Geoserver SLD with CQL

Question

I am trying various ways to have a programatic way of combining various bands of a tiff file into the red, green, blue channels.

I tried the CQL way, eg see below

This is my SLD

<?xml version="1.0" encoding="UTF-8"?>
<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>custom_raster</Name>
    <UserStyle>
      <FeatureTypeStyle>
        <Rule>
          <Name>rule1</Name>
          <Title>Opaque Raster</Title>
          <Abstract>A raster with 100% opacity</Abstract>
          <RasterSymbolizer>
            <Opacity>1.0</Opacity>
            <ChannelSelection>
              <RedChannel>
                <SourceChannelName>${env('red',0)}</SourceChannelName>
              </RedChannel>
              <GreenChannel>
                <SourceChannelName>${env('green',1)}</SourceChannelName>
              </GreenChannel>
              <BlueChannel>
                <SourceChannelName>${env('blue',2)}</SourceChannelName>
              </BlueChannel>
            </ChannelSelection>
          </RasterSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

I call it using ENV=red:1;blue:3;green:5 but I am getting the following message

Caused by: java.lang.IllegalArgumentException: Band number -1 is not valid.
...
Caused by: java.lang.NumberFormatException: For input string: "${env('red',0)}"
...

It doesn't seem to allow CQL expressions where I am placing them... yet doesn't complain when I validate. Any idea?

Answer 1

The SLD specification allows only static names in the SourceChannelName element. GeoServer/GeoTools have not yet extended them to support expressions, it would be possible with some resourcing, see here: https://github.com/geoserver/geoserver/wiki/Successfully-requesting-and-integrating-new-features-and-improvements-in-GeoServer

Btw, why are you getting no errors? Because the spec says the channel name is a string, and "${env('red',0)}" is a string. The special CQL syntax is only used in selected attributes, as an extension, because there is no way to allow a normal OGC expression.