Metaball based on particle movement. So basically the centers of the metaball are particles which attracted to each other.
Metaball is defined in the mathematical world as n-dimensional objects. The main inputs for creating metaball are points, section planes and metaball treshold. Because of the complexity of the form which usually have more than one genus, creating the skin of a complex metaball are relatively hard. Therefore metaballs usually displayed as series of section lines rather than a single two or three genus surface. There are already some algortihm used in creating metaball skin such as: Marching Cube algorithm (this is the most popular one). But in this case we have to deal with the voxel space.
I am still trying to write a marching cube algorithm and will post it as soon as I finished it. For now I just extrude the section lines and cap the hole, which create a contour like form.
This is a video of growing metaball controlled by particle movement based on brownian motion.