mapnik/tests/visual_tests/styles/map-background-image-compositing.xml

<Map srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs"
  background-color="green"
  background-image-opacity=".8"
  background-image-comp-op="screen"
  background-image="../data/pattern.png">
    <Parameters>
        <Parameter name="sizes">512,512</Parameter>
    </Parameters>

    <Style name="ellipse">
        <Rule>
            <MarkersSymbolizer
              width="240"
              height="240"
              fill="steelblue"
              fill-opacity=".2"
              stroke="yellow"
              stroke-width="16"
              stroke-opacity=".3"
            />
        </Rule>
    </Style>

    <Layer name="layer" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
        <StyleName>ellipse</StyleName>
        <Datasource>
            <Parameter name="type">csv</Parameter>
            <Parameter name="inline">
x,y
2.5,2.5
            </Parameter>
        </Datasource>
    </Layer>

    <!-- points to frame data view -->

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

    <Layer name="frame" srs="+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs">
        <StyleName>frame</StyleName>
        <Datasource>
            <Parameter name="type">csv</Parameter>
            <Parameter name="inline">
x,y
0,0
5,0
0,5
5,5
            </Parameter>
        </Datasource>
    </Layer>

</Map>