[geos-devel] improvement of point/line intersection

[Rémi Cura]

Yep,
thanks for the answer,

First *there is already an implicit tolerance*, only it is not enforced
coherently for each function.
The absolute minimum is 10^-17, so 2 points having the same first 17 digits
and then different digits will be seen as identical. In some case it can be
up to the first 15 digits.


Second, I totally agree this should be defined by user, so in my 10 lines
of code,
there is a variable named
"

  double min_precision = pow(10,-12) ;

"
It should be set at build I guess. As you know I'm not too familiar with C
dev, and so I don't know if it should be preprovessor, or global variable,
or macro or plain parameter ...


About the value of "min_precision",  min value is 10^-17 , theoretically we
should go as far as 10^-8 to guarantee correct result on max 8 digits in
(x2-x1) or (y2-y1).
I choose 10^-12 because theoretically error is less than 1/ 10^7, and it
makes the line very very thin (10^-12*(P1-P2)) (example : 2 points are 1000
km from one other, the thickness of the line would be 10 micro meters )


Cheers,

Rémi-C


2013/11/25 Sandro Santilli <strk at keybit.net>

> Thanks for the contribution Remi, but I wouldn't accept a change
> that would make such a tolerance implicit. Not at that low level.
>
> If the tolerance value is configurable via a parameter that'd
> be fine. Note that a parameter should already exist in form
> of a "precision model", but I haven't looked at the details
> of neither the current GEOS code nor your patch yet.
>
> --strk;
>
> On Fri, Nov 22, 2013 at 05:25:00PM +0100, Rémi Cura wrote:
> > Hey dear list,
> >
> > after much worries,
> > I just submitted a pull request to propose a very tiny change in geos
> > LineIntersector.cpp file.
> >
> > Now a point is considered to be on a line much more often, as line have a
> > controlled thickness.
> >
> > The thickness of a line is defined so that a point that shall be on the
> > line with the correct first 12 digits would be on it, even if the other
> > digits are wrong.
> >
> > All lines have not the same thickness is real world units, because the
> > tickness depends of the number of digits in coordinates.
> >
> >
> > This improves a lot of function working with points and lines, and
> > shouldn't change computation time (there where already 2 tests).
> >
> >
> > With synthetic data precise to the millimeter across all France, I had no
> > error for 1 million lines.
> > Theoretically there could still be some lines that have no points. (one
> in
> > 10 million or so?)
> > I can't solve this without changing much more the current RobustDet
> > algorithm.
> >
> >
> > Cheers,
> >
> >
> > Rémi-C
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