geotiff: inherit nodata + rich-tag set on overview reads (#1739)

`extract_geo_info_with_overview_inheritance` was inheriting only the
CRS-side fields from the level-0 IFD. The COG writer also emits
GDAL_NODATA, GDAL_METADATA, the resolution tags, and the extra-tag
pass-through fields (colormap, image_description, extra_samples) only
on level 0, so an `open_geotiff(path, overview_level=N)` for N >= 1
returned a DataArray whose `attrs['nodata']` was None even when the
file declared a sentinel. The overview's on-disk pixels still carried
that sentinel (writer rewrites NaN to sentinel before reducing), so
downstream stats / threshold / plot pipelines silently folded sentinel
values into their results.

Fix: extend the inheritance helper to fill in the per-IFD pass-through
tags whenever the overview IFD lacks its own value. Mirrors the
existing CRS-side inheritance pattern (only inherit fields the
overview doesn't already carry), so an overview that does re-declare
any of these tags keeps its own value.

Affects all four backends (numpy, dask+numpy, cupy, dask+cupy) since
they all go through this helper. Adds a 4-backend regression test
suite that exercises nodata, gdal_metadata, and the resolution tags
across overview levels, and a contract test pinning the symmetric
diff of the attrs keyset to zero between level 0 and overview reads.

Updates one existing test in test_cog_overview_nodata_1613.py that
was inadvertently asserting the buggy behaviour (raw sentinel in the
overview output instead of NaN); the post-fix expectation matches
the level-0 behaviour.

Author: brendancol

.claude/sweep-metadata-state.csv

@@ -1,3 +1,4 @@
 module,last_inspected,issue,severity_max,categories_found,notes
 geotiff,2026-05-12,1710,MEDIUM,2,"open_geotiff/read_geotiff_dask/read_geotiff_gpu windowed reads of non-georef TIFFs produced float64 half-pixel-shifted coords while full reads produced int64 [0,1,2,...] coords. Affected every backend the same way; not a backend parity bug, a windowed-vs-full inconsistency. _populate_attrs_from_geo_info also fabricated an identity transform attr on non-georef files. Fixed by threading has_georef through all windowed coord paths and through the transform attr emitter (#1710)."
+geotiff,2026-05-12,1739,HIGH,1;4,"COG overview reads dropped attrs['nodata'] from level 0, so the writer-baked sentinel survived as raw data in the overview pixels (silent numerical corruption). extract_geo_info_with_overview_inheritance was inheriting CRS-side fields only; extended to per-IFD pass-through tags (nodata, gdal_metadata*, resolution*, colormap, extra_tags, image_description, extra_samples). All four backends affected (numpy/dask/cupy/dask+cupy). Fixed in #1739."
 reproject,2026-05-10,1572;1573,HIGH,1;3;4,geoid_height_raster dropped input attrs and used dims[-2:] for 3D inputs (#1572). reproject/merge ignored nodatavals (rasterio convention) when rioxarray absent (#1573). Fixed in same branch.

xrspatial/geotiff/_geotags.py

@@ -703,24 +703,39 @@ def extract_geo_info_with_overview_inheritance(
     """Extract geo metadata, inheriting from level 0 when the IFD lacks it.
 
     Wraps :func:`extract_geo_info` for overview reads. GDAL-style COG
-    writers (including this package's :func:`to_geotiff`) put the
-    GeoKeyDirectory, ModelPixelScale and ModelTiepoint only on the
-    level-0 IFD. Calling ``extract_geo_info`` directly on an overview
-    IFD therefore returns a default :class:`GeoTransform` with
-    ``has_georef=False`` and no CRS, so overview reads silently lose
-    their georeferencing.
+    writers (including this package's :func:`to_geotiff`) put a handful
+    of tags only on the level-0 IFD:
 
