[geos-devel] SnapPointToLine : GEOS : where is the code for st_intersects between line and point?

[Sandro Santilli]

On Fri, Nov 08, 2013 at 01:01:56PM +0100, Rémi Cura wrote:

> "Are you saying you have a point and a line and you can mathematically
> prove that the point is on the line while GEOS says otherwise ?"
> Yep, it's all the problem ! in fact according to geos (st_intersect) lines
> have *very* few points, and some line have no points except extremities !

You mean they have no _representable_ points.
You're right you can mathematically represent them,
only not in floating point notation.

> AS I said the
> spacing of the point found bi st_intersects seems to follow the
> prime decomposition of the slope (or 1/slope) of the segment. Problem
> occurs when slope is not a finite number.
> 
> The whole problem comes from using exact math formula with inexact
> computations means. This has been somehow tackled by using a "robust"
> determinant sign, unfortunately this robustness is about saying left or
> right.
> 
> :-) Thanks for the typs about DEBUG, I'll try to find this , and also the
> place where postgres store it's log.
>
> Many thanks for your patience with a beginner like me ^^

Thank you for going deep into this !

--strk;