[postgis-devel] ST_ConcaveHull in GEOS

[Martin Davis]

Regina:

Yes, I'm working on updating the docs, and the examples

On Wed, Jan 26, 2022 at 1:07 PM Regina Obe <lr at pcorp.us> wrote:

> Martin,
>
>
>
> Feel free to take a stab at updating the docs, but what you describe below
> sounds good to me to detail the change in shrink behavior.
>
> I think the new GEOS concave hull is far more scientific about it than the
> old, so most people will prefer the new behavior.
>
>
>
>
>
> *From:* postgis-devel [mailto:postgis-devel-bounces at lists.osgeo.org] *On
> Behalf Of *Martin Davis
> *Sent:* Wednesday, January 26, 2022 2:55 PM
> *To:* PostGIS Development Discussion <postgis-devel at lists.osgeo.org>
> *Subject:* Re: [postgis-devel] ST_ConcaveHull in GEOS
>
>
>
> As part of the replacement effort, the PostGIS doco needs some changes to
> accurately describe the new Concave Hull behaviour.   In particular, there
> are a few key differences:
>
> - the target_percent parameter now defines a length ratio, rather than an
> area ratio (see doc below).  This should make it easier to choose a
> target_percent value that applies to a wider range of potential inputs,
> since edge length is a less-constraining criterion than area.
>
> - the concave hull algorithm only uses the vertices of the input
> geometry.  It does not respect the geometry linework (if any).  I think
> this is a change from the current behaviour?
>
> - because only vertices are considered, there is no longer a need to use
> ST_Union to compute the hull of a set of polygons (and the input does not
> even have to be valid)
>
>
>
> New doc (comments welcome):
>
> --------------------------------------------
>
> Description
> A concave hull of a geometry is a possibly concave geometry that encloses
> the vertices of the input geometry. The result is a single polygon, line or
> point. It will not contain holes unless the optional allow_holes argument
> is specified as true.
>
> One can think of a concave hull as a geometry obtained by
> "shrink-wrapping" a set of points. This is different to the convex hull,
> which is more like wrapping a rubber band around the points. The concave
> hull generally has a smaller area and represents a more natural boundary
> for the input points.
>
> The target_percent controls the concaveness of the computed hull. A value
> of 1 produces the convex hull. A value of 0 produces a hull with maximum
> concaveness (but still a single polygon). Values between 1 and 0 produce
> hulls of increasing concaveness. Choosing a suitable value depends on the
> nature of the input data, but often values between 0.4 and 0.2 produce
> reasonable results.
>
> Technically, the target percent determines a length as a fraction of the
> difference between the longest and shortest edges in the Delaunay
> Triangulation of the input points. Edges longer than this length are
> "eroded" from the triangulation. The triangles remaining form the concave
> hull.
>
> This is not an aggregate function. To compute the concave hull of a set of
> geometries use ST_Collect (e.g. ST_ConcaveHull( ST_Collect( geom ), 0.80).
>
> ---------------------
>
>
>
> On Tue, Jan 25, 2022 at 2:38 PM Paul Ramsey <pramsey at cleverelephant.ca>
> wrote:
>
> Hey all,
> The native ST_ConcaveHull in GEOS is in GEOS main now, so I have hooked it
> up into PostGIS in this branch
>
> https://github.com/pramsey/postgis/tree/master-concavehull
>
>
> Under this function for now.
>
> CREATE OR REPLACE FUNCTION _ST_GEOSConcaveHull(
>   geom geometry,
>   area_ratio float8,
>   allow_holes boolean DEFAULT false)
>
> I would like to just flip the old ST_ConcaveHull out and replace it with
> the new one, for folks with GEOS 3.11+, but I figured an interval to look
> at them side by side for a while on a test branch might be appreciated.
> Currently the case of point inputs is worth looking at. Polygon inputs that
> respect the polygon are (hull doesn't cross interior of input polygon)
> coming down the line.
>
> P
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/postgis-devel
>
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/postgis-devel
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-devel/attachments/20220126/84d399a1/attachment-0001.html>