Hi,

I'm using gdal2tiles but I'm having problems with the alpha on some of the 
tiles output. The base PNG has some transparent areas but the final tiles 
output have those areas as opaque black.

Here's the base image:
  http://dev.andrewnicolaou.co.uk/m/#base
  
These are the steps I'm doing:

1. Create a VRT from base image:

 gdal_translate -of VRT -a_srs EPSG:4326 -gcp 0 0 -10 65 -gcp 512 0 0 65 -gcp 
512 768 0 50 base.png base.vrt

2. Translating the base image into EPSG:900913:

  gdalwarp -of VRT -t_srs EPSG:900913 base.vrt base_mercator.vrt

3. Generate the tiles:
  
  gdal2tiles.py --zoom=0-9 --no-kml --webviewer=openlayers base_mercator.vrt

The problem is that the transparent area in the base image appears as solid 
black on the tiles:
  http://dev.andrewnicolaou.co.uk/m/#tile

If I don't generate the tiles, but just a single translated image, then the 
transparency is maintained:
  http://dev.andrewnicolaou.co.uk/m/#mercator

Can anyone see what I'm doing wrong? I'd expect that the base image 
transparency would carry over to the output tiles.

Many thanks,
Andrew

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