Variable substitution in SLD for geoserver 2.13 RasterSymbolizer ChannelSelection

Question

I'm trying to publish a layer with a style that allows the extended WMS request to specify what image bands to map to the RGB values. Based on existing examples I assumed that my SLD should have something like the following RasterSymbolizer element:

<RasterSymbolizer>            
        <ChannelSelection>
          <RedChannel>                  
            <SourceChannelName>
              <ogc:Function name="env">
               <ogc:Literal>redchan</ogc:Literal>
               <ogc:Literal>1</ogc:Literal>
              </ogc:Function>
            </SourceChannelName>
          </RedChannel>...

But the SLD validation fails with the following error:

Element 'SourceChannelName' is a simple type, so it must have no element information item [children]

I am following this documentation:

http://docs.geoserver.org/stable/en/user/styling/sld/reference/rastersymbolizer.html http://docs.geoserver.org/stable/en/user/styling/sld/extensions/substitution.html

Is there a way to do this or is it not supported for the ChannelSelection element? Are there any more examples and/or documentation for SLD variable substitution that I can look at?

Answer 1

The OGC specification for SLD is quite restrictive about where functions (or expressions) can be used. GeoServer relaxes those restrictions in a lot of places but not here.

You would need to extend the GeoTools SLD implementation to expect an expression there, rather than a simple type.