[geos-devel] [ANN] Bug #87 - Strange result of isValid() for LinearRing

[Stephen Woodbridge]

Mateusz Łoskot wrote:
> Hi,
> 
> I've reported new bug on the BT:
> 
> http://postgis.refractions.net/bugs/bug.php?op=show&bugid=87&pos=4
> 
> I think it may need some wider discussion.
> 
> Cheers

Hey Mateusz,

I think your test case is bad.

/ 2. Add 4 points to get closed ring
pseq->add(Coordinate(0, 0, 0));
pseq->add(Coordinate(5, 5, 5));
pseq->add(Coordinate(10, 10, 10));
pseq->add(Coordinate(0, 0, 0)); // <--- [1]

The points (0,0,0), (5,5,5), (10,10,10) are all in a straight line (ie: 
they are co-linear) and then you return to the start point which falls 
on  top of the first two segments so you have a case of a collapsed ring 
where all the point of the ring fall on a single line in space.

This is indeed self intersecting. and isValid() should report it as false.

-Steve