(require picturing-programs)

(define (limit n)
 (real->int
  (max 0
       (min 255 n))))

(define (distance x1 y1 x2 y2)
 (sqrt (+ (sqr (- x1 x2))
          (sqr (- y1 y2)))))

(define center-x 250)
(define center-y 200)
(define (amount-of-color x y k)
(* k (distance x y center-x center-y)))
(define (red-f x y)
 255)

(define (green-f x y k)
 (limit (amount-of-color x y k)))

(define (blue-f x y k)
 (limit (amount-of-color x y k)))

(define (make-pixel x y k)
  (make-color (red-f x y)
              (green-f x y k)
              (blue-f x y k)))

(define (make-gradient model)
  (build-image/extra 500 400 make-pixel model))

(define grad (make-gradient 2))

(define (mouse-handler model x y event)
  (* 1/150 (distance x y center-x center-y)))

(define (draw-handler model)
  (crop/align "center" "center" 500 400
              (overlay (scale (max model 0.001) grad)
                       (rectangle 500 400 "solid" "white"))))

(big-bang 1
  (on-mouse mouse-handler)
  (on-draw draw-handler))