[postgis-devel] ST_CurveToLine extended API proposal

[Sandro Santilli]

On Tue, Jun 13, 2017 at 11:27:10AM +0200, Tom van Tilburg wrote:
> For what it's worth: I noticed that the old behaviour of segs_per_quadrant
> caused some topologies to break because the segments were not of equal
> length. When polygons are neighbouring eachother in a topological manner,
> the new segments vertices will not align with the neighbour when running
> ST_CurveToLine on both polygons, since the line direction is opposite.
> (GDAL does it correct, making every segment in the former curve of equal
> length).
> If this behaviour can be tackled in new functionality that would be great.

I think the "balanced" argument would take care of that.

I was actually thinking of making first and last segment of
equal length rather than all segments of the same length, but
your suggestion seems indeed better. I don't think we need
two different policies, what do you think ?

--strk;

> 
> On Tue, Jun 13, 2017 at 11:12 AM, Sandro Santilli <strk at kbt.io> wrote:
> 
> > I've started work on extended functionality for ST_CurveToLine [1]
> > to allow for specifying max-error instead of segments-per-quadrant [2]
> > and balancing of output segments [3].
> >
> > [1] https://postgis.net/docs/ST_CurveToLine.html
> > [2] https://trac.osgeo.org/postgis/ticket/2464
> > [3] https://trac.osgeo.org/postgis/ticket/3772
> >
> > Current API is:
> >
> >   ST_CurveToLine(geom) -- defaults to 32 segs-per-quadrant
> >   ST_CurveToLine(geom, integer segs_per_quadrant)
> >
> > The proposed new API would be:
> >
> >   ST_CurveToLine(geom geometry,
> >           tolerance float8 = 32,
> >           tolerance_interpretation int = 0,
> >           balanced bool=true)
> >
> > The tolerance_intepretation parameter could initially
> > support these values:
> >
> >   0 - segments per quarter circle
> >   1 - maximum distance beween segment and curve
> >
> > Leaving the door open to futher extensions (for example
> > QGIS also supports "maximum angle between segments").
> >
> > The default values would also allow for the new signature
> > to replace the existing ones, so to have a single signature.
> >
> > Of course the code would then need to perform some consistency
> > checking, like not accepting a non-integer value as tolerance
> > when "segments-per-quarter-cirlce" interpretation is requested.
> >
> > What do you think about the proposal ?
> >
> > --strk;
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