[postgis-devel] Re: [postgis-users] Modifying a linestring

[strk at refractions.net]

On Thu, Aug 04, 2005 at 11:17:51AM -0400, Stephen Woodbridge wrote:
> strk at refractions.net wrote:
> >On Thu, Aug 04, 2005 at 11:29:52AM +0300, Ehud Shabtai wrote:
> >
> >>Hi,
> >>
> >>I need to modify a linestring (actually change the first or last
> >>point), in a postgres trigger. Is there a simple way to do it using
> >>the sql language?
> >
> >
> >Nope, but it would probably be useful.
> >
> >The interface might be something like:
> >
> >	UpdateLinePoint(line, pointN, point);
> >	InsertLinePoint(line, pointN, point); -- add before pointN
> >	AppendLinePoint(line, pointN, point); -- add after pointN
> >	DeleteLinePoint(line, pointN, point);
> >
> >Similar editing functions would be nice to have for
> >polygons as well.
> >
> >Comments ? Volunteers ? Founders ?
> 
> Comment :)
> 
> Why would you not want to make this generic like:
> 	UpdateGeomPoint(geometry, itemN, pointN, point);
> 	AddPointBefore(geometry, itemN, pointN, point);
> 	AddPointAfter(geometry, itemN, pointN, point);
> 	DeleteGeomPoint(geometry, itemN, pointN, point);
> 
> where itemN would be a ring or item in MULTI*, well it is obvious that 
> you can create objects that are too complex for this to work, but itemN 
> could also be an ARRAY() of indexes to select a simple geometry.

Sounds complex... anyway we already used the ARRAY() method for
the Dump(geometry) function, so we might as well use that to
address complex geometries elements. 

A simpler syntax would be:

	UpdateExteriorRing(poly, line); 
	UpdateInteriorRingN(poly, ringN, line);
	AddInteriorRing(poly, line); -- does position really matters ?
	DeleteInteriorRingN(poly, ringN);

	InsertGeometry(multi*, path); -- add before geom in path
	AppendGeometry(multi*, path); -- add after geom in path
	UpdateGeometry(multi*, path, single*);
	GetGeometry(multi*, path);

Example:

	geom := 'GEOMETRYCOLLECTION(
		POINT(0 0),
		MULTILINESTRING((0 0, 2 2),(1 1, 3 3))
	)';

Paths:
[ SELECT path(Dump(geom), AsText(geom(Dump(geom)) AS geom ]

 	path    geom
	------+---------------------
 	{1}   | POINT(0 0)
 	{2,1} | LINESTRING(0 0,2 2)
 	{2,2} | LINESTRING(1 1,3 3)

Update of first point of last linestring:

	UpdateGeometry(geom,
		{2,2},
		UpdateLinePoint(
			GetGeometry(geom, {2,2}),
			1,
			'POINT(3 3)'
		)
	);

These, as you say, would be less efficient, but more intuitive.
In your terms this would be easy:

 	UpdateGeomPoint(geom, {2,2}, 1, 'POINT(3 3)');

But how would it be to update the point of a polygon hole ?

--strk;



> In general this speaks to having general purpose constructors and 
> de-constructors for all geometry objects that are fast and efficient. 
> Most of the pieces are there for this, but it is not always obvious how 
> to do this otherwise the response would be you modify the a line by:
> 
>   points := getLinePoints(line);
>   points[0] := new_point;
>   line := makeline(points);
> 
> or something to this effect.
> 
> -Steve W.
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