gdaladdo -r cubic / -r lanczos produce out-of-range pixel values on Byte rasters

[zdila]

What is the bug?

gdaladdo -r cubic and -r lanczos produce visibly anomalous pixels in lower overview levels of a Byte RGBA hillshade GeoTIFF - sparse pixels that are noticeably darker or brighter than any of their neighbours. The pixels are clearly not real terrain features. -r cubicspline and -r average on the same file are clean.

Same world location, same zoom level (1/16 overview), three resamplers:

Note the stray dark/yellow pixels mid-frame in cubic and lanczos, absent in cubicspline.

Detail of lanczos result:

Sample files (20 km × 20 km crop of a hillshade GeoTIFF, 4-band RGBA Byte, lossless DEFLATE):

• report_crop20_no_overviews.tif — source, no overviews built yet (116 MB)
• report_crop20_cubic.tif — gdaladdo -r cubic applied (168 MB) — bug visible
• report_crop20_cubicspline.tif — gdaladdo -r cubicspline applied (164 MB) — clean
• report_crop20_lanczos.tif — gdaladdo -r lanczos applied (169 MB) — bug more pronounced

Steps to reproduce the issue

To reproduce from scratch:

gdaladdo \
  --config COMPRESS_OVERVIEW DEFLATE \
  --config PREDICTOR_OVERVIEW 2 \
  -r cubic \
  report_crop20_no_overviews.tif

Then open the result in QGIS and zoom to the 1/16 overview level. Stray dark/bright pixels appear scattered across the image, not at edges.

Repeat with -r cubicspline on a fresh copy - artifacts gone.

Versions and provenance

• OS: Debian GNU/Linux forky/sid (testing/unstable)
• GDAL: GDAL 3.12.4 "Chicoutimi", released 2026/04/22
• Source: conda-forge (gdal-3.12.4-np2py314h5ae587a_2)
• Viewer: QGIS

Additional context

No response