(define (array->list a)
	(let* 
		(
			(i (- (length a) 1)) 
			(lout ())
		)
		(while (<= 0 i)
			(set! lout (cons (aref a i) lout))
			(set! i (- i 1)))
		lout))
		
(define (squared a) (* a a))

(define (benday img base-layer dot-size intensity-fn)
	(gimp-image-undo-freeze img)
	(let* 
		(
			(w (car (gimp-drawable-width base-layer))) 
			(h (car (gimp-drawable-height base-layer)))
			(layer (car (gimp-layer-copy base-layer 0)))
			(x 0)
			(y 0)
			(bw (* 1.1 1.7321 dot-size))
			(bh (* 0.55 dot-size))
			(points (cons-array 2 'double))
			(one-two 0)
		)
		(gimp-selection-all img)
		(gimp-edit-clear layer)
		(gimp-image-add-layer img layer -1)
		(while 
			(< y h)
			(set! one-two (list (* 0.5 bw) 1))
			(while 
				(not (or (null? one-two) (>= y h)))
				(set! x (car one-two))
				(aset points 1 y)
				(while 
					(< x w)
					(aset points 0 x)
					(let*
						((r (* dot-size (sqrt (- 1.0 (intensity-fn base-layer x y))))))
						(if (< 2 r)
							(begin
								(gimp-ellipse-select img (- x (* 0.5 r)) (- y (* 0.5 r)) r r 2 1 0 0)
								(gimp-edit-fill layer 0)))
					)
					(set! x (+ x bw))
				)
				(set! one-two (cdr one-two))
				(set! y (+ y bh))
			)
		)
		(gimp-selection-all img)
		(plug-in-blur 1 img layer)
		(gimp-selection-clear img)
	)
	(gimp-image-undo-thaw img)
)

(let*
	(
		(img (aref (cadr (gimp-image-list)) 0)) ;  assumes at least one image
		(base-layer (car (gimp-image-get-active-layer img))))
	(benday 
		img
		base-layer
		19
		(lambda (layer x y) 
			(let*
				( 
					(a (array->list (cadr (gimp-drawable-get-pixel layer x y)))) 
					(px (mapcar (lambda (b) (if (< 0 b) b (- 128 b))) (butlast a))) 
					(return (/ (apply + px) (length px) 255)) ; assumes px not empty
				)
				(min 1.0 (max 0.0 return))
			)
		)
	)
)