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Annotation of 43BSD/usr.lib/libplot/hp7221/arc.c, revision 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 "hp7221.h"
        !            12: 
        !            13: /* 
        !            14:  * 7221 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:        /* Not quite implemented...
        !            49:        printf("C(A%d c)[%d,%d]", angle, xbeg, ybeg);
        !            50:        */
        !            51: }

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