SLD Style rendering transform value on GeoServer

Question

I am using GeoServer 2.19, and I have a wind coverage view from a grib file (uwnd,vwnd components), and I want it to become a wind speed raster layer.

When I preview it with a raster style, I realised that the color of raster just draw with only uwnd component, not the value of wind speed value(sqrt(uwnd^2 + vwnd^2)).

I saw rendering-transform can do what I want(something like value transform?!), but the example on it is not my needs. (I just need a raster layer of wind speed value, neither wind barbs nor wind arrows)

so I tried to edit it, then it doesn't worked.

My SLD style:

<StyledLayerDescriptor
    version="1.0.0"
    xmlns="http://www.opengis.net/sld"
    xmlns:gml="http://www.opengis.net/gml"
    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 ./StyledLayerDescriptor.xsd">
    <NamedLayer>
        <Name>wind</Name>
        <UserStyle>
            <Title>wind</Title>
            <FeatureTypeStyle>
                <Transformation>
                    <ogc:Function name="Categorize">
                        <!-- Value to transform -->
                        <ogc:Function name="sqrt">
                            <ogc:Add>
                                <ogc:Mul>
                                    <ogc:PropertyName>uwnd</ogc:PropertyName>
                                    <ogc:PropertyName>uwnd</ogc:PropertyName>
                                </ogc:Mul>
                                <ogc:Mul>
                                    <ogc:PropertyName>vwnd</ogc:PropertyName>
                                    <ogc:PropertyName>vwnd</ogc:PropertyName>
                                </ogc:Mul>
                            </ogc:Add>
                        </ogc:Function>
                        <ogc:Literal>8</ogc:Literal>
                        <ogc:Literal>1.543333332</ogc:Literal>
                        <ogc:Literal>32</ogc:Literal>
                    </ogc:Function>
                </Transformation>
                <Rule>
                    <RasterSymbolizer>
                        <Size>
                            <ogc:Function name="Categorize">
                                <!-- Value to transform -->
                                <ogc:Function name="sqrt">
                                    <ogc:Add>
                                        <ogc:Mul>
                                            <ogc:PropertyName>uwnd</ogc:PropertyName>
                                            <ogc:PropertyName>uwnd</ogc:PropertyName>
                                        </ogc:Mul>
                                        <ogc:Mul>
                                            <ogc:PropertyName>vwnd</ogc:PropertyName>
                                            <ogc:PropertyName>vwnd</ogc:PropertyName>
                                        </ogc:Mul>
                                    </ogc:Add>
                                </ogc:Function>
                                <ogc:Literal>8</ogc:Literal>
                                <ogc:Literal>1.543333332</ogc:Literal>
                                <ogc:Literal>32</ogc:Literal>
                            </ogc:Function>
                        </Size>
                        <Graphic>
                            <ColorMap type="ramp">
                                <ColorMapEntry color="#f9f9f9" quantity="0" label="0m/s"/>
                                <ColorMapEntry color="#d860aa" quantity="2" label="2m/s"/>
                                <ColorMapEntry color="#9757cc" quantity="4" label="4m/s"/>
                                <ColorMapEntry color="#5774cc" quantity="6" label="6m/s"/>
                                <ColorMapEntry color="#57cccc" quantity="8" label="8m/s"/>
                                <ColorMapEntry color="#aacc57" quantity="10" label="10m/s"/>
                                <ColorMapEntry color="#ccc457" quantity="12" label="12m/s"/>
                                <ColorMapEntry color="#cc8b57" quantity="14" label="14m/s"/>
                                <ColorMapEntry color="#cc5757" quantity="16" label="16m/s"/>
                                <ColorMapEntry color="#775046" quantity="18" label="18m/s"/>
                                <ColorMapEntry color="#000000" quantity="20" label="20m/s"/>
                            </ColorMap>
                        </Graphic>
                    </RasterSymbolizer>
                </Rule>
            </FeatureTypeStyle>
        </UserStyle>
    </NamedLayer>
</StyledLayerDescriptor>

Error message:

code="internalError"Rendering process failed. Layers:wind-2020-11-23
Don't know how to handle the results of the transformation, the supported result types are FeatureCollection, GridCoverage2D and GridCoverage2DReader, but we got: class java.lang.String

Is anyone have experience of it?

Answer 1

I found the solution to the above problem using ras:Jiffle script as follows:

<Transformation>
  <ogc:Function name="ras:Jiffle">
    <ogc:Function name="parameter">
      <ogc:Literal>coverage</ogc:Literal>
    </ogc:Function>
    <ogc:Function name="parameter">
      <ogc:Literal>script</ogc:Literal>
      <ogc:Literal>
        u = src[0];
        v = src[1];
        dest = sqrt(u*u+v*v) * 0.01;               
      </ogc:Literal>
    </ogc:Function> 
  </ogc:Function>
</Transformation>
<sld:Rule>
  <sld:RasterSymbolizer>   
    <ColorMap>
      <ColorMapEntry color="#5E70B4" quantity="0" opacity="1.0"/>
      <ColorMapEntry color="#3B68A1" quantity="1" opacity="1.0"/>
      <ColorMapEntry color="#4078A5" quantity="2" opacity="1.0"/>              
    </ColorMap>
  </sld:RasterSymbolizer>