SLD Point Stacker

<?xml version="1.0" encoding="ISO-8859-1"?>
<StyledLayerDescriptor version="1.0.0"
xsi:schemaLocation="http://www.opengis.net/sld StyledLayerDescriptor.xsd"
xmlns="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc"
xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <NamedLayer>
    <Name>g_stacker</Name>
    <UserStyle>
      <Title>Point Stacker</Title>
      <Abstract>A sample style that aggregates points</Abstract>
      <FeatureTypeStyle>
        <Transformation>
          <ogc:Function name="gs:PointStacker">
            <ogc:Function name="parameter">
              <ogc:Literal>data</ogc:Literal>
            </ogc:Function>
            <ogc:Function name="parameter">
              <ogc:Literal>cellSize</ogc:Literal>
              <ogc:Literal>50</ogc:Literal>
            </ogc:Function>
            <ogc:Function name="parameter">
              <ogc:Literal>outputBBOX</ogc:Literal>
              <ogc:Function name="env">
                <ogc:Literal>wms_bbox</ogc:Literal>
              </ogc:Function>
            </ogc:Function>
            <ogc:Function name="parameter">
              <ogc:Literal>outputWidth</ogc:Literal>
              <ogc:Function name="env">
                <ogc:Literal>wms_width</ogc:Literal>
              </ogc:Function>
            </ogc:Function>
            <ogc:Function name="parameter">
              <ogc:Literal>outputHeight</ogc:Literal>
              <ogc:Function name="env">
                <ogc:Literal>wms_height</ogc:Literal>
              </ogc:Function>
            </ogc:Function>
          </ogc:Function>
        </Transformation>
        <Rule>
          <PointSymbolizer>
            <Graphic>
              <Mark>
                <WellKnownName>circle</WellKnownName>
                <Fill>
                  <CssParameter name="fill">#0000FF</CssParameter>
                  <CssParameter name="fill-opacity">0.5</CssParameter>
                </Fill>
              </Mark>
              <Size><ogc:Mul><ogc:PropertyName>countunique</ogc:PropertyName><ogc:Literal>3</ogc:Literal></ogc:Mul></Size>
            </Graphic>
          </PointSymbolizer>
          <TextSymbolizer>
            <Label>
              <ogc:PropertyName>countunique</ogc:PropertyName>
            </Label>
            <Font>
              <CssParameter name="font-family">Arial</CssParameter>
              <CssParameter name="font-size">12.0</CssParameter>
              <CssParameter name="font-style">normal</CssParameter>
              <CssParameter name="font-weight">normal</CssParameter>
            </Font>
            <LabelPlacement>
              <PointPlacement>
                <AnchorPoint>
                  <AnchorPointX>
                    <ogc:Literal>0.5</ogc:Literal>
                  </AnchorPointX>
                  <AnchorPointY>
                    <ogc:Literal>0.5</ogc:Literal>
                  </AnchorPointY>
                </AnchorPoint>
                <Rotation>
                  <ogc:Literal>0.0</ogc:Literal>
                </Rotation>
              </PointPlacement>
            </LabelPlacement>
            <Halo>
              <Radius>
                <ogc:Literal>2</ogc:Literal>
              </Radius>
              <Fill>
                <CssParameter name="fill">#FFFFFF</CssParameter>
              </Fill>
            </Halo>
            <Fill>
              <CssParameter name="fill">#000000</CssParameter>
            </Fill>
          </TextSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

GeoServer Integration: rendering transformations - GeoServer Training

Geometry transformations have been added almost one year ago to GeoTools to allow the application of a filter function to the geometry before rendering it. This allows for quite a bit of flexibility, yet still fails to address some needs: transformations that do need to take into account more than one feature at a time transformations that perform raster to vector transformations, like contouring.

https://docs.geoserver.geo-solutions.it/edu/en/wps/rendering_tx.html