afielden@cbnewsb.cb.att.com (andrew.j.fielden) (03/28/91)
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.commouse@lightning.mcrcim.mcgill.EDU (der Mouse) (04/02/91)
> 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. [example] To test whether region A is inside region B, you intersect A and B and check whether the result is equal to A. This will work but is likely to be inefficient. I would recommend using XSubtractRegion to subtract B from A, then checking the result to see whether it's empty. (This should also keep you from being bothered by the bug you found.) > region a = (10,20),(30,20),(70,70),(50,70) > region A = (10,20),(30,20),(70,50),(50,50) > 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' ! Your test program (once I changed the #include directives) exhibited the behavior you describe here as well. Dumping the internal representations of the regions involved makes it clear what's going on: A size 50 numRects 50 extents X: 10 70 Y: 20 70 rects: 10 30 X 20 21 11 31 X 21 22 12 32 X 22 23 13 33 X 23 24 NOTE> 14 34 X 24 25 NOTE> 14 34 X 25 26 15 35 X 26 27 16 36 X 27 28 17 37 X 28 29 NOTE> 18 38 X 29 30 NOTE> 18 38 X 30 31 19 39 X 31 32 20 40 X 32 33 [many lines deleted] B size 1 numRects 1 extents X: 5 80 Y: 5 80 rects: 5 80 X 5 80 C size 41 numRects 41 extents X: 10 70 Y: 20 70 rects: 10 30 X 20 21 11 31 X 21 22 12 32 X 22 23 13 33 X 23 24 NOTE> 14 34 X 24 26 15 35 X 26 27 16 36 X 27 28 17 37 X 28 29 NOTE> 18 38 X 29 31 19 39 X 31 32 20 40 X 32 33 [more lines deleted] So it is clear that the problem is a bug in the Region implementation. Either XPolygonRegion should merge those mergeable rectangles or XEqualRegion should take care to do the comparison properly. (Personally, I prefer the first fix, but the second is probably easier to implement.) I'll send a bug report to xbugs, since this bug appears in R4. der Mouse old: mcgill-vision!mouse new: mouse@larry.mcrcim.mcgill.edu