Lisp Game Programming <Step4>

64×64の大きさのmapchipを、(0,-64)の位置から横８個、縦に9個並べ、1dotずつ縦にスクロールさせていき、mapchipが96個分動いたら０にリセット

追加するプログラムは以下

;; step 4 <Scroll>
;; -----------------------------------------------------------------------------------------------
(defvar *scroll* 0) ; scroll counter

(defun Scroll (obj)
  "draw background"
  (setf (y obj) (+ -64 (mod *scroll* 64))) ; scroll start from y(-64) to y(0)
  (incf *scroll*) ; scroll counter
  (when (= *scroll* 3072)
    (setf *scroll* 0)) ; mapchip 96 pieces move
    (dotimes (i 9)
      (setf (x obj) 0)
      (dotimes (j 8)
        (Draw obj)
        (incf (x obj) 64))
      (incf (y obj) 64)))

 メインルーチンは以下

(defun Common-shooter ()
  "main routine"
  (sdl:with-init (sdl:sdl-init-video sdl:sdl-init-audio) ; use video and audio
  (sdl:window 640 480 :position #(192 50) ; size 640*480, position x(192) y(50)
                                       :title-caption "THE SHOOTER"
                                       :icon-caption "THE SHOOTER"
                                       :double-buffer T)
                                     ; :fullscreen T)
   ; step 3
  (Initialize) ; graphics initialize
  ; step 2
  (Open-sound) ; open audio and load sound data
  ; step 2
  (Play-music *bg-music*) ; play music
  ; step 3 - 4
  (let ((ship (make-instance 'entity :imageid 0 :id 0 :x 224 :y 416
                                                     :width 32 :height 32 :dx 4 :dy 4 :state 1))
         (mapchip (make-instance 'object :imageid 1 :id 0)))

  (sdl:update-display)
  (sdl:with-events (:poll)
  (:quit-event ()
  ; step 2
    (Stop-sound) ; sample and music stop
    (Close-sound) ; close audio
      t)

    (:idle ()
    ; Game body

    ; step 4
    (Scroll mapchip) ; scroll background

    ; step 3
    (when (= (state ship) 1)
      (Draw ship)) ; draw ship

  (sdl:update-display))))))

以外と簡単に背景画面が縦スクロールするプログラムが完成した