[gdal-dev] Question on gdalwarp, srcnodata and dstalpha

[Daniele Romagnoli]

Hi Even,
thanks for the feedback.

On Sun, Nov 6, 2016 at 5:33 PM, Even Rouault <even.rouault at spatialys.com>
wrote:

> Le jeudi 03 novembre 2016 18:19:36, Daniele Romagnoli a écrit :
> > Hi List,
> > I was checking a gdalwarp (GDAL 2.1.2) invokation with a colleague and I
> > have a question.
> >
> > I know that when warping a geotiff with internal NODATA_VALUES=0 0 0
> > metadata, only RGB pixels = 0 0 0 are handled as nodata whilst pixels
> like
> > "0 2 4", "50 0 0" will not right?
> >
> > In the past, I had issues using -scrnodata 0 instead of embedded
> > NODATA_VALUES=0 0 0, getting results with sparse nodata around due to
> > band_i = 0 being interpreted as a nodata pixels. This is why I usually
> > suggest to define the NODATA_VALUES metadata to make sure only the exact
> > triplet is handled as nodata.
> >
> > However, today we have processed a simple UTM RGB dataset (without
> > NODATA_VALUES metadata defined) using this command:
> > gdalwarp -t_srs EPSG:4326 -srcnodata 0 -dstnodata None -dstalpha
> >
> > And the result is properly rendered: the alpha mask is zero only where
> the
> > input pixels are "0 0 0" which is exactly what we needed.
> > Do something has been changed in the latest versions? Is the alpha band
> > mask being set to zero only when all input pixels are zero?
>
> Hi Daniele,
>
> This is a good question, and I would have thought the same as you. But when
> digging further, the nodata values must be indeed fully matched for all
> components to consider a pixel as transparent. And this has been the case
> for
> the last 11 years :-)) : https://trac.osgeo.org/gdal/changeset/8286
>
> My confusion came from the fact that the default behaviour of the warping
> engine used by gdalwarp is not that one (see UNIFIED_SRC_NODATA option in
> http://www.gdal.org/structGDALWarpOptions.html#
> a0ed77f9917bb96c7a9aabd73d4d06e08),
> but gdalwarp override it to have the UNIFIED_SRC_NODATA=YES behaviour.
>

Thanks for the investigation.

At this point, if this behavior is there since 11 years, I guess that the
cause of the issues I had in the past was probably involving a gdaladdo
after a gdalwarp in my processing. So that gdalwarp was internally
considering nodata a fully matching 0 triplets to produce the high res
output whilst gdaladdo was considering nodata on band basis, separately,
producing scattered no data.

Cheers,
Daniele


>
> I've also just added support in trunk to be able to specify
> UNIFIED_SRC_NODATA=NO explicitly if wished.
>
> See :
>
> Let create a dataset with a single pixel R,G,B=10,20,30 with
> gdal_translate byte.tif rgb.tif -b 1 -b 1 -b 1 -scale_1 0 255 10 10 \
>     -scale_2 0 255 20 20  -scale_3 0 255 30 30 -srcwin 0 0 1 1
>
> By default:
> gdalwarp rgb.tif out.tif -overwrite  -srcnodata 10 -dstnodata None
> -dstalpha
> lead to (R,G,B,A)=(10,20,30,255)
>
> And (in trunk):
> gdalwarp rgb.tif out.tif -overwrite  -srcnodata 10 -dstnodata None
> -dstalpha \
>                  -wo UNIFIED_SRC_NODATA=NO -wo INIT_DEST=127
> lead to (R,G,B,A)=(127,20,30,255)
>
> ==> Nodata is applied band per band, and alpha is considered to be 255 as
> soon
> as at least one band is valid.
>
> Even
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
>



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