5.6.1 Using Flipping Ink

(defun cmd-rubberband ()

  (let ((x1 0)        ; x1, y1 represents the fix point 

        (y1 0)

        (x2 0)        ; x2,y2 represents the point that is changing

        (y2 0)

        (mouse-button-press nil)

        ;; press to select pivot

        (stream (get-frame-pane *application-frame* 'main)))

    (tracking-pointer (stream)

                      (:pointer-button-press

                       (event x y )

                       (setf x1 x y1 y x2 x y2 y)

                       (draw-line* stream x1 y1 x2 y2

                                   :ink +flipping-ink+)

                       (setf mouse-button-press t))

                      (:pointer-motion

                       (window x y)

                       (when Mouse-button-press

                         ;;erase

                         (draw-line* stream x1 y1 x2 y2

                                     :ink +flipping-ink+)

                         ;; draw

                         (draw-line* stream x1 y1 x y

                                     :ink +flipping-ink+)

                         (setf x2 x y2 y)))

                      (:pointer-button-release

                       (event x y )

                       (cond

                        ((eq mouse-button-press t)

                         (return

                          (list x1 y1 x2 y2))))))))