How it works
Overlapping circles are painted as radial gradient fans into an offscreen buffer. The brightness of that buffer is then read as an angle, which turns a picture of some blobs into a flow field shaped like those blobs. The visible canvas is only ever two flat colours — inside the shapes and outside them — and a brush walks the field twice, once in each region, laying down chains of wobbly dots in the opposite colour.
Each blade of a fan is a single canvas gradient, and a blob is a few hundred blades. The brush asks two questions before every dot: how far am I from the edge of the region I am in, which a distance transform of the mask answers, and is anything already here, which an index of the dots laid down so far answers.
Everything the brush decides from — the field, the region mask, the distance to the nearest edge — is worked out on a fixed grid rather than at canvas resolution, and every step it takes is measured in reference pixels rather than real ones. That is what makes an A2 the same picture as the gallery thumbnail instead of a denser relative of it, and it is why the sheet costs about the same to draw whatever size it is asked for.