Setting transparency in an SLD file for a 4 channel GEOTIFF

Question

I'm struggling to set the a background colour to transparent in an SLD file for a 4-channel GEOTIFF using GeoServer 2.1.4.

I think I've set my background colour to transparent alpha layer in a graphics package correctly.

I've set up an SLD file to read Red/Green/Blue channels of my GEOTIFF = this resolves colours correctly for the RGB image = but still shows the background black colour (see below).

When I do a GetFeatureInfo request on the WMS, I can see an ALPHA_BAND=0 where I want the transparency and ALPHA_BAND =255 where I do not want transparency (see attached figure).

I was hopeing to do something like setting the ALPHA_BAND=0 to Opacity=0 in the SLD or something. However, there's no option in SLD schema that I can see ?

I've read the SLD cookbook and the SLD reference documentation. Alpha transparency is referred to but only with ColorMap' property. Can I mix and match 'ColorMap' and ChannelSelection' properties in the SLD ?

Am I going about this the right way ? Thanks in advance for assistance!

This is the SLD I'm using:

    <?xml version="1.0" encoding="ISO-8859-1"?>
    <StyledLayerDescriptor version="1.0.0" xmlns="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc"
      xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.opengis.net/sld http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd">
      <NamedLayer>
            <Name>bathy example</Name>
            <UserStyle>
              <Name>RGB image 3 channels</Name>
              <Title>RGB image</Title>
              <FeatureTypeStyle>
                    <Rule>
                      <RasterSymbolizer>
                            <ChannelSelection>
                              <RedChannel>
                              <SourceChannelName>1</SourceChannelName>
                              </RedChannel>
                              <GreenChannel>
                              <SourceChannelName>2</SourceChannelName>
                              </GreenChannel>
                              <BlueChannel>
                              <SourceChannelName>3</SourceChannelName>
                              </BlueChannel>            
                            </ChannelSelection>
                      </RasterSymbolizer>
                    </Rule>
              </FeatureTypeStyle>
            </UserStyle>
      </NamedLayer>
    </StyledLayerDescriptor>

Answer 1

I faced quite a similar problem and found here and here the following statements of people working for GeoSolutions:

"The colormap element can be used only on a single band to generate an RGB(A) raster where quantities can be mapped to colors."

"You cannot apply the ColorMap element when having 3 bands, you need only 1."

I found two ways to be able to add transparency using GDAL tools:

1. using gdalwarp as described in the link above: This adds a 4th band to the file containing a transparency mask.

   • Set nodata masking values:

     $ gdalwarp -of GTiff -srcnodata 0 -dstalpha inputfile.tif outputfile.tif
     

   • After that, setting the default raster style for the Geoserver layer is enough to have a transparent background.

   It is also mentioned that there are "extensions to SLD to support multiband color maps" or one could write custom WPS processes but I did not go further into that.

2. using gdal_translate as mentioned by @ian in the comments above: This sets 'no data' values to every band (I don't really understand whats going on there).

   • Assign a specified nodata value to output bands:

     $ gdal_translate -a_nodata 0 inputfile.tif outputfile.tif
     

   • After that, you can set the InputTransparentColor in the Layer properties in your Geoserver Web Application to the desired value (here: 000000).

   My OpenLayers preview could not deal with such a layer when the underlying GeoTIFF was of 16bit color depth (check with gdalinfo). One can reduce it to a manageable 8bits with $ gdal_translate -ot Byte inputfile.tif outputfile.tif.