[ODE] Networking (Client-Side Prediction)

[Hampus Soderstrom]

Hi Bruno,

To be absolutely sure that the game world is persistent setting FPU  
state is not enough.

Periodically sending complete world states and have the clients  
resync is a must I think.

/Hampus

27 apr 2006 kl. 06.01 skrev STenyaK (Bruno Gonzalez):

> I'm most interested in this issue. Unfortunately i can't help at  
> all with
> it (yet, and prolly for at least 1 or 2 more years... my sim is  
> still to
> yound and it's a spare time project).
> Please write any interesting info in the wiki, i can help  
> reformatting it
> there... or whatever. It's not much, i know, but i'm afraid that's  
> all i
> can do for now... Maybe i can test things for you, if it doesn't take
> "much" time: i have 5 computers at home, with 5 different OSs, half
> Pentium (PI, PIII), half AMD (1700XP, 3000Barton, 3200XP). Simply  
> let me
> know if i can help with anything.
>
> On Thu, 27 Apr 2006 00:30:48 +0200, Jon Watte (ODE)
> <hplus-ode at mindcontrol.org> wrote:
>
>
>>
>> Perhaps the client uses a DLL (such as Direct3D) that changes the FPU
>> precision or rounding mode? I've found that you must verify that  
>> the FPU
>> internal precision is set to your chosen bits mantissa and the  
>> rounding
>> mode is set to your preference, before each simulation step (or  
>> any math
>> that affects body state).
>>
>> Also note that AMD and Intel machines have different FPU hardware  
>> (Intel
>> can go to 80 bits doubles internally; AMD stays at 64 bits). Again,
>> setting the precision mode explicitly fixes this problem most of the
>> time.
>>
>> The networking forum at www.gamedev.net discussed these issues at  
>> length
>>   sometime last year, if you want to go archive spelunking.
>>
>> Cheers,
>>
>>             / h+
>>
>>
>> Robert Pröpper wrote:
>>
>>> Hi,
>>>
>>> I am trying to program a multiplayer game using ODE (float version).
>>> I've got many things running already, most important to this  
>>> question
>>> is that I can jump to a previously saved state, simulate the
>>> intermediate steps again and arrive at EXACTLY the same result.
>>> I have now begun to build a Client-Server System. The client  
>>> sends only
>>> his Inputs(with a timestamp) to the server. When he receives an  
>>> Input,
>>> the server jumps back to the tick where the input happened and
>>> re-simulates. Now, since I can get the same results from the same
>>> starting conditions in my "Single Player" application, I would  
>>> expect
>>> that the server also arrives at the same results, but this is not  
>>> the
>>> case.
>>>
>>> For example, I have turned off gravitation in my application. As  
>>> long
>>> as no force is applied to my body, the client and the server stay
>>> perfectly in sync (obviously, neither the position nor the rotation
>>> changes). They also stay the same if I apply a force at
>>> the center of the body (like gravitation) - I get a linear  
>>> velocity and
>>> the positions stay the same (the rotations again don't change).  
>>> Now, as
>>> soon as I apply a force at any other point than the center, the two
>>> simulations go out of sync.
>>> For testing purposes, I only apply the force for 1 step.  
>>> Immediately,
>>> the client and the server rotations are very slightly off - in the
>>> order of 1e-8.
>>> I can't explain this, I have followed all the Save and Restore  
>>> FAQ and,
>>> as I mentioned, have no problems in the same application or until  
>>> the
>>> body begins to rotate. For a good client-side prediction, I need the
>>> server and the client to arrive at the exactly same results, so this
>>> inaccuracies are a big problem for me.
>>>
>>> So, do you have any ideas what could be causing this problem, or  
>>> even
>>> better, how to solve it?
>>>
>>>
>>> PS: For the moment, both the client and the server run on the same
>>> computer.
>>> _______________________________________________________________
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>>>
>>>
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>
>
>
> -- 
> Saludos,
>      STenyaK
>
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