[unix-history] / usr / src / lib / libplot / gigi / arc.c

/*
* Copyright (c) 1980 Regents of the University of California.
* All rights reserved.  The Berkeley software License Agreement
* specifies the terms and conditions for redistribution.
*/

#ifndef lint
static char sccsid[] = "@(#)arc.c       5.1 (Berkeley) %G%";
#endif not lint

#include "gigi.h"

/* 
* gigi requires knowing the anlge of arc.  To do this, the triangle formula
*      c^2 = a^2 + b^2 - 2*a*b*cos(angle)
* is used where "a" and "b" are the radius of the circle and "c" is the
* distance between the beginning point and the end point.
*
* This gives us "angle" or angle - 180.  To find out which, draw a line from
* beg to center.  This splits the plane in half.  All points on one side of the
* plane will have the same sign when plugged into the equation for the line.
* Pick a point on the "right side" of the line (see program below).  If "end"
* has the same sign as this point does, then they are both on the same side
* of the line and so angle is < 180.  Otherwise, angle > 180.
*/
  
#define side(x,y)       (a*(x)+b*(y)+c > 0.0 ? 1 : -1)

arc(xcent,ycent,xbeg,ybeg,xend,yend)
int xcent,ycent,xbeg,ybeg,xend,yend;
{
       double radius2, c2;
       double a,b,c;
       int angle;

       /* Probably should check that this is really a circular arc.  */
       radius2 = (xcent-xbeg)*(xcent-xbeg) + (ycent-ybeg)*(ycent-ybeg);
       c2 = (xend-xbeg)*(xend-xbeg) + (yend-ybeg)*(yend-ybeg);
       angle = (int) ( 180.0/PI * acos(1.0 - c2/(2.0*radius2)) + 0.5 );

       a = (double) (ycent - ybeg);
       b = (double) (xcent - xbeg);
       c = (double) (ycent*xbeg - xcent*ybeg);
       if (side(xbeg + (ycent-ybeg), ybeg - (xcent-xbeg)) != side(xend,yend))
               angle += 180;
       
       move(xcent, ycent);
       printf("C(A%d c)[%d,%d]", angle, xbeg, ybeg);
}