On Thu, Jun 16, 2011 at 4:08 AM, Mark Hammond <[email protected]> wrote:
> I’ve published two GeoTIFFs to my Geoserver install (both on ESPG:27700) and
> added them as non-base layers on a Google Map base in OpenLayers.
>
> Where Geoserver is re-projecting the tiffs, it’s leaving a honking great
> black border. I’ve added transparency requests on the layer, but they appear
> to be ignored.
>
> Could anyone suggest what I’ve done wrong?

I've run into an issue like this myself. Basically, for this to work
properly, your original GeoTIFF needs to have an alpha channel. The
reprojection occurs before the GeoTIFF is actually painted onto the
layer, and it occurs in the native pixel format of the GeoTIFF. So, if
the original has no alpha channel, there will be none after
reprojection either, and any dead space created by warping the image
gets filled with black pixels.

If you add an alpha channel to the GeoTIFF then during reprojection
the dead space will get filled with transparent pixels, and then when
it's painted onto the layer it will behave like you want it to.

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