[ODE] Re: Deterministic simulation on multiple clients

[Sean Butterworth]

I have worked on commercial quality networked games before so I'll add my 
two cents.

For starters, yes, the two machines will be deterministic, and will not 
experience a difference in rounding errors, if the hardware is the same.

Also, the easiest way to network will be to share the input and have all the 
machines deterministically achieve the identical results like you are doing.

Also, the two machines will need to have some sort mechanism for controlling 
the sync state, in case one runs slightly faster than the other, which can 
happen on identical hardware due to load speeds, background processes, etc. 
Something like a lock step protocol, etc.

Its been my experience that the single most important thing is to ensure the 
random number generator stays in sync.  You cant always just make sure they 
are seeded at the same time and forget about them.  What is typically done 
is the random number generator is reseeded every call with the previous 
results.  You make a function call that does this, and whenever a random 
number is generated that can affect the sync state (ie randomly decide what 
direction to move) you use this new call.  Whenever you need a random number 
for something less important (ie number of particles in an explosion) you 
can revert back to the standard call.  Any calls buried in libraries, etc 
will still be using the standard call. I am no expert on PC games, my 
experience being with consoles, but could another process running on the pc 
affect the standard random?

Anyways, in your game code, it should call the new random call, which 
reseeds itself with the previous call's result.  This way, your game's sync 
state will not be affected by what libraries you use, or any other process 
that is running, that might be also using the standard library random.

I hope this help.

Sean