Extending GeoServer Styling with Geometry Transformations

While designing a map style, sometimes it is necessary to manipulate the geometries representation on-the-fly. For instance a straight line should be drawn as a sinuous line or plain building footprints should be decorated with a 3D effect. ESRI developers might be familiar with the various rendering extension interfaces in ArcObjects API.

With GeoTools/Geoserver there are similar extension points as well – although somewhat hidden. So here is a little sample for a custom geometry transformation function one can add to the GeoTools rendering module.

First create a java class that extends from org.geotools.filter.FunctionExpressionImpl and implement your geometry manipulation function inside the #evaluate method. If you need some examples, look at the existing classes that extend from FunctionExpressionImpl.

Second, to register that class to the GeoTools service provider interface: add a file named “org.opengis.filter.expression.Function” to the “META-INF/services” directory of your JAR file and add a line with the full class name of the java class (that one extending from FunctionExpressionImpl) to this file.

Third, create a JAR with your class and the META-INF and add it to the Geoserver’s classpath.

Fourth, create a SLD style where this new geometry function is used:

  <FeatureTypeStyle>
    <Rule>
      <PolygonSymbolizer>
        <Geometry>
           <ogc:Function name="magic">
              <ogc:PropertyName>the_geom</ogc:PropertyName>
              <ogc:PropertyName>bld_height</ogc:PropertyName>
           </ogc:Function>
        </Geometry>
        <Stroke>
          <CssParameter name="stroke">#888888</CssParameter>
          <CssParameter name="stroke-width">0.6</CssParameter>
        </Stroke>
      </PolygonSymbolizer>
    </Rule>
  </FeatureTypeStyle>

Voilá… oh I forgot to say, that of course you can pass other attributes from the feature to your function class – like with my example the attribute “building height” to control the height of the 3D wire frame.

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