;; Outline effect - shows only edges
;; @param thickness int [1, 10] default 2
;; @param threshold int [20, 300] default 100
;; @param color list default (0 0 0)
;; @param fill_mode string default "original"

(define-effect outline
  ((thickness 2) (threshold 100) (color (list 0 0 0)) (fill_mode "original"))
  (let* ((edge-img (edges frame (/ threshold 2) threshold))
         (dilated (if (> thickness 1)
                      (dilate edge-img thickness)
                      edge-img))
         (base (cond
                 ((= fill_mode "original") (copy frame))
                 ((= fill_mode "white") (make-image (width frame) (height frame) (list 255 255 255)))
                 (else (make-image (width frame) (height frame) (list 0 0 0))))))
    (map-pixels base
      (lambda (x y c)
        (let ((edge-val (luminance (pixel dilated x y))))
          (if (> edge-val 128)
              color
              c))))))