[gdal-dev] downsampling geotiff with a low-pass filter

[Even Rouault]

> If I understand this correctly, the effective radius signifies the
> number of spline coefficients contributing to the interpolated value, it
> does not imply a low-pass filter. If the spline (and I suspect it's a
> B-spline) is created by first prefiltering the data and then evaluating
> the spline at the desired target locations, the interpolation criterion
> is fulfilled and there is no smoothing.
> 
> Omitting the prefiltering and using the original data as spline
> coefficients does some smoothing, but from a cubic spline the effect is
> small and does not vary with the scale of the downsampling, so it would
> not work equally well for different scale changes.

There is no explicit code to do smoothing. For each target pixel, given the 
radius of the filter and the resampling factor, it determines the index of the 
first and last contributing source pixels and compute the associated weight 
using a filter specific function.

> 
> Maybe you can point me to the place in the source code where the
> relevant math happens? I had a look but it's very complex and I did not
> find my way in easily.

For gdal_translate/gdaladdo computation, the function is there:
https://github.com/OSGeo/gdal/blob/master/gdal/gcore/overview.cpp#L1859

There is functionnaly equivalent code for gdalwarp in
https://github.com/OSGeo/gdal/blob/master/gdal/alg/gdalwarpkernel.cpp#L3825 
for example (we don't use the same code, as the code can be more optimized in 
the gdal_translate/gdaladdo case as a source line gives a target line for 
gdal_translate, whereas for gdalwarp given that coordinate reprojection may 
occur, we cannot assume that, so must process each target pixel independently 
of its neighbours)

One of the function used to compute weights:
https://github.com/OSGeo/gdal/blob/master/gdal/alg/gdalwarpkernel.cpp#L3163

> 
>  > They are for sure widely used
> 
> This doesn't automatically make them correct.

Indeed.

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com