mapnik/tests/data/good_maps/raster-alpha-gradient.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="blue">

<Style name="transp">
  <Rule>
    <RasterSymbolizer opacity="1" scaling="bilinear" />
  </Rule>
</Style>
<Layer name="transp"
  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>transp</StyleName>
    <Datasource>
       <Parameter name="file">../raster/transp.tiff</Parameter>
       <Parameter name="type">gdal</Parameter>
    </Datasource>
  </Layer>

</Map>