COG Quality

Resampling with reprojecton and overview creation

creating COGs using a command very similar to

gdal_translate --version

COMPRESS=webp
BLOCKSIZE=256

for RESAMPLE in cubic lanczos bilinear
do
gdal_translate \
    -of COG \
    -co COMPRESS=$COMPRESS\
    -co NUM_THREADS=ALL_CPUS \
    -co BIGTIFF=YES \
    -co TILING_SCHEME=GoogleMapsCompatible \
    --config BIGTIFF_OVERVIEW YES \
    -co ALIGNED_LEVELS=1 \
    -co SPARSE_OK=TRUE \
    -co ADD_ALPHA=YES \
    -co BLOCKSIZE=$BLOCKSIZE \
    -co RESAMPLING=$RESAMPLE \
    -co QUALITY=90 \
    -co ZOOM_LEVEL_STRATEGY=UPPER \
    $PWD/$TIF_FOLDER/.vrt $PWD/$TIF_FOLDER.$COMPRESS.google.r_$RESAMPLE.bs_$BLOCKSIZE.aligned.cog.tif
done

Overviews

When creating COGs using the different resampling methods the output quality differs a lot.

Here are some overviews created using some of the different resampling methods:

Lanczos

Provides quite a nice and crisp output

Cubic

Creates weird artifacts around the edges

Bilinear

looks blury

Using lanczos looks best for our overview creation.

Reprojection resampling

However when we use lanczos for the cog creation we start getting jagged lines inside the high resolution images

Here are some higher resolution aerial imagery image 0 with different resampling methods, these are tiles taken directly from a tiff and converted to webp.

Lanczos

Creates a jaggeredness, look at the street markings and the edges of the buildings

Bilinear

Creates a nice image

For our aerial imagery reprojection seems to be best done as bilinear, where as Overview creation seems to be best done as lanczos.

Using GDAL 3.2.x+ this can be configured with as it was fixed in osgeo/gdal#2671

gdal_translate -co WARP_RESAMPLING=bilinear -co OVERVIEW_RESAMPLING=lanczos