mapnik/tests/data/good_maps/agg_poly_gamma_map.xml

<?xml version="1.0" encoding="utf-8"?>
<Map srs="+proj=merc +lon_0=0 +lat_ts=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs" bgcolor="steelblue">
    <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 +lon_0=0 +lat_ts=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs">
        <StyleName>style</StyleName>
        <Datasource>
            <Parameter name="file">../../data/shp/world_merc</Parameter>
            <Parameter name="type">shape</Parameter>
        </Datasource>
    </Layer>
</Map>