[postgis] 'real' contains, touches, overlaps etc.

[mbdavis]

Aw c'mon - robust determinants aren't *that* hard.  8^)     

In JTS we're using the approach of Avanim et al. 
(described at http://www-sop.inria.fr/prisme/logiciel/determinant.html
This is a very nice tutorial about why robustness is a serious problem, as
well).

This is only part of what you need to do to correctly handle boundaries,
however.  There's a lot of complex code needed to keep track of the
combinatorial aspects of the problem.  (At least there is in our
implementation - if someone has a simple method of doing this I'd like to
see it!).

Martin

-----Original Message-----
From: Dave Blasby [mailto:dblasby at refractions.net]
Sent: Tuesday, August 28, 2001 11:39 AM
To: postgis at yahoogroups.com
Subject: Re: [postgis] 'real' contains, touches, overlaps etc.


> today I implemented the contains and the overlaps function (it was not so
> hard while using the PIP stuff already there. I test it for a while and
send
> you the code soon (If you want it ;-) ...

Yes - send it in and I'll add it.  I'm looking forward to it.
 
> But I recognized one thing:
> 
> The PIP test does just divide between two cases -> inside and outside
> One problem (concerning to OGC conformity) is that we need a on-boundary
> operator to implelement touches(g1,g2) and the others in the coorect way.
> 
> I'm thinking about that (maybe I should wait, but I need it before ..)

Yes, there's lots of research into robust sign of determinates. 
Basically, if you can do that, you can robustly answer the question:
"does this point lies to the left, to the right, or on this line?"

Its very difficult to find out if a point is on a line or not because
the precision of the calculations (and the actual numbers) in the
computer are not exact. 

dave


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