[postgis-users] Postgis 3.0: How to convert liblwgeom to librttopo?

Mon Jun 27 11:14:49 PDT 2022

Hi,

Your function sounds a lot like existing
https://postgis.net/docs/ST_AsMVTGeom.html

On Mon, Jun 27, 2022 at 9:10 PM <pt122 at gmx.net> wrote:

> Thank you for your feedback. Since I see no practical way for me to use
> liblwgeom functionality after installing postgis 3.x I tried another
> approach and added my function to the normal postgis make process which
> works fine for me:
>
> - put my c file into /postgis
> - add it in postgis/Makefile
> - append a create function in postgis/postgis.sql.in
> - ./configure, make and make install
> - then in psql: drop+create extension postgis to reflect changes and test.
>
> Two more questions:
>
> 1. I'd like to ask if I could contribute with my function (since I think
> that the existing st_methods are not really that what I want): It is a
> short preprocessing function for subsequent actions like fast rendering of
> high resolution data within a bbox. It aims to reduce as many coordinates
> as possible, and as fast as possible. It is not topology-safe, works
> currently for multipolygons and multilines, and offers different subtasks
> that may be combined:
>
> a) scale and remove coordinates in a sequence that lie on same grid cells
> (similar to st_snaptogrid and st_reduceprecision)
> b) sorting out all points outside a bbox that are irrelevant for rendering
> data within this bbox (similar to st_clipbybox2d but without computing any
> new intersection points)
> c) remove small parts with dimensions below a threshold (similar to
> st_box2d component checks, and simpler+faster than expensive st_area
> computations)
>
> 2. Are there any examples on how to use the officially recommented
> librttopo (as part of the default package-based postgis installation) for
> compile own c-based libraries after installation of postgis?
>
> Thank you, Sven
>
>
>
> Gesendet: Samstag, 25. Juni 2022 um 23:48 Uhr
> Von: "Regina Obe" <lr at pcorp.us>
> An: "'PostGIS Users Discussion'" <postgis-users at lists.osgeo.org>
> Betreff: Re: [postgis-users] Postgis 3.0: How to convert liblwgeom to
> librttopo?
>
> liblwgeom.so is not separate anymore and liblwgeom.h is not installed
> either. They are all just object libraries, statically linked in all the
> .sos we expose as needed.
> You can take a look at https://trac.osgeo.org/postgis/ticket/4260  and
> https://github.com/postgis/postgis/pull/348/[https://github.com/postgis/postgis/pull/348/]
> where it was removed and back track those changes.
>
> By the way
> https://postgis.net/docs/ST_ReducePrecision.html[https://postgis.net/docs/ST_ReducePrecision.html]
> is a better alternative to ST_SnapToGrid as it’s much less likely to
> produce invalid geometries, but as mentioned only available if you have
> GEOS 3.9+
>
>
> Hope that helps,
> Regina
>
>
>
>
> From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On
> Behalf Of pt122 at gmx.net
> Sent: Friday, June 24, 2022 5:01 AM
> To: postgis-users at lists.osgeo.org
> Subject: Re: [postgis-users] Postgis 3.0: How to convert liblwgeom to
> librttopo?
>
>
> Hi Regina, thank you very much for your valuable and fast reply. What I
> want to
>
> achive is quite similar to st_snaptogrid(). I'll surely give it a try to
> replace our function
>
> (developed in 2013) with one or more of these functions you mentioned at
> some
>
> point in the future. But, for now, I'd like to try to go forward with
> liblwgeom in the
>
> first place and I am glad for your hint that it is still a usable part of
> PostGIS 3.x.
>
>
>
> So, two questions arise:
>
>
>
> (How) can I build PostGIS via gcc and special flags from source to get the
> required
>
> liblwgeom.h and liblwgeom.so files?
>
>
>
> Or is liblwgeom already contained in the "normal" installable package,
> with some
>
> way to access it?
>
>
>
> Thank you, Sven
>
>
>
> Gesendet: Freitag, 24. Juni 2022 um 02:47 Uhr
> Von: "Regina Obe" <lr at pcorp.us[mailto:lr at pcorp.us]>
> An: "'PostGIS Users Discussion'" <postgis-users at lists.osgeo.org[mailto:
> postgis-users at lists.osgeo.org]>
> Betreff: Re: [postgis-users] Postgis 3.0: How to convert liblwgeom to
> librttopo?
>
> liblwgeom is still part of PostGIS but not exposed anymore as a standalone
> library and is always statically linked. The reason is it caused too much
> headache since we often change it even in micro versions so didn't want
