gdal2tiles: Fix case where --exclude still writes fully transparent tiles

[kevinmehall]

The alpha channel is only checked for a fully transparent tile when the source region is of nonzero size. When there are zero pixels to copy, the tile will also be transparent and should be skipped with --exclude.

This occurs when the edge of the input is aligned with the tile grid. To reproduce the issue, create a GeoTIFF with:

$ gdal_create -outsize 1536 1536 -a_srs EPSG:3857 -a_ullr -114.65554242953658 114.65554242953658 114.65554242953658 -114.65554242953658 -bands 4 -burn 0 -co COMPRESS=DEFLATE empty.tif

This will be fully transparent so should produce no tiles with --exclude, but currently produces tiles along one edge:

$ python3 swig/python/gdal-utils/osgeo_utils/gdal2tiles.py --exclude --xyz -w none empty.tif -z 11-20 /tmp/tiled_out 
...
$ find /tmp/tiled_out/20
/tmp/tiled_out/20
/tmp/tiled_out/20/524288
/tmp/tiled_out/20/524288/524291.png
/tmp/tiled_out/20/524287
/tmp/tiled_out/20/524287/524291.png
/tmp/tiled_out/20/524285
/tmp/tiled_out/20/524285/524291.png
/tmp/tiled_out/20/524289
/tmp/tiled_out/20/524289/524291.png
/tmp/tiled_out/20/524286
/tmp/tiled_out/20/524286/524291.png
/tmp/tiled_out/20/524284
/tmp/tiled_out/20/524284/524291.png
/tmp/tiled_out/20/524290
/tmp/tiled_out/20/524290/524291.png

[rouault]

@kevinmehall Your fix is reasonable, but I'm wondering if an even better fix wouldn't be to add a check before

gdal/swig/python/gdal-utils/osgeo_utils/gdal2tiles.py

Line 2807 in 3dfece7

# Read the source raster if anything is going inside the tile as per the computed

to avoid adding such tiles at all into the tile_details[] list. The --exclude option was designed more to exclude fully transparent tiles "in the middle" of the covered area, rather than tiles at the edges that have no area at all.

[kevinmehall]

Agreed. Added a commit (you can squash-merge them if you'd prefer).

I'm also not sure whether it should check self.options.exclude_transparent or be enabled unconditionally. I didn't dig into geo_query deep enough to tell whether this can result in a non-rectangular set of tiles, or if it will always cut off an entire row or column of tiles that is only there due to rounding differences.