Parameter substitution in SLD¶

Parameter substitution in SLD is a GeoServer specific functionality (starting from version 2.0.2) allowing to pass down parameter values from the WMS request into an SLD style, allowing to dynamically change colors, fonts, sizes and filter thresholds dynamically.

The parameters are specified on GetMap requests using the env parameter:

...&env=paramName:value;otherParam=otherValue&...

and can be accessed from the SLD using the env function. In the simplest form the env function will retrieve the value of a specific parameter:

<ogc:Function name="env">
   <ogc:Literal>size</ogc:Literal>
</ogc:Function>

Alternatively a default value can be provided, to be used if the parameter value was not provided along with the GetMap request:

<ogc:Function name="env">
   <ogc:Literal>size</ogc:Literal>
   <ogc:Literal>6</ogc:Literal>
</ogc:Function>

The function can be called in any place where an OGC expression is used, so for example CSSParameter, sizes and offsets, and filter themselves. The SLD parser accepts it even in some places where an expression is not formally accepted, as the mark well known names.

A working example¶

The following symbolizer has been parametrized in three places, every time with fall backs (full SLD is also available):

<PointSymbolizer>
  <Graphic>
    <Mark>
      <WellKnownName><ogc:Function name="env">
            <ogc:Literal>name</ogc:Literal>
            <ogc:Literal>square</ogc:Literal>
         </ogc:Function>
      </WellKnownName>
      <Fill>
        <CssParameter name="fill">
          #<ogc:Function name="env">
            <ogc:Literal>color</ogc:Literal>
            <ogc:Literal>FF0000</ogc:Literal>
         </ogc:Function>
        </CssParameter>
      </Fill>
    </Mark>
    <Size>
       <ogc:Function name="env">
          <ogc:Literal>size</ogc:Literal>
          <ogc:Literal>6</ogc:Literal>
       </ogc:Function>
    </Size>
  </Graphic>
</PointSymbolizer>