mapnik/tests/visual_tests/styles/polygon-symbolizer-expressions.xml

<Map
    background-color="#eee"
    srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
    <Parameters>
        <Parameter name="sizes">256,256</Parameter>
    </Parameters>

    <Style name="bug">
        <Rule>
            <Filter>[id] = 'fill'</Filter>
            <PolygonSymbolizer fill='[value]' />
        </Rule>
        <Rule>
            <Filter>[id] = 'fill-opacity'</Filter>
            <PolygonSymbolizer fill='#0000ff' fill-opacity='[value]' />
        </Rule>
        <Rule>
            <Filter>[id] = 'gamma'</Filter>
            <PolygonSymbolizer fill='#0000ff' gamma='[value]' />
        </Rule>

        <Rule>
            <TextSymbolizer face-name='DejaVu Sans Book' halo-fill='#ffffff' halo-radius='1'>[id]</TextSymbolizer>
        </Rule>
    </Style>

    <Style name="frame">
        <Rule>
            <PolygonSymbolizer />
        </Rule>
    </Style>

    <!-- 
      frame is a layer with a single polygon that is used to ensure that m.zoom_all() will zoom the
      map to a reasonable bounding extent to make the test easy to view with tools like nik2img.py
      (which calls m.zoom_all() by default). Another approach in >= Mapnik 2.3.x is to just sent a
      manual `extent` for the CSV datasource.
    -->

    <Layer name="frame" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
        <StyleName>frame</StyleName>
        <Datasource>
            <Parameter name="type">csv</Parameter>
            <Parameter name="inline">
wkt|name
Polygon((-180.0 -125.0, -180.0 125.0, 180.0 125.0, 180.0 -125.0, -180.0 -125.0))|bounds
            </Parameter>
        </Datasource>
    </Layer>

    <!-- 
      "bug" is a layer that should provide sample data that triggers the bug in question. It is
      listed after the "frame" layer so that it renders on top and is visible.
    -->

    <Layer name="bug" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
        <StyleName>bug</StyleName>
        <Datasource>
            <Parameter name="type">csv</Parameter>
            <Parameter name="inline">
id|value|wkt
fill|#0000ff|Polygon((-160 -120,-20 -120,-40 -80,-100 -90,-160 -120))
fill-opacity|0.5|Polygon((-160 -70,-20 -70,-40 -30,-100 -40,-160 -70))
gamma|1.0|Polygon((-160 -20,-20 -20,-40 20,-100 10,-160 -20))
            </Parameter>
        </Datasource>
    </Layer>

</Map>