> anyone directly relying on it or our own code hooking onto the wrong
> version
> installed in system.
>
> Yes librttopo is a fork and is maintained separate from PostGIS here -
>
> https://git.osgeo.org/gitea/rttopo/librttopo[https://git.osgeo.org/gitea/rttopo/librttopo]
> .
>
> I'm not sure if it has a hard requirement on GEOS. It definitely has pieces
> that use GEOS, so at very least a soft requirement. It is a subset of
> liblwgeom, that subset that was useful for topology work in particular, but
> it probably includes the subset you need.
>
> It's unclear to me what your function does, but some possibly useful
> functions to achieve the same goal are:
>
>
> https://postgis.net/docs/ST_Subdivide.html[https://postgis.net/docs/ST_Subdivide.html]
>
> https://postgis.net/docs/ST_ClipByBox2D.html
>
> https://postgis.net/docs/ST_SquareGrid.html
>
> https://postgis.net/docs/ST_HexagonGrid.html
>
>
> Also if you are running GEOS 3.9 (+ PostGIS 3.1+) or higher, using
>
> https://postgis.net/docs/ST_ReducePrecision.html[https://postgis.net/docs/ST_ReducePrecision.html]
> (and/or the newer ST_Union,
> ST_Intersection, ST_Subdivide) which has reduce precision logic in them
> might solve some of your problems)
>
> Hope that helps,
> Regina
>
> > -----Original Message-----
> > From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org
> [mailto:postgis-users-bounces at lists.osgeo.org]] On
> Behalf
> > Of pt122 at gmx.net[mailto:pt122 at gmx.net]
> > Sent: Thursday, June 23, 2022 4:52 PM
> > To: postgis-users at lists.osgeo.org[mailto:postgis-users at lists.osgeo.org]
> > Subject: [postgis-users] Postgis 3.0: How to convert liblwgeom to
> librttopo?
> >
> > Dear Postgis community, I'm not sure if I'm in the right forum but I want
> to
> > give it a try. We want to upgrade from PostGIS 2.x to PostGIS 3.x and
> take
> > over a self-written library (.so). This library was compiled with
> liblwgeom of
> > PostGIS 2.x. liblwgeom seems to be not part of PostGIS 3.x anymore
> > (/include/liblwgeom.h is missing for instance).
> >
> > According to the comment in
> https://postgis.net/2019/10/20/postgis-3.0.0/[https://postgis.net/2019/10/20/postgis-3.0.0/]
> ,
> > liblwgeom should be replaced with librttopo (which seems to be a fork of
> > liblwgeom according to
> https://github.com/r-spatial/lwgeom/issues/30[https://github.com/r-spatial/lwgeom/issues/30]
> ).
> >
> > Our function that makes use of our lib, used for fast scaling, clipping
> and
> > sorting-out coordinates, looks like:
> >
> > CREATE OR REPLACE FUNCTION preprocess(geometry, box2d, ...) RETURNS
> > geometry AS 'path/to/lib.so', 'preprocess'
> > LANGUAGE c;
> > and in the c code are some LWGEOM*, LWMLINE*, LWMPOLY* and lwfree(..)
> > calls.
> >
> > My questions are:
> >
> > - is lwgeom still part of Postgis 3.x? (I still see liblwgeom.h.in as
> part
> of
> > postgis-3.2.1.tar.gz, so it seems that is not fully removed yet)
> >
> > - if not, what would be a good starting point for conversion?
> >
> > - are there any mappings of old and new functions or types available?
> >
> > -are there any packages (besides librttopo and librttopo-devel) that must
> be
> > installed additionally with librttopo (like GEOS or similar)?
> >
> > -what are the PostGIS built-in alternatives for scaling and clipping?
> >
> > Thank you for any hints, Peter
> >
> > (See original post on
> >
> https://gis.stackexchange.com/questions/434369/postgis-3-0-how-to-convert-[https://gis.stackexchange.com/questions/434369/postgis-3-0-how-to-convert-]
> > liblwgeom-to-librttopo, but my problem is likely is too specific to ask
> in
> > stackexchange...) _______________________________________________
> > postgis-users mailing list
> > postgis-users at lists.osgeo.org[mailto:postgis-users at lists.osgeo.org]
> >
> https://lists.osgeo.org/mailman/listinfo/postgis-users[https://lists.osgeo.org/mailman/listinfo/postgis-users]
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org[mailto:postgis-users at lists.osgeo.org]
> https://lists.osgeo.org/mailman/listinfo/postgis-users
>
>
>
>
> _______________________________________________ postgis-users mailing list
> postgis-users at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/postgis-users[https://lists.osgeo.org/mailman/listinfo/postgis-users]
>
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/postgis-users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20220627/a7a56d66/attachment.htm>