[ODE] eliminating all state

[Chris Campbell]

> is done, I take the best ones and "play them back" with graphics and
> logging and other fun stuff.  What I'd really like is for the 
> simulation
> to go through exactly the same calculations with exactly the 
> same roundoff
> errors, but at the moment there are small differences that 
> creep up over
> time.  Russ, you mentioned that ODE uses the correction at 

Using fixed timestep? I am making a racing game using ODE 
for physics which records only keyboard and mouse inputs to
make a replay. That is, to play the replay the entire simulation
is recalculated. Initially I was a little worried that error might 
creep in after a while but after a LONG time experimenting
I convinced myself that there was no state held by ODE, and
that the idea should work. It does, but only with fixed timestep.
Also, a recording made with the debug binary will not replay
properly with the release binary and vice versa. 
Another thing which seems to be necessary is that the cpu
should conform to IEEE floating point standards in order 
for replays to work deterministically on a different computer.

ChrisC