```   1: /*
2:  * Copyright (c) 1980 Regents of the University of California.
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);
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

arc defined in line 18; used 2 times

#### Defined variables

sccsid defined in line 8; never used

#### Defined macros

side defined in line 15; used 2 times
 Last modified: 1985-05-01 Generated: 2016-12-26 Generated by src2html V0.67 page hit count: 677