Init

ucbcmd/plot/libplot/bitgraph/arc.c

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+/*	Copyright (c) 1984, 1986, 1987, 1988, 1989 AT&T	*/
+/*	  All Rights Reserved  	*/
+
+/*	THIS IS UNPUBLISHED PROPRIETARY SOURCE CODE OF AT&T	*/
+/*	The copyright notice above does not evidence any   	*/
+/*	actual or intended publication of such source code.	*/
+
+#ident	"@(#)arc.c	1.1	90/08/19 SMI"	/* SVr4.0 1.1	*/
+
+/*
+ * Copyright (c) 1980 Regents of the University of California.
+ * All rights reserved. The Berkeley software License Agreement
+ * specifies the terms and conditions for redistribution.
+ */
+
+/*
+ * Copyright (c) 1983, 1984 1985, 1986, 1987, 1988, Sun Microsystems, Inc.
+ * All Rights Reserved.
+ */
+
+
+#include "bg.h"
+
+/* should include test for equality? */
+#define side(x,y)	(a*(x)+b*(y)+c > 0.0 ? 1 : -1)
+
+/* The beginning and ending points must be distinct. */
+arc(xc,yc,xbeg,ybeg,xend,yend)
+int xc,yc,xbeg,ybeg,xend,yend;
+{
+	double r, radius, costheta, sintheta;
+	double a, b, c, x, y, tempX;
+	int right_side;
+
+	int screen_xc = scaleX(xc);
+	int screen_yc = scaleY(yc);
+
+	/* It is more convienient to beg and end relative to center. */
+	int screen_xbeg = scaleX(xbeg) - screen_xc;
+	int screen_ybeg = scaleY(ybeg) - screen_yc;
+
+	int screen_xend = scaleX(xend) - screen_xc;
+	int screen_yend = scaleY(yend) - screen_yc;
+
+	/* probably should check that arc is truely circular */
+	r = sqrt( (double) (screen_xbeg*screen_xbeg + screen_ybeg*screen_ybeg) );
+
+	/*
+	This method is reasonably efficient, clean, and clever.
+	The easy part is generating the next point on the arc.  This is
+	done by rotating the points by the angle theta.  Theta is chosen
+	so that no rotation will cause more than one pixel of a move.
+	This corresponds to a triangle having x side of r and y side of 1.
+	The rotation is done (way) below inside the loop.
+
+	Note:  all calculations are done in screen coordinates.
+	*/
+	if (r <= 1.0) {
+		/* radius is mapped to length < 1*/
+		point(xc,yc);
+		return;
+		}
+
+	radius = sqrt(r*r + 1.0);
+	sintheta = 1.0/radius;
+	costheta = r/radius;
+
+	/*
+	The hard part of drawing an arc is figuring out when to stop.
+	This method works by drawing the line from the beginning point
+	to the ending point.  This splits the plane in half, with the
+	arc that we wish to draw on one side of the line.  If we evaluate
+	side(x,y) = a*x + b*y + c, then all of the points on one side of the
+	line will result in side being positive, and all the points on the
+	other side of the line will result in side being negative.
+
+	We want to draw the arc in a counter-clockwise direction, so we
+	must find out what the sign of "side" is for a point which is to the 
+	"right" of a line drawn from "beg" to "end".  A point which must lie 
+	on the right is [xbeg + (yend-ybeg), ybeg - (xend-xbeg)].  (This
+	point is perpendicular to the line at "beg").
+
+	Thus, we compute side of the above point, and then compare the
+	sign of side for each new point with the sign of the above point.
+	When they are different, we terminate the loop.
+	*/
+
+	a = (double) (screen_yend - screen_ybeg);
+	b = (double) (screen_xend - screen_xbeg);
+	c = (double) (screen_yend*screen_xbeg - screen_xend*screen_ybeg);
+	right_side = side(screen_xbeg + (screen_yend-screen_ybeg),
+			  screen_ybeg - (screen_xend-screen_xbeg) );
+
+	x = screen_xbeg;
+	y = screen_ybeg;
+	move(xbeg, ybeg);
+	do {
+		currentx = screen_xc + (int) (x + 0.5);
+		currenty = screen_yc + (int) (y + 0.5);
+		putchar( ESC );
+		printf(":%d;%dd", currentx, currenty);
+		tempX = x;
+		x = x*costheta - y*sintheta;
+		y = tempX*sintheta + y*costheta;
+	} while( side(x,y) == right_side );
+}