Rotation in Geoserver SLD based on complex Math Function

Question

I try to visualize windspeed and -direction using geoserver's build-in windbarbs. Therefor I have a rasterlayer with two bands "u" and "v", representing the lat and lon vector forces of the wind.

I use the "ras:RasterAsPointCollection"-function to transform the raster to use the PointSymbolizer with which i'm able to plot the windbarbs with different windspeeds. However I wasn't successful trying to use the atan2 function within the Rotation.

The following code resulted in an error message

Filter Function problem for function atan2 argument #1 - expected type double

          <Rotation>
            <ogc:Function name = "atan2">
              <ogc:Function name ="parameter">
                <ogc:Literal>x</ogc:Literal>
                <ogc:PropertyName>U</ogc:PropertyName>
              </ogc:Function>
              <ogc:Function name ="parameter">
                <ogc:Literal>y</ogc:Literal>
                <ogc:PropertyName>V</ogc:PropertyName>
              </ogc:Function>
            </ogc:Function>
          </Rotation>

I checked the datatype of the layer (layer > click on layername > bottom of page, details of bands) which says

Real 64 bits

I've tried both "u" and "U" in PropertyName but I guess there's still something off with the syntax

Here is the full SLD

<?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>uv</Name>
    <UserStyle>
      <FeatureTypeStyle>
        <Title>spd_dir</Title>
        <Transformation>
          <ogc:Function name="ras:RasterAsPointCollection">
            <ogc:Function name="parameter">
              <ogc:Literal>data</ogc:Literal>
            </ogc:Function>
          </ogc:Function>
        </Transformation>
        <Rule>
          <PointSymbolizer>
            <Graphic>
              <Mark>
                <WellKnownName>
                windbarbs://default(${sqrt((u*u) + (v*v))})[m/s]
                </WellKnownName>
                <Fill>
                  <CssParameter name="fill">
                    <ogc:Literal>#ffffff</ogc:Literal>
                  </CssParameter>
                </Fill>
                <Stroke>
                  <CssParameter name="stroke">#000000</CssParameter>
                  <CssParameter name="stroke-width">0.1</CssParameter>
                </Stroke>
              </Mark>
              <Rotation>
                <ogc:Function name = "atan2">
                  <ogc:Function name ="parameter">
                    <ogc:Literal>x</ogc:Literal>
                    <ogc:PropertyName>U</ogc:PropertyName>
                  </ogc:Function>
                  <ogc:Function name ="parameter">
                    <ogc:Literal>y</ogc:Literal>
                    <ogc:PropertyName>V</ogc:PropertyName>
                  </ogc:Function>
                </ogc:Function>
              </Rotation>
            </Graphic>
          </PointSymbolizer>
        </Rule>
      </FeatureTypeStyle>
    </UserStyle>
  </NamedLayer>
</StyledLayerDescriptor>

Answer 1

There is documentation here: docs. Try removing the parameter function and the literal:

      <Rotation>
        <ogc:Function name="toDegrees">
          <ogc:Function name = "atan2">
             <ogc:PropertyName>U</ogc:PropertyName>
             <ogc:PropertyName>V</ogc:PropertyName>
          </ogc:Function>
        </ogc:Function>
      </Rotation>

I have not tested. This is similar format to this answer here: Rotating sld with expression?