Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!ucbvax!bloom-beacon!dont-send-mail-to-path-lines
From: mouse@lightning.mcrcim.mcgill.EDU (der Mouse)
Newsgroups: comp.windows.x
Subject: Re:  Xlib regions
Message-ID: <9104011929.AA04956@lightning.McRCIM.McGill.EDU>
Date: 1 Apr 91 19:29:15 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 81

> 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