[gdal-dev] gdal_merge.py alpha mask

[Stefan Klug]

Hi,

gdal_merge.py doesn't handle tiff files with alpha masks so I've added 
this functionality.

My patch changes the behaviour of gdal_merge as follows:
- If a band is recognized as Alpha Band it is automatically treated as 
having a NODATA value of 0.
I'm not sure if this is the way to go. But it makes some sense, as the 
resulting alpha channel is a correct alpha merge, independent of the 
nodata value used for the other bands.
- If a band has mask flag GMF_ALPHA set, the mask band controls the 
parts that get written to the destination

Would be great if someone could review and possibly checkin the patch.

Regards
     Stefan



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