;; Scanlines effect - VHS-style horizontal line shifting
;; @param amplitude float [0, 100] default 10
;; @param frequency float [1, 100] default 10
;; @param randomness float [0, 1] default 0.5

(define-effect scanlines
  ((amplitude 10) (frequency 10) (randomness 0.5))
  (map-rows frame
    (lambda (y row)
      (let* ((sine-shift (* amplitude (sin (/ (* y 6.28) (max 1 frequency)))))
             (rand-shift (random (- amplitude) amplitude))
             (shift (floor (lerp sine-shift rand-shift randomness))))
        (roll row shift 0)))))