# [ODE] Increasing number of particales changes physics

Megan Fox shalinor at gmail.com
Sun Aug 13 13:38:21 MST 2006

```Assuming you're handling particles as, say, sparks, bits of snow, etc,
it seems as though the collision physics will be identical to spheres.
The collisions will be one-sided in the forces they generate, but the
little particles will need all the same dependentrating forces as a
normal sphere - the forces will simply apply only to the particle,
rather than to both sides.

You could even do the same tweaks you do to normal collisions to
convince the collisions to be more bouncy (sparks) or more thud-y
(snow).

On 8/13/06, Roland Kindermann <iyo at gmx.de> wrote:
> I was rather thinking about collision detection. I think, it should in
> most cases be much faster, to test, whether a point is inside a volume,
> than to check, whether to volumes intersect. Also calculating normal and
> depth should be faster. Or am I wrong?
> Cheers,
> Roland Kindermann
>
> Megan Fox schrieb:
> > This may be obtuse, but I'm curious about something... joint
> > contraints work by removing axis of freedom from a given object,
> > constraining them to a particular orientation or region.  As I
> > understand it, the more constraints, the faster the joint and less
> > amount of impact on the sim speed.
> >
> > Given that, would it be possible to make a "point" contact joint that
> > acted to correct the position of the body in question, but with fewer
> > axis of freedom (eg. no rotation), thereby acting faster than a
> >
> >
> > Seemingly, you'd use the same collider either way (unless the collider
> > is doing something odd a sphere and a point collision should be doing
> > the same thing - which is to say, not much really, radius + center
> > point), but this would be the difference between a point and a sphere?
> >
> > On 8/12/06, Roland Kindermann <iyo at gmx.de> wrote:
> >> Hi,
> >> I work with particles too. I think it would be nice to have a Point
> >> Geometry (which only consists of one point) for particles. It would
> >> probably be faster than a small Sphere.
> >> Cheers,
> >> Roland Kindermann
> >>
> >> Jacob Ole Juul Kolding schrieb:
> >> > OK I have a fixed step time and that works fine.
> >> >
> >> > The next problem is when I enable collision (So that the particles
> >> > bounce against a floor). When the simulation has up to 350 particles it
> >> > seem to run smoothly and stable, but above 350 it will eventually
> >> suffer
> >> > a slowdown death causing the app to be unresponsive.
> >> >
> >> > I switched to HashSpace and set the category/collide bits so that the
> >> > particles should not collide with each other but only the floor.
> >> >
> >> > Is this a bottleneck?
> >> >
> >> > /Jacob Kolding
> >> >
> >> >
> >> > On 8/11/06, *Jon Watte* <hplus at mindcontrol.org
> >> > <mailto:hplus at mindcontrol.org>> wrote:
> >> >
> >> >     Correct. You need to use a fixed time step. Such as in
> >> >     http://www.mindcontrol.org/~hplus/graphics/game_loop.html
> >> >     <http://www.mindcontrol.org/%7Ehplus/graphics/game_loop.html>
> >> >
> >> >     Jacob Ole Juul Kolding wrote:
> >> >     >  I'm implementing a particle system in ODE and have
> >> encountered the
> >> >     >  problem that when the number of particles increase the
> >> physics change
> >> >     >  in the the way the even though the same force is applied, the
> >> more
> >> >     >  particles are in the simulation the longer they will travel.
> >> >     >
> >> >     >  My guess is that since i step the world each frame, when the
> >> frame
> >> >     >  rate drops the step time increase which changes the outcome
> >> of the
> >> >     >  simulation?
> >> >     >
> >> >     >  Anyone have an idea how to help this problem?
> >> >     >
> >> >     >  /Jacob Kolding
> >> >     >
> >> >
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--
-Megan Fox
Idyllon, LLC
http://shalinor.circustent.us/megan/resume/
```