gdal2tiles: add --nodata-values-pct-threshold option

This is similar to the --excluded-values-pct-threshold option added in
OSGeo#9631, but here this controls the
minimum amount of transparent/invalid/nodata source pixels to cause the
target pixels to be transparent.
Only used currently for average resampling

autotest/pyscripts/test_gdal2tiles.py

@@ -630,6 +630,61 @@ def test_gdal2tiles_excluded_values(script_path, tmp_path):
     )
 
 
+@pytest.mark.require_driver("PNG")
+def test_gdal2tiles_nodata_values_pct_threshold(script_path, tmp_path):
+
+    input_tif = str(tmp_path / "test_gdal2tiles_nodata_values_pct_threshold.tif")
+    output_folder = str(tmp_path / "test_gdal2tiles_nodata_values_pct_threshold")
+
+    src_ds = gdal.GetDriverByName("GTiff").Create(input_tif, 256, 256, 1, gdal.GDT_Byte)
+    src_ds.GetRasterBand(1).SetNoDataValue(20)
+    src_ds.GetRasterBand(1).WriteRaster(
+        0, 0, 2, 2, struct.pack("B" * 4, 10, 20, 30, 40)
+    )
+    srs = osr.SpatialReference()
+    srs.ImportFromEPSG(3857)
+    src_ds.SetSpatialRef(srs)
+    MAX_GM = 20037508.342789244
+    RES_Z0 = 2 * MAX_GM / 256
+    RES_Z1 = RES_Z0 / 2
+    # Spatial extent of tile (0,0) at zoom level 1
+    src_ds.SetGeoTransform([-MAX_GM, RES_Z1, 0, MAX_GM, 0, -RES_Z1])
+    src_ds = None
+
+    test_py_scripts.run_py_script_as_external_script(
+        script_path,
+        "gdal2tiles",
+        f"-q -z 0-1 {input_tif} {output_folder}",
+    )
+
+    ds = gdal.Open(f"{output_folder}/0/0/0.png")
+    assert struct.unpack("B" * 2, ds.ReadRaster(0, 0, 1, 1)) == (
+        round((10 + 30 + 40) / 3),
+        255,
+    )
+
+    test_py_scripts.run_py_script_as_external_script(
+        script_path,
+        "gdal2tiles",
+        f"-q -z 0-1 --nodata-values-pct-threshold=50 {input_tif} {output_folder}",
+    )
+
+    ds = gdal.Open(f"{output_folder}/0/0/0.png")
+    assert struct.unpack("B" * 2, ds.ReadRaster(0, 0, 1, 1)) == (
+        round((10 + 30 + 40) / 3),
+        255,
+    )
+
+    test_py_scripts.run_py_script_as_external_script(
+        script_path,
+        "gdal2tiles",
+        f"-q -z 0-1 --nodata-values-pct-threshold=25 {input_tif} {output_folder}",
+    )
+
+    ds = gdal.Open(f"{output_folder}/0/0/0.png")
+    assert struct.unpack("B" * 2, ds.ReadRaster(0, 0, 1, 1)) == (0, 0)
+
+
 @pytest.mark.require_driver("JPEG")
 @pytest.mark.parametrize(
     "resampling, expected_stats_z0, expected_stats_z1",

doc/source/programs/gdal2tiles.rst

@@ -22,7 +22,9 @@ Synopsis
                   [-w <webviewer>] [-t <title>] [-c <copyright>]
                   [--processes=<NB_PROCESSES>] [--mpi] [--xyz]
                   [--tilesize=<PIXELS>] --tiledriver=<DRIVER> [--tmscompatible]
-                  [--excluded-values=<EXCLUDED_VALUES>] [--excluded-values-pct-threshold=<EXCLUDED_VALUES_PCT_THRESHOLD>]
+                  [--excluded-values=<EXCLUDED_VALUES>]
+                  [--excluded-values-pct-threshold=<EXCLUDED_VALUES_PCT_THRESHOLD>]
+                  [--nodata-values-pct-threshold=<NODATA_VALUES_PCT_THRESHOLD>]
                   [-g <googlekey] [-b <bingkey>] <input_file> [<output_dir>] [<COMMON_OPTIONS>]
 
 Description
@@ -155,13 +157,25 @@ can publish a picture without proper georeferencing too.
   that pixels matching one of the excluded value tuples are still considered
   as valid, when determining the target pixel validity/density.
 
+  .. versionadded:: 3.9
+
 .. option:: --excluded-values-pct-threshold=EXCLUDED_VALUES_PCT_THRESHOLD
 
   Minimum percentage of source pixels that must be set at one of the --excluded-values to cause the excluded
   value, that is in majority among source pixels, to be used as the target pixel value. Default value is 50(%)
 
   .. versionadded:: 3.9
 
+.. option:: --nodata-values-pct-threshold=<NODATA_VALUES_PCT_THRESHOLD>
+
+  Minimum percentage of source pixels that must be at nodata (or alpha=0 or any
+  other way to express transparent pixel) to cause the target pixel value to
+  be transparent. Default value is 100 (%), which means that a target pixel is
+  transparent only if all contributing source pixels are transparent.
+  Only taken into account for average resampling.
+
+  .. versionadded:: 3.9
+
 .. option:: -h, --help
 
   Show help message and exit.

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

@@ -885,12 +885,17 @@ def scale_query_to_tile(dsquery, dstile, options, tilefilename=""):
     )
     dstile.SetGeoTransform((0.0, 1.0, 0.0, 0.0, 0.0, 1.0))
 
-    if options.resampling == "average" and options.excluded_values:
+    if options.resampling == "average" and (
+        options.excluded_values or options.nodata_values_pct_threshold < 100
+    ):
 
         gdal.Warp(
             dstile,
             dsquery,
-            options=f"-r average -wo EXCLUDED_VALUES={options.excluded_values} -wo EXCLUDED_VALUES_PCT_THRESHOLD={options.excluded_values_pct_threshold}",
+            options="-r average "
+            + f"-wo EXCLUDED_VALUES={options.excluded_values} "
+            + f"-wo EXCLUDED_VALUES_PCT_THRESHOLD={options.excluded_values_pct_threshold} "
+            + f"-wo NODATA_VALUES_PCT_THRESHOLD={options.nodata_values_pct_threshold}",
         )
 
     elif options.resampling == "average":
@@ -1872,6 +1877,13 @@ def optparse_init() -> optparse.OptionParser:
         default=50,
         help="Minimum percentage of source pixels that must be set at one of the --excluded-values to cause the excluded value, that is in majority among source pixels, to be used as the target pixel value. Default value is 50 (%)",
     )
+    p.add_option(
+        "--nodata-values-pct-threshold",
+        dest="nodata_values_pct_threshold",
+        type=float,
+        default=100,
+        help="Minimum percentage of source pixels that must be at nodata (or alpha=0 or any other way to express transparent pixel) to cause the target pixel value to be transparent. Default value is 100 (%). Only taken into account for average resampling",
+    )
 
     # KML options
     g = optparse.OptionGroup(