Thanks!  That looks like it might help in more than one place... I'll give
it a go.

--

Mark Hanford
http://www.mrhanford.com/povray


"Mark Lewin" <m_j### [at] yahoocomau> wrote in message
news:3C9AA765.EEFD3B57@yahoo.com.au...
>
>
> Mark Hanford wrote:
>
> > global_settings{waffle on}
> >
> > Okay, so I've made a start on my particle system, but would like a
couple of
> > pointers (or even cheeky samples).  Here's what I've got so far...
> >
> > Make Some Particles (on first frame):
> >     pos1=rand(...)
> >     velocity1=rand(...)
> >     force1=0
> >     mass1=1
> > do for each frame_number
> >     Load Particle Data (pos, velocity, force, mass, ...extras...)
> >     Draw The Particles
>
> I tend to draw the particles last, but so long as you are consistent...
>
> >
> >     Calculate New Positions
> >     Save Particle Data
> > next frame_number
> >
> > Now this seems like a logical choice of steps, so I assume (?) my
troubles
> > are with the calculations.  It must have taken even Him some time to get
the
> > rules right...
> >
> > The only interaction I'm looking at is the attraction due to each of the
> > other particles, with BigG being a guess for this system:
> > //newforce = oldforce + direction*(Universal Gravitation)
> > #declare force1 = force1 +
> >     vnormalize(pos2-pos1) * ((BigG * mass1 * mass2) /
vlength(pos2-pos1)^2);
>
> I hope you set each particle's force to <0,0,0> before accumulating the
> gravitational forces from the other particles. The total force should not
> accumulate between time steps, it should be evaluated at each time step
only.
>
> >
> >
> > which should give me a nice force pulling the particle around, yes?
This is
> > where I kind of "made it up" to get something that looked like it works:
> > #declare velocity1 = velocity1+force1;  //first leap of faith
> > #declare pos1 = pos1+velocity1;  //second leap of faith
> >
>
> Note that...
>
> velocity = velocity + acceleration*timestep;
>
> and...
>
> acceleration = Force / mass;
>
> Because you have mass = 1, this does not matter, but think of what should
happen
> when mass != 1.
>
> Note too that the timestep should be small (really small) for a good
simulation.
> This may mean performing more than one set of position calculations per
frame of
> animation; for my mini solar system anim, I think I used eight itterations
per
> frame.
>
> From the looks of it, you are only performing one itteration per frame, so
you
> might want to try more.
>
> >
> > Now while this can create some quite nice animations, it seems heavily
> > dependent on correct settings for BigG with relation to ParticleCount
and
> > mass.
> > So, is this to be expected?
>
> Your BigG is scaling your force, so you should expect BigG to make a big
> difference. And, the more particles you have, the more forces from
neighbours
> each individual particle will experience.
>
> Note that BigG in Newtonian physics = 6.67 x 10-11 Nm2/kg2 regardless of
the
> number of bodies (particles) in the system. My tip is to stick to one BigG
value
> and adjust the mass of the particles.
>
> > Particles either achieve escape velocity
> > immediately, or clash together and head off together into the distance.
>
> > Any help greatfully received, although just putting all this down in
words
> > seems to help sometimes too ;)
>
> Good luck with it. I'm sure others will be able to give you a better
explaination
> than myself.
>
> MJL
>

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