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From: afielden@cbnewsb.cb.att.com (andrew.j.fielden)
Newsgroups: comp.windows.x,alt.toolkits.xview
Subject: Xlib regions
Keywords: regions graphics
Message-ID: <1991Mar28.094108.4281@cbfsb.att.com>
Date: 28 Mar 91 09:41:08 GMT
Sender: news@cbfsb.att.com
Followup-To: comp.windows.x
Organization: AT&T Bell Laboratories
Lines: 64


I'm writing an xview program that uses Xlib graphics primitives. I have found
a problem when trying to use Regions. What I am trying to do is detect whether
one region is contained within another region. I wrote a small program to
illustrate the problem.
I create a region with co-ordinates (5,5),(80,5),(80,80),(5,80) - call this
region b.
Now I try to determine if the following regions are contained within the above
region :-
          region a  = (10,20),(30,20),(70,70),(50,70)
          region A  = (10,20),(30,20),(70,50),(50,50)

So what I do is calculate the intersection of the regions 
(region [aA] and region b)
If region a is contained in region b, then this intersected region should be
exactly equal to region a.
Problem is, this method works for region 'a', but does not work for region 'A' !

I am a newcomer to X-Windows, so maybe I am overlooking something, but I would
appreciate any help anyone could offer.
Thanks in advance

Andrew.
==============================  CUT  =======================================

#include "/usr/openwin/include/X11/Intrinsic.h"
#include "/usr/openwin/include/X11/Xutil.h"

main()
{
  XPoint pt_a[4], pt_b[4];
  Region region_a, region_b, region_c;

/* (doesn't work) */
  pt_a[0].x = 10; pt_a[0].y = 20;
  pt_a[1].x = 30; pt_a[1].y = 20;
  pt_a[2].x = 70; pt_a[2].y = 70;
  pt_a[3].x = 50; pt_a[3].y = 70;

/* (works) */
/*
  pt_a[0].x = 10; pt_a[0].y = 20;
  pt_a[1].x = 30; pt_a[1].y = 20;
  pt_a[2].x = 70; pt_a[2].y = 50;
  pt_a[3].x = 50; pt_a[3].y = 50;
*/
  pt_b[0].x = 5; pt_b[0].y = 5;
  pt_b[1].x = 80; pt_b[1].y = 5;
  pt_b[2].x = 80; pt_b[2].y = 80;
  pt_b[3].x = 5; pt_b[3].y = 80;

  region_a = XPolygonRegion(pt_a, 4, EvenOddRule);
  region_b = XPolygonRegion(pt_b, 4, EvenOddRule);
  region_c = XCreateRegion();

  XIntersectRegion(region_b, region_a, region_c);
  if (XEqualRegion(region_a, region_c))
    printf("region a is in region b\n");
  else
    printf("region a is NOT in region b\n");
}
-- 
Andrew Fielden. AT&T Network Systems UK.
afielden@hvmpa.att.com