On Sat, Jan 4, 2014 at 9:49 PM, Rahkonen Jukka <[email protected]>wrote:
> Hi,
>
> ST_Simplify tends to return every now and then invalid geometries from
> polygon layers. What happens when Geoserver meets for example a big
> simplified lake with self-intersection? I know that Mapserver is sensitive
> for self-intersections and the lake might be dropped or even the whole
> request can fail. On the other hand, Thomas Bonfort wrote:
>
> "MapServer already simplifies geometries at the pixel level
> before sending them to the renderers, and from my testing at the time
> was more efficient at doing so than using st_simplify(), with the
> added bonus that the cellsize (resolution, units per pixel) does not
> need to be factored in the actual SQL query, and that the
> simplification applies to all data backends.
> Doing the simplification inside postgis does however make sense if the
> data transfer from db to mapserver is a bottleneck, and/or potentially
> if reprojection is involved (as less points would need to be fed into
> proj)".
>
> Have we considered pixel level simplification, what ever it is?
>
GeoServer was the first open source server to have it (that I know of), and
in the past I also
tried what Thomas tried, and came to the same conclusions (but I was trying
to use JTS
Douglas Peuker simplification), it was not helping (it was actually making
things slower).
However, our renderer should not be affected by invalid geometries (believe
we stripped all
topological operations from it), and I cannot questions the numbers I've
got during the benchmarks,
the speedup on complex geometries is _very_ significant.
Given that when ST_Simplify was tried by Mapnik, it was actually tried by a
PostGIS developer,
maybe they improved ST_Simplify performance in the process? Not sure, I
also plan to
check the code of ST_Simplify and see if/how it differs from the JTS
implementation.
Cheers
Andrea
--
*== GeoSolutions will be closed for seasonal holidays from 23/12/2013 to
06/01/2014 ==*
Ing. Andrea Aime
@geowolf
Technical Lead
GeoSolutions S.A.S.
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