in the area. In fact, if the LOS check is successful at the beginning of the throw, the plane creation is not even necessary. If an agent is going to yell to his squadmates about the throw, you don’t need to do LOS checks for them at all – they already know. It would take some testing to find out how much overhead this eats up, but the result would be that you could do stealth/surprise grenades in a much more realistic fashion rather than the current “I saw it coming around the corner” effect.

(More observations of “Call of Duty 2″ for Xbox 360)

Call of Duty 2: Omniscience and Invulnerability

Wednesday, November 28th, 2007

I was playing Call of Duty 2 on XBox 360 last night. I’ve already gone through the full campaign on the normal difficulty and am now about halfway on “Veteran” (“…you will not survive”).

Enemy Invulnerability:
“I can’t be bothered right now.”

I was on the level where your convoy through the town in Tunisia gets ambushed. I had to start it over and over because I was getting pelted by the guys on top of the walls. I finally got down where I should hide and who I should tag first. One thing kept bugging me. There was a string of guys appearing on one wall in order. Most of them were gunners shooting at me – but there was one that had a Panzerschreck. I would try to shoot him over and over and he wouldn’t die until he had fired at one of the tanks/trucks in my convoy and blown it up. In the mean time, another one would pop up and take me down. I realized that it was pointless to try to kill this dude at all. He wasn’t going to kill me and that tank/truck was going to blow up anyway. It was part of the level.

What annoys me, however, is that it took me a number of times before I realized that, until that guy got his shot off, he was invulnerable. As soon as he was done with his job (that I was trying to prevent) he was now able to be shot. I wasted valuable time and got very frustrated by the fact that the level designers had decided that this was such a required series of actions on the level that they would break the rules.

I have found other instances where I have tried to peg some dude that was threatening me and was dead in my sights only to find that he had some sort of mission that couldn’t be stopped. That really exposes the scripting in the game. I understand why the scripting is there – and, in general, it is very well done in the game. I very much love some of the actions that happen despite the fact that I can tell you the exact line I crossed in order to trigger the action – but usually they are too fast for me to react to or something that is meant to be just watched anyway. Don’t let me point at a guy and unload an entire clip into his spleen while he doesn’t seem to care that I am there.

To me, it seems like this is a case of the “anti-sandbox” concept. Sure, a somewhat linear game like CoD isn’t meant to be a sandbox – and there are certain things that have to happen to advance the plot. That’s fine, but I always feel cheated when I can’t change the series of events even if I do the right thing to disrupt it.

Enemy Omniscience:
“Am I wearing an orange hunting vest?”

The other thing that I noticed the other day is that, while I can sneak up on some people from the side once in a while – which is very satisfying – there are other times when someone will spin 45 degrees to point right at me despite the fact that I am peeking between some crates or barrels or sandbags. It is almost as if I just barely moved into a place where the ray-trace succeeded and told the enemy that I was now visible. This is fine if I walk around the corner or pop up behind something, but it seems odd when only 2% of my body is visible. The result is that it seems like the AI is omniscient (i.e. cheating) as to my whereabouts. In some respects it creates excitement, in others frustration. I know that they aren’t trying to create Thief or Splinter Cell. Stealth isn’t the focus of CoD. Still, there are times when hiding is a requirement of the game. Don’t cheat me out of those brief moments of respite.

A possible solution to this is to measure how much of me is visible and then combine that with a coefficient based on how far “off center” I am from their current direction of vision. Perhaps another factor based on movement. I know that is a bit expensive to calculate for each AI (since their fields of view would all be different).

Another potential solution is to cast multiple rays to different parts of my body – perhaps shoulders, head and a couple of lower torso spots. If more than ONE is visible, then I can be seen. Not knowing exactly what mechanism they are using, it’s difficult to know how to improve upon it.

This is even more alarming when it is obvious that there is a bias towards firing at ME. I may have 10 squad members all hidden behind objects and taking pot shots at the enemy – or even running around in the open, but damn it if the AI doesn’t want to fire at me instead. It doesn’t happen all the time, but it is obviously biased more towards me than my squad-mates. This is an obvious design decision to make it more exciting. I understand that. However, when combined with the omniscience above, it’s kinda creepy.

[note: I've realized that waiting to write one complete writeup on a game is sometimes prohibitive - so I will write as I think of things... and tag them by game so you can find all relevant stuff]

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