[gdal-dev] GPKG nodata isn't applied in all tiles

[Even Rouault]

On jeudi 28 septembre 2017 09:08:00 CEST Roarke Gaskill wrote:
> Hi,
> 
> It seems the nodata value is not being used as "fill" completely around
> tiled images.
> 
> To reproduce:
> 
> http://download.osgeo.org/geotiff/samples/spot/chicago/SP27GTIF.TIF
> 
> gdal_translate  -ot Float32 -of GPKG -co
> "TILING_SCHEME=GoogleMapsCompatible" SP27GTIF.TIF chi1.gpkg
> gdal_translate -a_nodata "-nan" -ot Float32 -of GPKG -co
> "TILING_SCHEME=GoogleMapsCompatible" SP27GTIF.TIF chi2.gpkg
> 
> gdal_translate -of GTIFF -oo "USE_TILE_EXTENT=YES" chi1.gpkg chi1.tif
> gdal_translate -of GTIFF -oo "USE_TILE_EXTENT=YES" chi2.gpkg chi2.tif
> 
> Notice that chi1.tif has 0 values all the way around the image.  As I would
> expect because a nodata value was not applied when creating the gpkg.
> Howeverm in chi2.tif it appears the nodata value of nan was applied on the
> left and top of the image but not on the right and bottom.  Though on the
> right it does appear to have applied some nodata values but not completely
> to the right edge of the image.

Roark,

there are 2 issues:

* NaN isn't a supported value for GeoPackage (just added a warning in that case). This due to 
the value being stored in the Real data_null column of the 
gpkg_2d_gridded_coverage_ancillary SQLite3 table, but SQLite3 doesn't support NaN values. 
They are stored like a SQL NULL value, so on reading GDAL (or other software) will not able 
to realize that there was a nodata value stored. That said, that should affect only the read 
side

* there was indeed an implementation issue with non-zero nodata value on edge tiles when 
using a TILING_SCHEME, which I've fixed in trunk.

So on writing this will work properly now, but when reading it back, the right bottom most 
tile which isn't serialized, but intersects the window of interest, will be read back with the 
default value of 0 instead of the expected NaN.

So I'd recommend you use a different value than NaN (like -1e10)

Also note that Float32 uses the implementation of the GeoPackage elevation extension at 
the time GDAL 2.2 was released. And this extension has finally be reworked, so 
interoperability with other software and potential future GDAL versions that might 
implement the final version of the extension is not guaranteed.

Even

-- 
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