In Flowfield Driven Drawing the computer creates a drawing where the lines follow a predetermined invisible flow field or flow map, very much like ink following the fibres of a highly effective blotting paper.
Even though the flow field is predefined, the user may have some influence on the drawing process.
This blog post explains the mechanism behind Flowfield Driven Drawings (or f2d2 for short) and provides a couple of Open Source examples to get you started rolling your own.
Hair Flow, my first exploration in flow fields was a very simple sketch submitted for the tiny sketch competition in 2009. It is quite impressive what you can get out of Processing, with as little as 200 characters of code. I highly recommend checking out the tiny sketch collection at Open Processing for more examples of Processing madness.
Doodle 3 (2009) was my first sketch using flow fields to control the drawing process, rather than just visualizing the fields.
[s] soft mode on/off
[d] dodge mode on/off
[c] crayon mode on/off
[f] forward (next doodle)
[+] and [-] to zoom
[space] to redraw
The stunning beauty of the result is really due to the ability of Processing, to control the opacity of the lines.
In soft mode the opacity of the lines is controlled by the flow field itsself: lines are drawn vigorous in regions of intense flow, while fading out in silent waters. In hard mode the opacity is controlled by the the drawing tool: opacity increases towards the middle of each stroke, then decreases again.
Like in the Hair Flow sketch the underlying flow field is generated by the Perlin noise function that comes with Processing.