I am developing an online real-time multiplayer game using Google Play Game Service and Cocos2D-X. The game has two players where each player control their own ball.

The each player sends their own ball’s velocity and position to the other connected device. In my game, all the physics calculations are done locally on each devices i.e. ball velocities after physical calculations are calculated locally on each devices.

Issue:?

When fast moving ball collides wth the opponent’s ball, there is no change in the movement velocity of the opponent’s ball. (According to real world physics, the fast moving ball should be able to push the opponent’s slowly moving ball.)

Reason:
During my code analysis, I understood that the velocity calculated by the local physics engine are over written by the data received from the opponent’s device.

For e.g.


Solution I tried:
To fix the problem, whenever the collision between balls occurs I am creating one of the device as server and other device as client. All the physics calculations occurs on the server device and data is send to the client device.

I have following questions:

1. Is the solution tried by me is optimal solution to fix the issue?
2. What other approaches can be tried to fix such type of issue??

I would appreciate any suggestions/thoughts on this topic. Thank you.

Thank you for the reply hplus0603. Your suggestions regarding flows in my current design are helpful.

Currently, the code is using extrapolation. The ball positions are updated using the physics body in the update loop. Hence, the ball position is updated on the remote device even if the remote device is not receiving data packets for the ball.

1. How can I ensure that the collision between balls happens at the same instant of time on both the devices so that local physics engine work properly?
2. or else how can I ensure same physical collision between balls (on both the devices) given I am sending the ball velocity to the other device?

How can I ensure that the collision between balls happens at the same instant of time on both the devices

You can't. To do so, you'd have to guarantee that both devices were perfectly synchronised in time, which is theoretically impossible to do perfectly (thanks to relativity) and practically nearly impossible for other reasons (network jitter, different device update rates). Systems like Network Time Protocol are specifically designed to synchronise computers across the internet to within "tens of milliseconds" across the internet - and that is not good enough for a fast-paced physics simulation.

You could probably do better and get within single digit milliseconds of each other, but not only is that still not good enough, but now the latency is a big problem - your balls might be in almost the same position on both devices but any player input cannot possibly be fully synchronised because it takes time to travel across the network.

I think the closest thing to a solution here is to have a 3rd process act as an authoritative server. Unfortunately Google Play doesn't offer this (the last time I checked). But it would allow you to have a single place where all physics is performed and where the results are broadcast out to players. Unfortunately this introduces extra latency between player input and the game's response to that input, but that can sometimes be mitigated or covered up.