[gdal-dev] Blue artifact dots in shadows when using GDAL to create mosaics

[Chastain, Raymond Lee (MU-Student)]

Process is very simple

  1.  Take our tiles and build a virtual raster:
gdalbuildvrt -r lanczos -resolution HIGHEST output.vrt -input_file_list input_file_list.txt
  2.  Convert our virtual raster to a GeoTIFF:
  3.  gdal_translate -of GTIFF ./output.vrt ./output.tif

The artifacts occur with all resolution methods (nearest, bilinear, cubic, cubicspline, lanczos, average, mode) to varying degrees.

It appears to worsen when more and more tiles overlay a location and when more scaling is needed.

I can provide a sample case that reproduces this:

Public Drive Link to archive: https://drive.google.com/file/d/1Tu4_A5NLiG0CQytuIQeeqr6JOVdAZDqu/view?usp=sharing

Archive contains 6 files:

img_1.tif - A low resolution (1m) GTIFF image to be rendered behind the second image.
img_2.tif - A high resolution (0.3m) GTIFF image to be rendered above the first image.
output.vrt - The VRT we generated using the above command from Step 1.
our_output.tif - The GTIFF mosaic generated using the above gdal_translate command from Step 2.
blue_dots_in_our_output.png - A zoomed in screenshot of some of the blue dots in the sample (the blue dots in this sample are unfortunately minor compared to what we have seen in our un-shareable data)
more_blue_dots_from_other_samples.png - A screenshot of some of the blue/yellow dots we see in some of our other samples which we cannot share.

Other relevant information:

  *   Attempted on GDAL 3.0.4, GDAL 3.2.1, and GDAL 3.2.3

Any help would be greatly appreciated,
Raymond
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