Physarum

A slime mould in raw WebGL2. No library: 262 144 agents living in two textures, growing a transport network out of noise, fed by your pointer.

Live: https://ronancamus.github.io/physarum/

Move the pointer to feed it. Press and hold for a feast. Keys 1 to 4 change its temperament (veins, lace, coral, storm), H shifts the hue, space pauses, R starts over. Left alone, the meal wanders on its own.

How it works

Physarum polycephalum explores by laying a chemical trail and following it. Jeff Jones reduced that to three rules in 2010, and three rules are enough:

1. Sense and move. Each agent reads the trail at three points ahead of it (straight, left, right), turns toward the strongest one, and takes a step. If the trail is stronger on both sides than ahead, it tosses a coin. A small random hesitation keeps the paths from locking.
2. Deposit. Each agent adds a little trail where it stands.
3. Spread. The trail blurs and fades. Food is just more trail, poured where the pointer is.

Agents are pixels of an RGBA32F texture (x, y, heading). The trail is a single R16F channel that wraps at the edges, so the network has no border. One simulation step is three draw calls: a full-screen pass over the agent texture, a gl.POINTS draw with one vertex per agent (position read with texelFetch in the vertex shader, additive blending), and a full-screen blur with four bilinear reads instead of nine. Sixty steps per second, fixed, whatever the display does.

Two details make the difference between a few fat tubes and a living network:

• Crowding. A sensor does not return the trail value v but v * exp(-0.55 v): a vein that is already crowded attracts less, so agents spread into secondary paths instead of collapsing onto the strongest one.
• Rebirth. Each step, 0.15 % of the agents are reborn at a random place. They are the ones who open new veins when the old ones have been eaten.

Rendering maps trail density to a cyan, magenta, orange palette, with a hot white core in the thickest veins, a soft vignette and 3 % of grain against banding.

The whole thing is about 200 lines, shaders included. It was written for the playground of ronancamus.com, where the same organism grows over a projection-mapped scene.

Run it

Open index.html through any static server, for example:

npx serve .

It needs WebGL2 with EXT_color_buffer_float, which every current desktop and mobile browser has.

License

MIT.