[gdal-dev] gdal_retile not generating alpha band

[Tom Wardrop]

Hi all,

I retiled whole lot of imagery over the weekend using the following gdal_retile command:


gdal_retile.py -v -r bilinear -levels 8 -ps 2048 2048 -s_srs EPSG:28355 -co "TILED=YES" -co "ALPHA=YES" -targetDir ~/output ~/input/*.tif

The result was good until I realised all the areas that should be transparent were instead black. It seems the problem is the source imagery didn’t have an alpha channel; it didn’t need one as there was no transparent data to represent. Because the source imagery didn’t fill the entire extent, gdal_retile had to create loads of transparent and partially-transparent tiles, which of course wound up black.

I was under the impression that "-co ALPHA=YES” would instruct gdal_retile to generate an alpha channel for all the generated tiles. Is this a bug? If not, how do I otherwise instruct gdal_retile to generate an alpha channel when required?

I’m working around this by using gdal_translate to re-save the source imagery with an (empty) alpha channel, so gdal_retile now outputs transparent tiles.

Any feedback is appreciated.

Cheers,
Tom


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