(define draw-one-point
  (lambda (image x y redfunc greenfunc bluefunc)
    (set-fgcolor (list (mod (redfunc x y) 256)
                       (mod (greenfunc x y) 256)
                       (mod (bluefunc x y) 256)))
    (blot image x y)))

(define draw-diagonal
  (lambda (image width height redfunc greenfunc bluefunc)
    (letrec ((kernel (lambda (percent)
                       (if (<= percent 1.0)
                           (let ((x (* percent width))
                                 (y (* percent height)))
                             (draw-one-point image 
                                              x y 
                                              redfunc greenfunc bluefunc)
                             (kernel (+ percent .10)))))))
      (kernel 0))))

(define draw-diagonals
  (lambda (image width height step redfunc greenfunc bluefunc)
    (letrec ((kernel (lambda (percent)
                       (if (<= percent 1.0)
                           (begin
                             (draw-diagonal image width (* height percent)
                                            redfunc greenfunc bluefunc)
                             (draw-diagonal image (* width percent) height
                                            redfunc greenfunc bluefunc)
                             (kernel (+ percent step)))))))
      (kernel 0))))