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Post-Play'em - Observations on Game AI


Posts Tagged ‘polling’

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)

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