Hello,
I've been looking over the docs and the code, but I'm still a little confused
about how masking and/or no data are handled during resampling in GDAL. I'm
interested mostly in the `cubic` and `average` resamples, but I assume the
answers will apply to the other modes just as well.
In my understanding:
* `cubic` will happily include "no data" values in the computation, likely
outputting not very useful results
* `average` will ignore no data input pixels; but how does average pick the
weights? The `gdal_translate` docs say "Starting with GDAL 3.1, this is a
weighted average taking into account properly the weight of source pixels not
contributing fully to the target pixel". I assume that's mostly unrelated.
* the `-srcnodata` docs and `gdalwarpkernel.cpp` suggest that masked pixels
are not included in the computation (unlike those of no data?)
I know two ways of handling masked/no data pixels during resampling:
* ignore them and rescale the remaining weights of the input pixels (e.g. a
4x4 grid in the `cubic` case), outputting no data if there's no valid pixel
among the inputs -- this seems "ideal"
* output no data if any of the input (e.g. 4x4 grid) pixels are no data
The latter method can be simulated in a reasonably easy way by making a
validity mask, resampling it with the same method, then thresholding it to get
a new mask which only keeps the "pure" pixels. It might even be possible (but
maybe not too easy) to implement do this with a VRT. But its disadvantage is
that it tends to expand the invalid areas.
So my main question if there's a supported way to handle this in GDAL (using
band masks?), and what does it do (will I get the "ideal" result from above?).
Regards,
Laurentiu Nicola
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