Merge pull request #305 from cogeotiff/patch/nodata-alpha-warning

raise warning with both nodata and alpha

Author: vincentsarago

CHANGES.md

@@ -1,5 +1,10 @@
 # Release Notes
 
+
+## 5.4.2 (2025-06-27)
+
+* add user warning when input dataset has both Nodata and internal Alpha/Mask band
+
 ## 5.4.1 (2024-12-16)
 
 * fix reading raster compression value with `rasterio>=1.4.3` (author @glostis, https://github.com/cogeotiff/rio-cogeo/pull/300)

rio_cogeo/cogeo.py

@@ -21,7 +21,7 @@
 from rasterio.vrt import WarpedVRT
 
 from rio_cogeo import models, utils
-from rio_cogeo.errors import IncompatibleOptions
+from rio_cogeo.errors import IncompatibleOptions, NodataAlphaMaskWarning
 
 IN_MEMORY_THRESHOLD = int(os.environ.get("IN_MEMORY_THRESHOLD", 10980 * 10980))
 
@@ -222,6 +222,12 @@ def cog_translate(  # noqa: C901
             alpha = utils.has_alpha_band(src_dst)
             mask = utils.has_mask_band(src_dst)
 
+            if nodata is not None and (alpha or mask):
+                warnings.warn(
+                    "Input dataset has both a nodata value and internal alpha/mask band. Nodata value will be prioritized.",
+                    NodataAlphaMaskWarning,
+                )
+
             if colormap and len(indexes) > 1:
                 raise IncompatibleOptions(
                     "Cannot add a colormap for multiple bands data."

rio_cogeo/errors.py

@@ -9,6 +9,10 @@ class IncompatibleBlockRasterSize(UserWarning):
     """Rio-cogeo module incompatible raster block/size warning."""
 
 
+class NodataAlphaMaskWarning(UserWarning):
+    """Dataset with nodata and alpha/mask warning."""
+
+
 class RioCogeoError(Exception):
     """Base exception class."""
 

tests/fixtures/image_rgba_nodata.tif

@@ -21,6 +21,7 @@
 
 FIXTURES_DIR = os.path.join(os.path.dirname(__file__), "fixtures")
 raster_path_rgba = os.path.join(FIXTURES_DIR, "image_rgba.tif")
+raster_path_rgba_nodata = os.path.join(FIXTURES_DIR, "image_rgba_nodata.tif")
 raster_path_rgb = os.path.join(FIXTURES_DIR, "image_rgb.tif")
 raster_path_nan = os.path.join(FIXTURES_DIR, "image_nan.tif")
 raster_path_nodata = os.path.join(FIXTURES_DIR, "image_nodata.tif")
@@ -920,3 +921,20 @@ def test_cog_values(src_path, runner):
             assert cog.colorinterp == source.colorinterp
             assert has_mask_band(cog)
             assert not has_alpha_band(cog)
+
+
+def test_nodata_alpha_incompatible(runner):
+    """test nodata/alpha warning."""
+    with runner.isolated_filesystem():
+        with pytest.warns(UserWarning):
+            cog_translate(
+                raster_path_rgba_nodata,
+                "cogeo.tif",
+                cog_profiles.get("deflate"),
+                quiet=True,
+            )
+            assert cog_validate("cogeo.tif")
+
+        with rasterio.open("cogeo.tif") as cog:
+            assert cog.nodata is not None
+            assert has_alpha_band(cog)