[ODE] Angular-velocity autodisabling considered broken

[Nguyen Binh]

Thanks Alen!
I might try it now....


On Wed, 15 Sep 2004 20:00:19 +0200, Alen Ladavac <alenl-ml at croteam.com> wrote:
> >1) If you sleep objects based on an instantaneous velocity reading, then
> 
> Did I mention using instantaneous velocities? Nope, I don't think so. :o)
> The key is that if an object is ready to sleep, it goes to sleep _and_ its
> velocity is forced to 0. Note that it is not necessarilly 0 ATM, but it is
> below some threshold (and has been so for a number of steps). Setting it to
> 0 removes its energy completely, and therefore decreases the chance that it
> will continue jittering.
> 
> >if one object is awake and moving, it will typically exert enough force on
> >the other bodies that the other bodies have to wake up. That's what we
> >mean when we say that when one body won't sleep, it'll keep the island
> >awake.
> 
> It doesn't have to be that way. You can have half of an island asleep and
> the other half moving. Actually, you don't win anything performance wise
> compared to letting entire island be awake - you still have to "wake" all
> the objects in order to perform collision checks and other calculations on
> them - but if this is done right, they are not moving/jittering. The gist of
> it is that the objects that were once asleep tend to stay that way, and the
> "sleeeping shockwave" propagates through the island, gradually stopping it
> over a several steps, or even several seconds. If you do it right, one
> jittering object can wiggle arund for seconds, without moving the rest of
> the island even a bit. If you insist on all-or-nothing sleeping, then
> situations like this (single object not being able to come to rest for
> seconds) will certainly blow the entire island into pieces.
> 
> The key is in setting velocity to 0 when an object is ready to sleep,
> regardless of what its neighbors do. So that you don't pull this step's
> errors into the next one. This thing helps a lot, IME.
> 
> HTH,
> 
> Alen
> 
> 
> 
> 
> ----- Original Message -----
> From: "Jon Watte" <hplus-ode at mindcontrol.org>
> To: "Alen Ladavac" <alenl-ml at croteam.com>; <ode at q12.org>
> Sent: Wednesday, September 15, 2004 5:56 PM
> Subject: RE: [ODE] Angular-velocity autodisabling considered broken
> 
> > One thing about jittery islands... don't know exactly how ODE handles
> > sleeping, but in our simulator, we noticed that it is better to handle
> > sleeping per-object than per island. An object that is determined to be
> > ready to sleep is put to sleep and its velocities are reset immediatelly.
> No
> > matter that the rest of island may still be moving. This removes energy
> from
> > islands that are on the verge of sleeping, making them less and less
> jittery
> > over time. And it prevents slow creeping of objects, because objects that
> > would otherwise move very slowly are forced to 0 velocity in every step,
> > hence they cannot move at all. You must be very careful about properly
> > waking up the objects in the next step, though. :)
> 
> There are two problems with this approach, if you were to use it for the
> general case (i e, built-into ODE).
> 
> 1) If you sleep objects based on an instantaneous velocity reading, then
> blocks that tip up on an edge, but will fall down again (eventually), will
> fall asleep and remain tilted forever.
> 
> 2) All sleeping approaches I know of apply to individual objects only. The
> problem is that you have to wake up objects when they are hit by other
> objects, or when forces are applied through other joints. Within an island,
> if one object is awake and moving, it will typically exert enough force on
> the other bodies that the other bodies have to wake up. That's what we
> mean when we say that when one body won't sleep, it'll keep the island
> awake.
> 
> Cheers,
> 
> / h+
> 
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> 



-- 
Nguyen Binh
www.glassegg.com
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...currently searching for a graduate school in physical simulation...
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