[OSM-dev] tirex, output from mapnik-rendering backend different from mapnik for GDAL raster source with opacity

[M∡rtin Koppenhoefer]

In my stylesheet I use some raster images which I combine with OSM
data with the "screen" and "multiply" method and with opacity values
<1.

When rendered with tirex and the mapnik rendering backend they do not
show up while the exact same stylesheet-file does behave as expected
with the "normal mapnik" (tested with 0.7.1 and 0.7.2 and
generate_image / tile).

I suspect the reason that they are not rendered lies in the
opacity-parameter, because I have other rasters with multiply (and
same scaling) and default opacity of 1 that do render well.


this is the style:

<Style name="bathymetry_m">
        <Rule>
            <MinScaleDenominator>100000000</MinScaleDenominator>
            <MaxScaleDenominator>250000000000</MaxScaleDenominator>
            <RasterSymbolizer>
                <CssParameter name="mode">multiply</CssParameter>
                <CssParameter name="scaling">bilinear</CssParameter>
                <CssParameter name="opacity">0.3</CssParameter>
            </RasterSymbolizer>
        </Rule>
    </Style>

and the layer:

    <Layer name="bathymetry_m" srs="+proj=merc +a=6378137 +b=6378137
+lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0
+units=m +nadgrids=@null +no_defs +over">
        <StyleName>bathymetry_m</StyleName>
        <Datasource>
            <Parameter name="file">OB_LR/OB_LR_multiply_.tif</Parameter>
            <Parameter name="format">tiff</Parameter>
            <Parameter name="type">gdal</Parameter>
        </Datasource>
    </Layer>


cheers,
Martin