mapnik/tests/visual_tests/styles/tiff-alpha-gdal.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE Map[]>
<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="red">
  <Parameters>
    <Parameter name="sizes">600,400</Parameter>
  </Parameters>

<Style name="white">
  <Rule>
    <RasterSymbolizer opacity="1" scaling="bilinear" comp-op="src-over"/>
  </Rule>
</Style>
<Layer name="white"
  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>white</StyleName>
    <Datasource>
       <Parameter name="file">../../data/raster/white-alpha-assoc-alpha-correct.tiff</Parameter>
       <Parameter name="type">gdal</Parameter>
    </Datasource>
  </Layer>

</Map>