Game network physics collision

Question

How to simulating two client-controlled vehicles colliding (sensibly) in a typical client/server setup for a network game? I did read this eminent blog post on how to do distributed network physics in general (without traditional client prediction), but this question is specifically on how to handle collisions of owned objects.

Example

Say client A is 20 ms ahead of server, client B 300 ms ahead of server (counting both latency and maximum jitter). This means that when the two vehicles collide, both clients will see the other as 320 ms behind - in the opposite direction of the velocity of the other vehicle. Head-to-head on a Swedish highway means a difference of 16 meters/17.5 yards!

What not to try

It is virtually impossible to extrapolate the positions, since I also have very complex vehicles with joints and bodies all over, which in turn have linear and angular positions, velocities and accelerations, not to mention states from user input.

Answer 1

I don't know of a perfect solution, and I have a feeling that one does not exist. Even if you could accurately predict the future position of the vehicle, you would be unable to predict the way the user will operate the controls. So the problem comes down to minimizing the negative effects of client/server lag. With that in mind, I would approach this from the position of the principle of least astonishment (paraphrased from Wikipedia):

In user interface design, the principle of least astonishment (or surprise) states that, when two elements of an interface conflict, or are ambiguous, the behaviour should be that which will least surprise the human user at the time the conflict arises.

In your example, each user sees two vehicles. Their own, and that of another player. The user expects their own vehicle to behave exactly the way they control it, so we are unable to play with that aspect of the simulation. However, the user can not know exactly how the other user is controlling their vehicle, and I would use this ambiguity to hide the lag from the user.

Here is the basic idea:

1. The server has to make the decision about an impending collision. The collision detection algorithm doesn't have to be 100% perfect, it just has to be close enough to avoid obvious inconsistencies.
2. Once the server has determined that two vehicles will collide, it sends each of the two users a message indicating that a collision is imminent.
3. On client A, the position of vehicle B is adjusted (realistically) to guarantee that the collision occurs.
4. On client B, the position of vehicle A is adjusted (realistically) to guarantee that the collision occurs.
5. During the aftermath of the collision, the position of each vehicle can be adjusted, as necessary, so that the end result is in keeping with the rest of the game. This part is exactly what MedicineMan proposed in his answer.

In this way, each user is still in complete control of their own vehicle. When the collision occurs, it will not be unexpected. Each user will see the other vehicle move towards them, and they will still have the feeling of a real-time simulation. The nice thing is that this method reacts well in low-lag conditions. If both clients have low-latency connections to the server, the amount of adjustment will be small. The end result will, of course, get worse as the lag increases, but that is unavoidable. If someone is playing a fast-paced action game over a connection with several seconds worth of lag, they simply aren't going to get the full exeperience.

Answer 2

Perhaps the best thing that you can do is not so show the actual collision real time, but give the illusion that things are happening in real time.

Since the client is behind the server (lag), and the server needs to show the result of the collision, perhaps what you can do, client side, is to show a flash or explosion or some other graphic to distract the user and buy enough time on the server side to calculate the result of the collision.. When you are finished with the prediction, you ship it back to the client side for presentation.