[postgis-users] New function ST_DumpSegments?? (was : utility function : linestring -> setof segment)

[Nicolas Ribot]

Hi,

To dump (multi)polygons boundaries, you can use:
st_exteriorRing((st_dumpRings((st_dump(geom)).geom)).geom)

I returns one linestring for each boundary, taking holes into account.

Nicolas

On 30 October 2013 14:34, Rémi Cura <remi.cura at gmail.com> wrote:
> Hey,
> here is the function, safe for geometry collection and multilinestring as
> well.
> It mimics ST_DumpPoints.
>
> It doesn't work on polygon as we would first need a method STDumpLines.
>
>
> Origine of segments is preserved inside the path when multi.
> Could it be added to core ?
> Shall I add a ticket?
>
> Cheers,
> Rémi-C
>
> /////////////////////////////////
>
> DROP FUNCTION IF EXISTS public.DumpSegments(line geometry ) ;
> CREATE OR REPLACE FUNCTION public.DumpSegments(_line geometry)
> RETURNS SETOF geometry_dump
> AS
> $BODY$
> --this function breaks a line into minimal segments and return it
> DECLARE
> _r record;
> BEGIN
>
> FOR _r in SELECT ST_Dump(ST_CollectionExtract (_line,2)) AS dp
> LOOP
> RETURN QUERY WITH line AS(
> SELECT --ST_GeomFromText('LINESTRING(12 1, 13 1, 14 2, 15 4)') AS line
> (_r.dp).path AS gpath, (_r.dp).geom AS line
> ),
> dump AS(
> SELECT gpath[1] AS gpath, (ST_DumpPoints(line)) as dp
> FROM line
> ),
> segments AS (
> SELECT CASE WHEN gpath IS NULL THEN ARRAY[(dp).path[1]-1] ELSE ARRAY[gpath,
> (dp).path[1]-1] END AS path,ST_MakeLine( lag((dp).geom , 1, NULL) OVER
> (ORDER BY  (dp).path),(dp).geom) AS geom
> FROM dump
> )
> SELECT path,geom
> FROM segments
> WHEre geom  IS NOT NULL;
> END LOOP;--loop if multi linestring
> RETURN;
> END;
> $BODY$
>  LANGUAGE plpgsql  IMMUTABLE STRICT;
>
>
> SELECT public.DumpSegments(line2)
> FROM ST_GeomFromText('MULTILINESTRING((12 1, 13 1, 14 2, 15 4),(1 1, 2 2,3
> 3))') AS line1,ST_GeomFromText( 'LINESTRING(12 1, 13 1, 14 2, 15 4)') AS
> line2
>
>
>
>
> //////////////////////////////////////
>
>
>
>
> 2013/10/30 Rémi Cura <remi.cura at gmail.com>
>>
>> Thanks,
>> wanted to do it the clean way (geos accelerated maybe?),
>>
>> but not so much a big deal, I'll go you way.
>>
>> Cheers,
>> Rémi-C
>>
>>
>> 2013/10/30 Nicolas Ribot <nicolas.ribot at gmail.com>
>>>
>>> Hi,
>>>
>>> I don't think such a function exists in postgis.
>>> (To achieve that, I usually dump line points and create new
>>> linestrings with vertices n, n+1)
>>>
>>> Nicolas
>>>
>>> On 30 October 2013 10:59, Rémi Cura <remi.cura at gmail.com> wrote:
>>> > Hey,
>>> > is somebodyers aware of a function breaking a linestring into a set of
>>> > segments (mathematical meaning),
>>> > without introducing new points?
>>> >
>>> > Cheers,
>>> >
>>> >
>>> > Rémi-C
>>> >
>>> > _______________________________________________
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>>
>>
>
>
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