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以上の拡張を行うと以下のリスプ関数を利用できるようになる．
(GRAPH-INIT)
(GRAPH-MOVETO x y)
(GRAPH-LINETO x y)
(GRAPH-CIRCLE x y radius)
(GRAPH-CLOSE)
(GRAPH-CLEAR)
(GRAPH-GETPIXEL x y)
(GRAPH-PUTPIXEL x y value)
graph-initにより初期化をし，graph-movetoでペンを移動し，graph-linetoで線
を描くという単純なモデルである．
Xwindow環境でmakeすればこれらの関数を使うことができるようになる．
C カーブは以下のように可能である．
(setq pi/4 (/ 3.1415926 4))
(setq 1/sqrt2 (/ 1.0 (sqrt 2.0)))
(defun float-vector (&rest l) (mapcar #'float l))
(defun point (x y) (float-vector x y))
(defun x (p) (car p))
(defun y (p) (cadr p))
(defun v+ (p1 p0)
(point (+ (x p1) (x p0)) (+ (y p1) (y p0))))
(defun v- (p1 p0)
(point (- (x p1) (x p0)) (- (y p1) (y p0))))
(defun scale (s v) (point (* (x v) s) (* (y v) s)))
(defun distance (p0 p1)
(let* ((dv (v- p1 p0)) (dx (x dv)) (dy (y dv)))
(sqrt (+ (* dx dx) (* dy dy)))))
(defun rotate (v rad)
(let ((x (x v)) (y (y v)))
(point (+ (* (cos rad) x) (* (sin rad) y))
(- (* (cos rad) y) (* (sin rad) x)))))
;;;
(defun draw-line (p0 p1)
(graph-moveto (x p0) (y p0))
(graph-lineto (x p1) (y p1)))
;;;
;;; C curve
;;;
(defun draw-ccurve (&optional
(p0 (point 190 300))
(p1 (point 450 300))
(min 20))
(cond
((< (distance p0 p1) min) (draw-line p0 p1))
(t (let ((p2 (v+ p0
(scale 1/sqrt2
(rotate (v- p1 p0) pi/4)))))
(draw-ccurve p0 p2 min)
(draw-ccurve p2 p1 min)))))
curve.lspファイルには，これらの関数定義がなされている．
~/lecture/soft3/note03/xlisp/xlisp2.1x> xlisp
XLISP version 2.1, Copyright (c) 1989, by David Betz
> (load "curve.lsp")
; loading "curve.lsp"
try (graph-init) and (draw-ccurve)
(graph-clear) , (draw-dragon)
T
> (graph-init)
__CYGWIN__ defined
T
> (draw-ccurve)
T
> (graph-clear)
T
> (draw-dragon)
T
> (exit)
generated through LaTeX2HTML. M.Inaba 平成18年5月7日
![\includegraphics[width=7.0cm]{/home/inaba/eps/lecture/fig/ccurvefine.eps}](img9.png)