[postgis-users] Bezier or Spline smoothing implementation

[James David Smith]

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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