mapnik/tests/data/good_maps/agg_poly_gamma_map.xml

<Map srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over" background-color="steelblue" minimum-version="0.7.2">
    <Style name="style">
        
        <!-- Asia - default dark color behind light polygon 
        leads to "ghost lines" show-through
        -->
        
        <Rule>
            <Filter>([REGION]=142)</Filter>
            <PolygonSymbolizer fill="yellow"/>
        </Rule>

        <!-- Europe, using Opacity avoids "ghost lines" 
        -->
        <Rule>
            <Filter>([REGION]=150)</Filter>
            <PolygonSymbolizer fill="rgb(176,151,238)" fill-opacity=".3"/>
        </Rule>
        
        <!-- Americas, using gamma of .65 to remove blue lines works as well..
        -->
        <Rule>
            <Filter>([REGION]=19)</Filter>
            <PolygonSymbolizer fill="rgb(136,172,2)" gamma=".6"/>
        </Rule>

        <!-- Africa, using gamma of .65 to remove blue lines works as well..
        -->
        <Rule>
            <Filter>([REGION]=2)</Filter>
            <PolygonSymbolizer fill="rgb(239,98,133)" gamma=".65"/>
        </Rule>

        <!-- Oceana, all disjunct polygons so it does not matter -->
        <Rule>
            <Filter>([REGION]=9)</Filter>
            <PolygonSymbolizer fill="rgb(212,9,14)"/>
        </Rule>
    </Style>
    <Layer name="world" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs +over">
        <StyleName>style</StyleName>
        <Datasource>
            <Parameter name="file">../../data/shp/world_merc</Parameter>
            <Parameter name="type">shape</Parameter>
        </Datasource>
    </Layer>
</Map>