How it works
The field is sampled at every node of a regular grid and drawn as a short needle at that angle, with a dot at the node itself. Then a handful of streamlines are traced through the same field, starting scattered, from the centre, or from the edges, and drawn heavier than the needles.
The angles come from one of two places. Perlin noise is the default, and is the same kind of field the other pieces walk their marks through. The alternative is a plain top-to-bottom ramp, where every needle in a row points the same way and the angle turns steadily down the sheet — worth switching to when a noise field has stopped looking like anything in particular, because the grid and the traces are then doing their work on something you can predict.