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Annotation of 43BSDTahoe/usr.lib/libplot/bitgraph/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      5.2 (Berkeley) 4/30/85";
                      9: #endif not lint
                     10: 
                     11: 
                     12: #include "bg.h"
                     13: 
                     14: /* should include test for equality? */
                     15: #define side(x,y)      (a*(x)+b*(y)+c > 0.0 ? 1 : -1)
                     16: 
                     17: /* The beginning and ending points must be distinct. */
                     18: arc(xc,yc,xbeg,ybeg,xend,yend)
                     19: int xc,yc,xbeg,ybeg,xend,yend;
                     20: {
                     21:        double r, radius, costheta, sintheta;
                     22:        double a, b, c, x, y, tempX;
                     23:        int right_side;
                     24: 
                     25:        int screen_xc = scaleX(xc);
                     26:        int screen_yc = scaleY(yc);
                     27: 
                     28:        /* It is more convienient to beg and end relative to center. */
                     29:        int screen_xbeg = scaleX(xbeg) - screen_xc;
                     30:        int screen_ybeg = scaleY(ybeg) - screen_yc;
                     31: 
                     32:        int screen_xend = scaleX(xend) - screen_xc;
                     33:        int screen_yend = scaleY(yend) - screen_yc;
                     34: 
                     35:        /* probably should check that arc is truely circular */
                     36:        r = sqrt( (double) (screen_xbeg*screen_xbeg + screen_ybeg*screen_ybeg) );
                     37: 
                     38:        /*
                     39:        This method is reasonably efficient, clean, and clever.
                     40:        The easy part is generating the next point on the arc.  This is
                     41:        done by rotating the points by the angle theta.  Theta is chosen
                     42:        so that no rotation will cause more than one pixel of a move.
                     43:        This corresponds to a triangle having x side of r and y side of 1.
                     44:        The rotation is done (way) below inside the loop.
                     45: 
                     46:        Note:  all calculations are done in screen coordinates.
                     47:        */
                     48:        if (r <= 1.0) {
                     49:                /* radius is mapped to length < 1*/
                     50:                point(xc,yc);
                     51:                return;
                     52:                }
                     53: 
                     54:        radius = sqrt(r*r + 1.0);
                     55:        sintheta = 1.0/radius;
                     56:        costheta = r/radius;
                     57: 
                     58:        /*
                     59:        The hard part of drawing an arc is figuring out when to stop.
                     60:        This method works by drawing the line from the beginning point
                     61:        to the ending point.  This splits the plane in half, with the
                     62:        arc that we wish to draw on one side of the line.  If we evaluate
                     63:        side(x,y) = a*x + b*y + c, then all of the points on one side of the
                     64:        line will result in side being positive, and all the points on the
                     65:        other side of the line will result in side being negative.
                     66: 
                     67:        We want to draw the arc in a counter-clockwise direction, so we
                     68:        must find out what the sign of "side" is for a point which is to the 
                     69:        "right" of a line drawn from "beg" to "end".  A point which must lie 
                     70:        on the right is [xbeg + (yend-ybeg), ybeg - (xend-xbeg)].  (This
                     71:        point is perpendicular to the line at "beg").
                     72: 
                     73:        Thus, we compute side of the above point, and then compare the
                     74:        sign of side for each new point with the sign of the above point.
                     75:        When they are different, we terminate the loop.
                     76:        */
                     77: 
                     78:        a = (double) (screen_yend - screen_ybeg);
                     79:        b = (double) (screen_xend - screen_xbeg);
                     80:        c = (double) (screen_yend*screen_xbeg - screen_xend*screen_ybeg);
                     81:        right_side = side(screen_xbeg + (screen_yend-screen_ybeg),
                     82:                          screen_ybeg - (screen_xend-screen_xbeg) );
                     83: 
                     84:        x = screen_xbeg;
                     85:        y = screen_ybeg;
                     86:        move(xbeg, ybeg);
                     87:        do {
                     88:                currentx = screen_xc + (int) (x + 0.5);
                     89:                currenty = screen_yc + (int) (y + 0.5);
                     90:                putchar( ESC );
                     91:                printf(":%d;%dd", currentx, currenty);
                     92:                tempX = x;
                     93:                x = x*costheta - y*sintheta;
                     94:                y = tempX*sintheta + y*costheta;
                     95:        } while( side(x,y) == right_side );
                     96: }

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