DLA - Diffusion Limited Aggregation

Many attractive images and life-like structures can be generated using models of physical processes from areas of chemistry and physics. One such example is diffusion limited aggregation or DLA which describes, among other things, the diffusion and aggregation of zinc ions in an electrolytic solution onto electrodes. "Diffusion" because the particles forming the structure wander around randomly before attaching themselves ("Aggregating") to the structure. "Diffusion-limited" because the particles are considered to be in low concentrations so they don't come in contact with each other and the structure grows one particle at a time rather then by chunks of particles. Other examples can be found in coral growth, the path taken by lightning, coalescing of dust or smoke particles, and the growth of some crystals. Perhaps the first serious study of such processes was made by Witten, T.A. and Sander, L. M. and published by them in 1981, titled: "Diffusion limited aggregation, a kinetic critical phenomena" in Physical Review Letters. number 47.

Another more colourful description involves a city square surrounded by taverns. Drunks leave the taverns and stagger randomly around the square until they finally trip over one their insensate companions at which time, lulled by the sounds of peaceful snoring, they lie down and fall asleep. The tendril like structure is an aerial view of the sleeping crowd in the morning.

There are a number of ways of simulating this process by computer, perhaps the most common is to start with a white image except for a single black pixel in the center. New points are introduced at the borders and randomly (approximation of Brownian motion) walk until they are close enough to stick to an existing black pixel. A typical example of this is shown below in figure 1. If a point, during its random walk, approaches an edge of the image there are two strategies. The point either bounces off the edge or the image is toroidally bound (a point moving off the left edge enters on the right, a point moving off the right edge enters on the left, similarly for top and bottom). In general new points can be seeded anywhere in the image area, not just around the border without any significant visual difference.

Figure 1. Point attractor

Another attractor geometry is a line, see figure 2. In this case the bottom row of pixels are initially black and new points enter from the top. The image is normally horizontally circular, that is, points move off the left appear on the right and visa-versa.

Figure 2. Line attractor

Figure 3 shows a box attractor, new points enter in the interior (the center of the image say). The source code supplied here generates the forms by a rather brute force method which is hardly the most efficient. A common efficiency measure is to start the new particles within a certain range of the current structure and abandon them if they move outside some other range.

Figure 3. Rectangle attractor

There are a number of ways of adding colour, figure 4 has a circular seed region (toroidal bounds) and the particles are coloured by the order in which they were introduced.

Figure 4. Circular attractor

Figure 5. Internal circular attractor

Another variation is to vary the so called "stickiness". In all the above example if the wandering particle struck part of the existing structure it always stuck (stickiness = 1). This can be modified by introducing a probability of sticking, the general result is to make the form more hairy and solid.

Figure 6. Stickines = 0.2

Figure 7. Stickines = 0.05

Figure 8. Stickines = 0.01