A tangle of coloured threads laid over the picture, like the fibres in banknote paper or a sheet of handmade card.
They are drawn by agents rather than by a formula. A field of gentle randomness is generated first, then several hundred agents are dropped onto it at random positions. Each one reads the value underneath itself, turns that into a direction, takes a short step, and draws a line as it goes. Because neighbouring agents read similar values, their paths sweep along together in currents.
The turn rate is what separates a fibre from a drift. The field value is multiplied before it becomes a heading, so small changes in the field swing an agent through many turns in a short distance and the threads curl rather than running straight. That multiplier is fixed here rather than exposed. Each strand also fades in and back out along its length, so the ends taper rather than stopping dead. density sets how many threads there are, alpha how strongly they sit over the picture, and seed picks a different tangle.