[postgis-users] Bezier or Spline smoothing implementation

[Stefan Keller]

Hi James

Good you pointed to ST_Line_Interpolate. The use case I have in mind is
that I have e.g. roads and want to smooth ithem before they are being
rendered.
The input is a linestring and the output is another linestring with about
same amount of coordinates (or less) - but smoothed.
Something like this
http://www.codeproject.com/Articles/31859/Draw-a-Smooth-Curve-through-a-Set-of-2D-Points-wit

Yours, Stefan


2013/9/18 James David Smith <james.david.smith at gmail.com>

> I'm sure that it's because I don't quite understand what you are
> trying to do, but why not just use the native PostGIS function -
> http://postgis.net/docs/manual-2.0/ST_Line_Interpolate_Point.html  ?
>
> On 18 September 2013 15:06,  <Michael.Scholz at dlr.de> wrote:
> > Hey!
> >
> > First of all I can recommend two tutorials about how to use PL/R in
> PostGIS
> > [1][2]. One possibility to interpolate splines with R is to use R's
> xspline
> > package [3]. Some theoretical background 'bout those x-splines you'll
> find
> > in the related paper [4]. I tried some other spline-methods in R but with
> > dubious results. xspline works fine for me. A wrapping PL/R-function
> could
> > look like this:
> >
> >
> >
> > /**
> >
> > * Interpolates a LineString using R's X-spline-method
> >
> > *
> >
> >  * Parameters:
> >
> > *   1st - A *single* PostGIS-geometry (POINT, LINESTRING or POLYGON) *as
> > text*: Use ST_AsEWKT() to pass a geometry as text.
> >
> > *   2nd - Shape parameter used in R's X-spline-method [-1.0,...,1.0]. A
> > shape of 0.0 does not interpolate anything. Default: 0.5.
> >
> > *
> >
> >  * Returns:
> >
> > *   A PostGIS-LINESTRING-geometry covering the interpolated points.
> >
> > *
> >
> >  * Usage:
> >
> > *   > SELECT interpolateXSpline(ST_AsEWKT(geom), 1.0) FROM your_table
> >
> > */
> >
> > create or replace function interpolateXSpline(text, float DEFAULT 0.5)
> > returns geometry as $$
> >
> >   # Select the points (also from line or polygon-boundary) into a data
> frame
> >
> >   points <- pg.spi.exec(
> >
> >     sprintf("WITH pts AS (SELECT
> (ST_DumpPoints(ST_GeomFromEWKT(%1$s))).geom
> > AS geom)
> >
> >       SELECT ST_X(geom) AS x, ST_Y(geom) AS y FROM pts;",
> >
> >         pg.quoteliteral(arg1)
> >
> >     )
> >
> >   )
> >
> >
> >
> >   # Interpolate spline
> >
> >   plot(points$x, points$y, type="n") # Dummy-call of plot() for xspline
> to
> > work
> >
> >   spline_pts <- xspline(points$x, points$y, shape=arg2, draw=FALSE)
> >
> >   # Here you could simply output spline_pts and return.
> >
> >   # Otherwise proceed to build a WKT linestring as shown below.
> >
> >
> >
> >   # LineString-creation: Build beginning of WKT string for output to
> PostGIS
> >
> >   out_line = "LINESTRING("
> >
> >     for (i in 1:(length(spline_pts$x)-1)) {
> >
> >       out_line = sprintf(
> >
> >         "%1$s %2$f %3$f,",
> >
> >         out_line, spline_pts$x[i], spline_pts$y[i]
> >
> >       )
> >
> >     }
> >
> >   # LineString-creation: Append middle part
> >
> >   out_line =
> >
> >     sprintf("%1$s %2$f %3$f)",
> >
> >       out_line,
> >
> >       spline_pts$x[length(spline_pts$x)],
> >
> >       spline_pts$y[length(spline_pts$y)]
> >
> >     )
> >
> >   # LineString-creation: Finalise (close)
> >
> >   oline <- pg.spi.exec(
> >
> >     sprintf("SELECT ST_GeomFromText('%1$s',
> ST_SRID(ST_GeomFromEWKT(%2$s)))
> > AS ln;",
> >
> >       out_line,
> >
> >       pg.quoteliteral(arg1)
> >
> >     )
> >
> >  )
> >
> >   return(oline$ln[1])
> >
> > $$ language 'plr' IMMUTABLE;
> >
> >
> >
> > I tried to get it to work with the 1st parameter as bytea to be able to
> pass
> > binary geometry but got unserialize-errors in R. So for now you have to
> > convert your input geometry to EWKT first and pass it as text.
> >
> >
> >
> > Have fun! Michi
> >
> >
> >
> > [1] http://www.bostongis.com/?content_name=postgresql_plr_tut01#87
> >
> > [2] http://www.bostongis.com/?content_name=postgresql_plr_tut02#98
> >
> > [3]
> http://stat.ethz.ch/R-manual/R-devel/library/graphics/html/xspline.html
> >
> > [4]
> >
> http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.44.5770&rep=rep1&type=pdf
> >
> >
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