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Annotation of 43BSDTahoe/usr.lib/libplot/gigi/arc.c, revision 1.1.1.1

1.1       root        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) 5/7/85";
                      9: #endif not lint
                     10: 
                     11: #include "gigi.h"
                     12: 
                     13: /* 
                     14:  * gigi requires knowing the anlge of arc.  To do this, the triangle formula
                     15:  *     c^2 = a^2 + b^2 - 2*a*b*cos(angle)
                     16:  * is used where "a" and "b" are the radius of the circle and "c" is the
                     17:  * distance between the beginning point and the end point.
                     18:  *
                     19:  * This gives us "angle" or angle - 180.  To find out which, draw a line from
                     20:  * beg to center.  This splits the plane in half.  All points on one side of the
                     21:  * plane will have the same sign when plugged into the equation for the line.
                     22:  * Pick a point on the "right side" of the line (see program below).  If "end"
                     23:  * has the same sign as this point does, then they are both on the same side
                     24:  * of the line and so angle is < 180.  Otherwise, angle > 180.
                     25:  */
                     26:    
                     27: #define side(x,y)      (a*(x)+b*(y)+c > 0.0 ? 1 : -1)
                     28: 
                     29: arc(xcent,ycent,xbeg,ybeg,xend,yend)
                     30: int xcent,ycent,xbeg,ybeg,xend,yend;
                     31: {
                     32:        double radius2, c2;
                     33:        double a,b,c;
                     34:        int angle;
                     35: 
                     36:        /* Probably should check that this is really a circular arc.  */
                     37:        radius2 = (xcent-xbeg)*(xcent-xbeg) + (ycent-ybeg)*(ycent-ybeg);
                     38:        c2 = (xend-xbeg)*(xend-xbeg) + (yend-ybeg)*(yend-ybeg);
                     39:        angle = (int) ( 180.0/PI * acos(1.0 - c2/(2.0*radius2)) + 0.5 );
                     40: 
                     41:        a = (double) (ycent - ybeg);
                     42:        b = (double) (xcent - xbeg);
                     43:        c = (double) (ycent*xbeg - xcent*ybeg);
                     44:        if (side(xbeg + (ycent-ybeg), ybeg - (xcent-xbeg)) != side(xend,yend))
                     45:                angle += 180;
                     46:        
                     47:        move(xcent, ycent);
                     48:        printf("C(A%d c)[%d,%d]", angle, xbeg, ybeg);
                     49: }

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