(define (read-ppm port)
  (parameterize ([current-input-port port])
    (define magic (read-line))
    (match-define (list w h) (string-split (read-line) " "))
    (define width (string->number w))
    (define height (string->number h))
    (define maxcol (string->number (read-line)))
    (define bm (make-object bitmap% width height))
    (define dc (new bitmap-dc% [bitmap bm]))
    (send dc set-smoothing 'unsmoothed)
    (define (adjust v) (* 255 (/ v maxcol)))
    (for/list ([x width])
      (for/list ([y height])
        (define red (read-byte))
        (define green (read-byte))
        (define blue (read-byte))
        (define color (make-object color% (adjust red) (adjust green) (adjust blue)))
        (send dc set-pen color 1 'solid)
        (send dc draw-point x y)))
    bm))

(define (image->bmp filename)
  (define command (format "convert ~a ppm:-" filename))
  (match-define (list in out pid err ctrl)  (process command))
  (define bmp (read-ppm in))
  (close-input-port in)
  (close-output-port out)
  bmp)

(image->bmp "input.jpg")