[gdal-dev] gdaladdo, overviews and NODATA

[Frank Warmerdam]

Armin,

Reviewing the overview building code in gdal/gcore/overview.cpp
it clearly tries to exclude nodata values from the averaging calculation.
What does the gdalinfo report look like for the file before you run
gdaladdo?

Best regards,

On Tue, Feb 21, 2012 at 12:36 PM, Armin Burger <armin.burger at gmx.net> wrote:
> Hi all
>
> I have a question regarding the gdaladdo tool and NODATA pixels:
>
> When using resampling like "average" it seems that along the border of DATA
> and NODATA pixels there happens an averaging also of some of the original
> NODATA pixels (looks like 1-2 pixel width). This way in the overviews they
> get values higher than 0 (0 is defined as NODATA). Using this type of data
> in mapserver leads to small black borders when the overviews are taken since
> the tag
>  OFFSITE 0 0 0
> does not work anymore for the small border of now only "nearly-black"
> pixels.
>
> I tried to set the metadata NODATA of the images to 0 for each band but it
> had no effect. Only when I use resampling "nearest" then every NODATA pixel
> of the overviews retains the full black 0 0 0.
>
> Is there a way to force to exclude NODATA pixels during the "average"
> resampling so that they retain their NODATA values?
>
> Thanks
> Armin
>
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