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From: stripes@eng.umd.edu (Joshua Osborne)
Newsgroups: comp.windows.x
Subject: Help!  My arc has fallen, and it can't get up!
Message-ID: <1991Jun28.062319.11565@eng.umd.edu>
Date: 28 Jun 91 06:23:19 GMT
Organization: College of Engineering, Maryversity of Uniland, College Park
Lines: 58

I am trying to draw the outline of a pie wedge.  I (think I) am using the
formula from the Digital Press "X Windows System, C Library and protocall
reference" by Scheifler, Gettys & Newman.  I coded it up like this:

#define RPI (64*180./3.14159)
#define PIPI (3.14159/2)

double ella(a, w, h)
double a;
int w, h;
{
	double skew;

	skew = atan(tan(a) * w/(h+0.0));
	skew += (a < PIPI) ? 0 : (a < 3 * PIPI) ? (2*PIPI) : (4*PIPI);

	return  skew;
}

Then armed with only a filled-out XArc structure, and a semester of Trig 4
years stale, I attempted many variations along the lines of:

v2_pie(xa)
XArc *xa;
{
	XPoint pts[3];
	double s, c;
	int r;

	r = (xa->width + xa->height) / 4;

	pts[1].x = xa->width / 2 + xa->x;
	pts[1].y = xa->height / 2 + xa->y;
	sincos(ella(xa->angle1/RPI, xa->width, xa->height), &s, &c);
	pts[0].x = pts[1].x + xa->width/2 * c;
	pts[0].y = pts[1].y + xa->height/2 * s;
	sincos(ella((xa->angle1+xa->angle2)/RPI, xa->width, xa->height), &s, &c);
	pts[2].x = pts[1].x + xa->width/2 * c;
	pts[2].y = pts[1].y + xa->height/2 * s;

	XSetFillStyle(dpy, drawGC, FillSolid);
	XDrawLines(dpy, win, drawGC, pts, 3, CoordModeOrigin);
	XDrawArcs(dpy, win, drawGC, xa, 1);
	reset_a();
}

I compiled the program, ran it, and it failed.  Miserably.  It's output was
hideous.  Women fainted.  Small children ran away.  Worst of all, the lines
that should have connected the arc endpoints had very little to do with the
arc endpoints.

Does anyone have any suggestions?  How can I compute the xy position of the
arc's endpoints?  Please mail to me; I will summarize any and all replies
to the net.
-- 
           stripes@eng.umd.edu          "Security for Unix is like
      Josh_Osborne@Real_World,The          Multitasking for MS-DOS"
      "The dyslexic porgramer"                  - Kevin Lockwood