Introduction#
The requirement is to have data for ocean features with 12 channels, where each channel corresponds to data for a specific month. After publishing the layer, it should be possible to select the corresponding channel using a style and display the data for a specific month. The straightforward approach would be to create 12 copies of the style. However, for easier maintenance (which will most likely be done by myself), I wanted to find a way similar to "dynamic styles" where parameters can be obtained externally and the same style can be used to select different channels based on different parameters.
Here, I fell into a pitfall of my own assumption: the production environment is using a relatively old version of GeoServer, 2.11.x. When I was reading the documentation, I was referring to the latest version, and after testing it didn't work, I looked at the documentation for version 2.11. Although the documentation had a similar usage, it was not available for channel selection.
Therefore, this approach is only applicable to newer versions.
Layer Publishing#
The layer publishing can be done for multi-channel images.
Setting the Style#
The general channel selection for band fusion is done in the following format1:
<ChannelSelection>
<RedChannel>
<SourceChannelName>1</SourceChannelName>
</RedChannel>
<GreenChannel>
<SourceChannelName>2</SourceChannelName>
</GreenChannel>
<BlueChannel>
<SourceChannelName>3</SourceChannelName>
</BlueChannel>
</ChannelSelection>
In the style, 1, 2, and 3 channels correspond to (R, G, B).
For selecting a single channel to display, use the Function to get the "environment variable" and replace the default value:
<RasterSymbolizer>
<Opacity>1.0</Opacity>
<ChannelSelection>
<GrayChannel>
<SourceChannelName>
<Function name="env">
<ogc:Literal>m</ogc:Literal>
<ogc:Literal>1</ogc:Literal>
</ogc:Function>
</SourceChannelName>
</GrayChannel>
</ChannelSelection>
</RasterSymbolizer>
In this case, the channel name is wrapped in a Function object, which provides a default value of 1 when the value of m in env is empty. If m is not empty, its value is used as the channel name.
To select channel 2 for display, add &env=m:2 to the WMS request:
http://localhost:8083/geoserver/wms?service=WMS&version=1.1.0&request=GetMap&layers=geosolutions:usa&styles=&bbox=-130.85168,20.7052,-62.0054,54.1141&width=768&height=372&srs=EPSG:4326&format=application/openlayers&env=m:2
Here is a complete style:
<?xml version="1.0" encoding="UTF-8"?>
<StyledLayerDescriptor 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" xsi:schemaLocation="http://www.opengis.net/sld
http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd" version="1.0.0">
<NamedLayer>
<Name>saltsld</Name>
<UserStyle>
<Title>A raster style</Title>
<FeatureTypeStyle>
<Rule>
<RasterSymbolizer>
<Opacity>1.0</Opacity>
<ChannelSelection>
<GrayChannel>
<SourceChannelName><ogc:Function name="env">
<ogc:Literal>m</ogc:Literal>
<ogc:Literal>1</ogc:Literal>
</ogc:Function></SourceChannelName>
</GrayChannel>
</ChannelSelection>
<ColorMap>
<ColorMapEntry color="#0000ff" quantity="28.0"/>
<ColorMapEntry color="#009933" quantity="30.0"/>
<ColorMapEntry color="#ff9900" quantity="32.0" />
<ColorMapEntry color="#ff0000" quantity="34.0"/>
</ColorMap>
</RasterSymbolizer>
</Rule>
</FeatureTypeStyle>
</UserStyle>
</NamedLayer>
</StyledLayerDescriptor>
Conclusion#
In the end, because it was not easy to update the production environment version, I ended up copying 12*2 styles myself.