4.3BSD - /usr/src/usr.lib/libplot/bitgraph/arc.c

   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: }

Defined functions

Defined variables

Defined macros