[ODE] Running ODE backwards in time?

[Adam Moravanszky [NovodeX]]

That is a bad example because we must assume a closed and fully 
simulated system in order to even be able to theorize about this, and in 
that case the hand that does the pushing would also have to be simulated 
(in reverse).  Therefore if we do not find a collision with a hand while 
doing the reverse simulation, we can assume the second case.

--Adam

Vrej Melkonian wrote:

>Having a residual velocity may not be enough.
>If you have a box on a table that is sliding, then how
>would you know what the original location was?
>
>The box may have been resting somewhere on the table,
>and someone pushed it with their hand.
>
>The box may have been thrown on the table and it hit
>the surface at a certain angle, bounced a bit while
>sliding, then it was in continuous sliding motion.
>
>
>
>
>--- Megan Fox <shalinor at gmail.com> wrote:
>
>  
>
>>This is probably a ridiculous question, but I'm
>>assuming you would
>>have physics turned completely off for the replay
>>(and have a movement
>>vector/orientation vector saved out for each
>>keyframe) - not plugging
>>in the keyframe'd reverse velocities and so on.
>>
>>-Megan Fox
>>
>>On Mon, 15 Nov 2004 13:16:34 +0100, Adam Moravanszky
>><adam.moravanszky at novodex.com> wrote:
>>    
>>
>>>Good point.  You would have to make sure that you
>>>      
>>>
>>never completely stop
>>    
>>
>>>-- you would have to maintain a tiny residual
>>>      
>>>
>>velocity.  Of course with
>>    
>>
>>>float rather than real numbers you would have data
>>>      
>>>
>>loss anyway as values
>>    
>>
>>>get truncated.  Anyway, this discussion is only
>>>      
>>>
>>theoretical anyway, in
>>    
>>
>>>practice there is no way you could pull this off. 
>>>      
>>>
>>In the real world you
>>    
>>
>>>would have to record motion keyframes when
>>>      
>>>
>>simulating forward and
>>    
>>
>>>interpolate backwards -- just like in games that
>>>      
>>>
>>have a rewindable
>>    
>>
>>>instant replay feature.
>>>
>>>--Adam
>>>
>>>
>>>
>>>Alen Ladavac wrote:
>>>
>>>      
>>>
>>>>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.
>>>>>_____________________
>>>>>
>>>>>
>>>>>
>>>>>          
>>>>>
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>>>
>>>      
>>>
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>>    
>>
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