Re: [Geoserver-users] polygon label repeated for each tile

[Martin D. <md...@op...>]

On Wed, Aug 15, 2012 at 9:31 AM, Andrea Aime
<and...@ge...>wrote:

> On Wed, Aug 15, 2012 at 4:40 PM, Rahkonen Jukka <Juk...@mm...
> > wrote:
>
>> I made a quick test with 22339 rather complicated polygons and
>> PointOnSurface function in Spatialite.  Result looks pretty good with all
>> the points well inside their polygons which was a nice surprise. I was
>> remembering from the past that point on surface is not usable for labeling
>> because when I last tried the same function with Oracle it simply placed
>> the points on surface into the first outer ring vertices. Perhaps Oracle
>> does it in a more clever way nowadays.  Somehow I feel that PostGIS behaves
>> a lot like Spatialite.
>>
>> I feel that point on surface could be a reasonable candidate for label
>> anchor point. Problem with Geoserver is that all backends would not behave
>> in a similar way. It could perhaps be possible to a sort of a simulation by
>> creating a point layer with PointOnSurface and  use that for labeling.
>> Perhaps it does not tell anything about how well partial labels from
>> adjacent tiles match.
>>
>
> If the spatialite algorithm works fine it's worth checking out how that is
> done. Is it using OGR maybe?
> There is a similar function in JTS, but now sure how well that matches a
> good label point:
>
> http://tsusiatsoftware.net/jts/javadoc/com/vividsolutions/jts/geom/Geometry.html#getInteriorPoint()
>

Looks to me like spatialite is using the GEOS PointOnSurface method, which
is the JTS interior point algorithm.

The JTS algorithm is intended just to find a point which is guaranteed to
be in the interior of a polygon, so it isn't trying very hard to find a
good point for labelling.  However, it does try and pick a point close to
the middle of the geometry if it can, so for many polygons this will be
reasonable.  (It would be a nice feature to provide a filter function which
explicitly attempts to find a good labelling point)

>
> We could roll a WPS process to do that, in case people want to use it as a
> starting point and then
> eventually manually edit the data.
> However the WPS process (chained with gs:Import to feed the data back in
> the db) would create
> a whole new layer, the user would have to drop the original one by hand.
>

Or since the JTS method is exposed as the filter function interiorPoint,
would it be possible to call this as a geometry transformation in the SLD?

-- 
>
Martin Davis
OpenGeo - http://opengeo.org
Expert service straight from the developers.