[gdal-dev] Potential issue with gdaladdo overviews setting alpha channel to nodata value instead of zero

[Tim Waters]

Even,

Many thanks! This did fix the issue.
Agreed, having both nodata and transparency would confuse things, and I see
that this way also ensures that it works well with cropped maps with pure
black pixels.

Best regards,

Tim



On 5 October 2017 at 22:21, Even Rouault <even.rouault at spatialys.com> wrote:

> On jeudi 5 octobre 2017 18:46:32 CEST Tim Waters wrote:
>
> > Hello,
>
> >
>
> > I'm seeing an issue which is present in 2.2.2 (tags/2.2.2 from github)
> and
>
> > seen in 1.11.3 (Ubuntu 16.04) but not present in 1.10.1 (Ubuntu 14.04).
>
> >
>
> > If a raster with an alpha channel has nodata set, and overviews via
>
> > gdaladdo are created, the overviews use the nodata value for the value of
>
> > the alpha channel. However, using the nearest resampling method correctly
>
> > sets the alpha to zero. All other resampling methods except nearest seem
> to
>
> > give the same wrong result.
>
> >
>
> > I've created a repository with all the files and readme containing the
>
> > workflow.
>
> > https://github.com/timwaters/gdal_gdaladdo_issue with screenshots of the
>
> > expected and observed files opened in Gimp to view the pixel values.
>
> >
>
> > The use case is mapwarper.net where users upload ungeoreferenced images,
>
> > they get converted to tifs and have overviews added, then they can mask
>
> > them (gdal_rasterize) and then warp the masked files, with overviews
> added
>
> > finally. I'm updating the server to run on Ubuntu 16.04 and will have to
>
> > compile gdal.
>
> >
>
> > Flow is:
>
> >
>
> > #Apply overviews to ungeoreferenced raster
>
> > gdaladdo -r average treasure_island.tif 2 4 8 16
>
> >
>
> > #Rasterise ungeoreferenced raster with the cropping mask
>
> > gdal_rasterize -i -b 1 -b 2 -b 3 -burn 17 -burn 17 -burn 17 20.gml -l
>
> > features treasure_island.tif
>
> >
>
> > #Georeference & warp the raster
>
> > gdal_translate -a_srs '+init=epsg:4326' -of VRT treasure_island.tif
>
> > temp.vrt -gcp 100.38, 83.096, -122.377, 37.830 -gcp 430.1966, 370.08,
>
> > -122.363, 37.8207 -gcp 313.69, 61.68, -122.368, 37.831
>
>
>
>
>
> > gdalwarp -rn -dstalpha -srcnodata '17 17 17' -s_srs 'EPSG:4326' temp.vrt
>
> > treasure_island_warped.tif -co TILED=YES -co COMPRESS=LZW
>
>
>
> Tim,
>
>
>
> The issue come from the above step. Basically this will add an alpha
> channel and also set implicitly a dest nodata value to 17. In GDAL extended
> TIFF, the nodata value applies to all channels. But when you have an alpha
> channel, having nodata values in addition doesn't really make sense and
> just confuse everything afterwards.
>
> If you add -dstnodata none, this will fix the issue.
>
> I think we should probably default to this behaviour when -dstalpha is
> specified.
>
>
>
> Even
>
>
>
> --
>
> Spatialys - Geospatial professional services
>
> http://www.spatialys.com
>
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