[gdal-dev] Warping quality with small geolocation arrays

[Frank Warmerdam]

On 12-11-26 12:11 PM, Knut-Frode Dagestad wrote:
> Dear list,
>
> Warping datasets with geolocation arrays works fine if the arrays have the same
> size as the other raster bands. However, if the geolocation arrays are much
> smaller, the quality is very bad.
>
> Say I have a 2000x2000 px dataset with 100x100 px sized geolocation arrays.
> Then a warped image have only 200x200 blocks of identical values, even if the
> size of the warped image is much larger. If I change the interpolation
> algorithm (neareast, cubic...) the values change slightly, but the real output
> resolution remains low. This is the same whether using Python API or gdalwarp.
> As a workaround I convert the geolocation arrays to GCPs. This gives decent
> warping quality, but is a bit awkward.
>
> Is this the expected behavior of the present implementation of geolocation
> arrays in GDAL?

Knut-Frode,

The intention is that the geolocation information will be linearly
interpolated from the geolocation array.  However, I gather you are not
seeing that effect.

Looking at the "backmap" it seems that this linear interpolation is not
being done, even though it is (after a fashion) in the forward case
transformation case.  I'd appreciate it if you could submit a bug on
this issue, and I'll look into correcting it.  It is a pretty serious
error.

Best regards,
-- 
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