xref: /csrg-svn/lib/libplot/hp2648/arc.c (revision 19975)
1 /*
2  * Copyright (c) 1980 Regents of the University of California.
3  * All rights reserved.  The Berkeley software License Agreement
4  * specifies the terms and conditions for redistribution.
5  */
6 
7 #ifndef lint
8 static char sccsid[] = "@(#)arc.c	5.1 (Berkeley) 05/07/85";
9 #endif not lint
10 
11 #include "hp2648.h"
12 
13 arc(xcent,ycent,xbeg,ybeg,xend,yend)
14 int xcent,ycent,xbeg,ybeg,xend,yend;
15 {
16 	double costheta,sintheta,x,y,xn,r;
17 	double x1,y1,x2,y2;
18 	int xi,yi,crosspflag,crossp;
19 
20 	r = (xcent-xbeg)*(xcent-xbeg)+(ycent-ybeg)*(ycent-ybeg);
21 	r = pow(r,0.5);
22 	if(r<1){
23 		point(xcent,ycent);
24 		return;
25 	}
26 	sintheta = 1.0/r;
27 	costheta = pow(1-sintheta*sintheta,0.5);
28 	xi = x = xbeg-xcent;
29 	yi = y = ybeg-ycent;
30 	x1 = xcent;
31 	y1 = ycent;
32 	x2 = xend;
33 	y2 = yend;
34 	crosspflag = 0;
35 	do {
36 		crossp = cross_product(x1,y1,x2,y2,x,y);
37 		if(crossp >0 && crosspflag == 0) crosspflag = 1;
38 		point(xcent+xi,ycent+yi);
39 		xn = x;
40 		xi = x = x*costheta + y*sintheta;
41 		yi = y = y*costheta - xn*sintheta;
42 	} while( crosspflag == 0 || crossp >0);
43 }
44 
45 cross_product(x1,y1,x2,y2,x3,y3)
46 double x1,x2,x3,y1,y2,y3;
47 {
48 	double z,a,b;
49 	a = (y3-y2)*(x2-x1);
50 	b = (x3-x2)*(y2-y1);
51 	z = a-b;
52 	if(z<0) return(-1);
53 	if(z>0) return(1);
54 	return(0);
55 }
56