How to rotate mark in Geoserver style?

Question

I want to make style like:

This is my style for start point:

<PointSymbolizer>
    <Geometry>
        <ogc:Function name="startPoint">
            <ogc:PropertyName>geom</ogc:PropertyName>
        </ogc:Function>
    </Geometry>
    <Graphic>
        <Mark>
            <WellKnownName>shape://oarrow</WellKnownName>
            <Stroke>
                <CssParameter name="stroke">
                    <ogc:Literal>#FF0000</ogc:Literal>
                </CssParameter>
                <CssParameter name="stroke-linecap">
                    <ogc:Literal>butt</ogc:Literal>
                </CssParameter>
                <CssParameter name="stroke-linejoin">
                    <ogc:Literal>miter</ogc:Literal>
                </CssParameter>
                <CssParameter name="stroke-opacity">
                    <ogc:Literal>1.0</ogc:Literal>
                </CssParameter>
                <CssParameter name="stroke-width">
                    <ogc:Literal>2</ogc:Literal>
                </CssParameter>
            </Stroke>
        </Mark>
        <Size>30</Size>
        <Rotation>
            <ogc:Function name="startAngle">
                <ogc:PropertyName>geom</ogc:PropertyName>
            </ogc:Function>
        </Rotation>
    </Graphic>
</PointSymbolizer>

And result:

How can i flip start mark?

Answer 1

The easiest way is to subtract 180 degrees, or probably to use endAngle for that end of the line.

          <Rotation> 
            <ogc:Add> 
              <ogc:Function name="startAngle"> 
                <ogc:PropertyName>the_geom</ogc:PropertyName> 
              </ogc:Function> 
              <ogc:Literal>180.0</ogc:Literal> 
            </ogc:Add> 
          </Rotation>