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

[Dave Blasby]

> 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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