Hi,

I've been exploring different methods for keeping particles spaced from one
another. There are many ways to go about it, so as a matter of course I thought
I'd try using a square falloff function for each particle. It works fairly well
and is less likely to produce abrupt changes than other methods. Initially the
square falloff was clipped between 0.0 - 1.0.

On a whim I thought I'd try changing 0.0 to a small negative value based on the
current particle's radius. What occurred next confirmed my suspicions: the
particles gravitate toward each other while still keeping their distances.

I might use this for dumping piles of rocks onto/into objects, making jars of
marbles, or whatever. In addition to the particle-particle square falloff
function, other types of collision detection *will* need to be used since it is
very difficult to make functions from, say, a mesh.

For this animation, the following rules were applied:

 1) a particle must always keep a relative distance between itself and another
particle
 2) a particle will become less responsive to change at higher masses (sizes)
 3) a particle will gravitate toward another particle based on the square
falloff determined by any particle's position and mass (size)

Momentum is not a factor here.

The reason they all go toward the center is because the averaged positions of
all particles would be around <0,0,0>.

An interesting thing to note is that the smaller particles will eventually reach
the center of the mass. This is partly caused by the instability of the system,
but it is also largely due to granular convection.
http://en.wikipedia.org/wiki/Granular_convection

I think I explained myself sufficiently ;) Sorry about the low video quality!

~Sam

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