[gdal-dev] Overview resampling generating non-existant values

[Justin Terry]

Hi,

I'm experiencing a strange issue I've never seen before where certain resampling methods that should only include values in the original dataset, like mode and nearest, are adding new values in the overviews.

For example, the original raster contains the following values:

[0,  1,  2,  4,  5,  8,  9, 10, 16, 17, 18, 20, 21, 32, 33, 34, 36, 37, 40, 41, 42, 64, 65, 66, 68, 69, 72, 73, 74, 80, 81, 82, 84, 85]

I resample it using the following command:

gdalwarp dataset.tif dataset_cog.tif -of COG -co COMPRESS=LZW -r mode

And when I look at overviews, I start these values, especially as it downsamples further:

[ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 90, 91]

I saw the same if I tried it with nearest resampling. Maybe I'm misunderstanding, but I would think these resampling methods would retain the original values, and not interpolate any in-between values.

I'm using GDAL version 3.9.1 on Mac M3 from Homebrew.

Thanks,
Justin

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