Interesting! Are you drawing the figure by hand (and the shader does only the "leaking") or have you encoded the "drawing", too? How do you achieve the effect?

I guess you use a "spray paint" brush bitmap which is written additively into an accumulating floating point target. You then use another target which is the target for the shader and you just push the values above a threshold x down in Y-direction, in an infinite ping-pong manner.