[ODE] Running ODE backwards in time?

[Alen Ladavac]

I'd say that this is insolvable for general case with dynamic friction. If a
box slides on a horizontal plane and comes to rest, how do you calculate
where it was supposed to come from, from its position only? ;)


----- Original Message -----
From: "Adam Moravanszky" <adam.moravanszky at novodex.com>
To: <mtoussai at inf.ed.ac.uk>
Cc: <ode at q12.org>
Sent: Monday, November 15, 2004 10:36
Subject: Re: [ODE] Running ODE backwards in time?


>     mtoussai at inf.ed.ac.uk wrote:
>
> Hi there!
>
> I need to run some simulations backwards in time (for some planning
> algorithms..). I know I could do this by inverting all dynamic properties
> ((angular) velocities, accelerations, etc.) of the system and running ODE
> forwards. But is there a simpler way? Just setting a negative time
interval in
> a step function doesn't work.
>
> Thanks!
> Marc.
> _____________________
>
>
>
> :-)  It is not that easy.  You would have to rewrite the physics engine,
> reversing all of the operations, including their order.  For example,
> imagine the super simple hypothetical physics engine for simulating a
> point mass falling due to gravity:
>
>  >> a = 10;
>  >> v = 0;
>  >> p = 0;  //acceleration, velocity, and position
>  >> v = v + a; p = p + v; [p v]   //matlab code
> ans =
>
>        10       10
>
>  >> v = v + a; p = p + v; [p v]
> ans =
>
>        30       20
>
>  >> v = v + a; p = p + v; [p v]
> ans =
>
>        60       30
>
>
> You can now make the particle go backwards in time if you reverse the
> whole process, but note how the code changes:
>
>  >> p = p - v; v = v - a; [p v]
> ans =
>
>        30       20
>
>  >> p = p - v; v = v - a; [p v]
> ans =
>
>        10       10
>
>  >> p = p - v; v = v - a; [p v]
> ans =
>
>         0        0
>
>
> --Adam
>
>
>
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