This sketch creates flow map based drawings.
A flow map is an image where hue indicates the direction of flow
and brightness indicates the amount of flow at any given pixel.
You can find some hires samples on flickr:
[space] switch flow map
 reset flow map position
[s] save image to disk
[d] dodge on/off
[b] brush on/off
[g] grain fine/rough
[l] lines short/long
[c] colors : daVinci, blackboard, crayons, fibreglass, rainbow
[w] whirl direction
[x] more x-hatch
[z] less x-hatch
To tweak this sketch, first download it.
Make your updates, then upload your tweak!