Somewhat rough implementation to generate Ford circles, http://en.wikipedia.org/wiki/Farey_sequence#Ford_circles.

ford-circles.clj

(ns ford-circles
  (:use quil.core quil.applet))

(def screen-w 1920)
(def screen-h 1080)

; This would likely be deemed a hack by a seasoned mathematician;
; I am sure there is a more efficient incantation to produce these numbers.
(defn farey-seq [n]
  (apply sorted-set
    (filter #(< % 1)
      (let [ns (range 1 (inc n))]
        (for [p ns q ns] (/ p q))))))

(defn setup []
  (background 0)
  (smooth)
  (no-stroke)
  (no-loop)
  )

; Circles correspondent with the same q value will be drawn with the same color;
; this makes the resultant drawing symmetric.
(defn draw []
  (let [[r b] [(random 256) (random 256)]]
    (doseq [n (farey-seq 20)]
      (let [p (numerator n)
            q (denominator n)
            d (/ screen-w (* q q))
            c (map #(mod % 255) [(* r q) 0 (* b q)])
            x (* screen-w n)
            y (- (* screen-h 0.67) (* 0.5 d))]
      (apply fill c)
      (ellipse x y d d))
      )
    )
  (save "ford-circles.png")
  )

(defsketch main
  :title "ford circles"
  :setup setup
  :draw draw
  :size [screen-w screen-h]
  :renderer :opengl
  )