Thursday, 5 September 2013

Stigmergic Particles

I've been getting a few questions on some recent work with indirect particle interaction so I figured I'd try to address them all in one fell swoop via blog post. Here goes.

Typically interactions among a population of particles occur directly. Particles refer to each other's properties (position, velocity, acceleration etc.) in order to build the forces which define their behaviour.

In both examples below, particles aren't particularly concerned with each other. Rather they look to a shared environment in deciding how to act. This environment takes the form of a 2d scalar field where values represent concentration of a diffusing communication medium or "pheromone". Particles build forces by locally sampling this field relative to their own positions. Depending on how they go about reading their environment, different collective patterns emerge over time. In fact this is the only significant difference between the following two systems.

In the first example, each particle samples the scalar field at regularly spaced points on a circle centered at its position. A force vector is calculated by summing the gradient vectors found at each sample point before being applied to the particle determining it's future position. The radius of the sample circle is being changed throughout the video causing the system to shift between a range of patterns.

The particles in the second example are less thorough in how they read the field. They only cast two sample points in front of themselves - one to the left and one to the right. The forward biasing of these points results in a completely different family of patterns than seen in the first example. More information on the inner workings of this system can be found here.

Particles can't successfully interact through a read-only environment however. Not much can happen if everyone is listening and no one is talking. In order to close the communicative feedback loop, particles must also write to their shared environment. This occurs through pheromone deposition whereby particles modify local values of the scalar field by either pulling them towards a target value or adding/removing a constant. Diffusion then helps these modifications propagate through the environment so others can take notice.

With both read and write mechanisms in place, particles are able to detect and react to each other's signals producing a variety of emergent patterns. Here the goal is simply the production of visual complexity. 
As mentioned in previous posts, however, I've been busy applying similar principles of indirect interaction among a population of autonomous entities to solving architectural design problems - specifically those relating to programmatic organization.

Platforms: Java, Processing

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