-    When ``ifd`` is a reduced-resolution overview (NewSubfileType bit 0
-    set) that lacks its own georef, we re-run ``extract_geo_info`` on
-    the first full-resolution IFD (NewSubfileType bit 0 clear, bit 2
-    clear) and rescale the pixel size by ``width_full / width_overview``
-    so coords cover the same extent as level 0.
+    * GeoKeyDirectory, ModelPixelScale, ModelTiepoint (georef)
+    * GDAL_NODATA, GDAL_METADATA (per-IFD pass-through tags)
+    * XResolution, YResolution, ResolutionUnit (resolution tags)
+    * ColorMap, ImageDescription, ExtraSamples (extra-tag pass-through)
+
+    Calling ``extract_geo_info`` directly on an overview IFD therefore
+    returns a default :class:`GeoTransform` with ``has_georef=False``,
+    no CRS, and a ``nodata=None`` field, so overview reads silently
+    lose their georeferencing and their nodata sentinel.
 
-    If the overview IFD already carries its own geokeys (some writers do
-    replicate them), this returns its own ``extract_geo_info`` output
-    unchanged. If no full-resolution sibling exists or the parent's geo
-    info is also missing, the overview's own (possibly empty) info is
-    returned -- callers get the same fallback behaviour they used to.
+    When ``ifd`` is a reduced-resolution overview (NewSubfileType bit 0
+    set), we re-run ``extract_geo_info`` on the first full-resolution
+    IFD (NewSubfileType bit 0 clear, bit 2 clear). Per-IFD pass-through
+    tags (nodata, GDAL metadata, resolution, colormap, extra tags,
+    image description, extra samples) are inherited when the overview
+    lacks its own value, regardless of whether the overview has its own
+    georef. The transform and CRS-side fields are additionally
+    inherited when the overview lacks its own georef, with the pixel
+    size rescaled by ``width_full / width_overview`` so coords cover
+    the same extent as level 0.
+
+    If the overview IFD already carries its own value for a given
+    field, that value wins -- inheritance is per-field and only fills
+    in missing entries. If no full-resolution sibling exists, the
+    overview's own (possibly empty) info is returned -- callers get the
+    same fallback behaviour they used to.
+
+    Inheriting nodata + the rich-tag set fixes #1739 (silent numerical
+    corruption when reading COG overview pixels because attrs['nodata']
+    was lost). The georef inheritance is the original fix from #1640.
 
     Parameters
     ----------
@@ -744,7 +759,7 @@ def extract_geo_info_with_overview_inheritance(
     # page IFDs (bit 1) are filtered out by ``select_overview_ifd``
     # before reaching here, so we never inherit a mask's geo info.
     is_overview = bool(ifd.subfile_type & 1)
-    if not is_overview or info.has_georef:
+    if not is_overview:
         return info
 
     # Find the level-0 IFD: NewSubfileType has bit 0 clear (not an
@@ -763,6 +778,51 @@ def extract_geo_info_with_overview_inheritance(
         return info
 
     base_info = extract_geo_info(base_ifd, data, byte_order)
+
+    # Inherit the per-IFD metadata that the COG writer emits only on the
+    # level-0 IFD: GDAL_NODATA, GDAL_METADATA, x/y resolution, colormap,
+    # extra tags, image description, extra samples. Without this block
+    # an overview read silently drops attrs['nodata'] (so the sentinel
+    # pixels the writer baked into the overview survive as ordinary data
+    # and poison downstream stats) and attrs['gdal_metadata'] (user
+    # metadata loss). See issue #1739.
+    #
+    # Each field is inherited only when the overview lacks its own
+    # value, so an overview IFD that does re-declare any of these keeps
+    # its own copy. Mirrors the gate the CRS-side inheritance applies
+    # below: prefer the overview's own value when present.
+    if info.nodata is None and base_info.nodata is not None:
+        info.nodata = base_info.nodata
+    if (info.gdal_metadata is None
+            and base_info.gdal_metadata is not None):
+        info.gdal_metadata = base_info.gdal_metadata
+    if (info.gdal_metadata_xml is None
+            and base_info.gdal_metadata_xml is not None):
+        info.gdal_metadata_xml = base_info.gdal_metadata_xml
+    if info.x_resolution is None and base_info.x_resolution is not None:
+        info.x_resolution = base_info.x_resolution
+    if info.y_resolution is None and base_info.y_resolution is not None:
+        info.y_resolution = base_info.y_resolution
+    if (info.resolution_unit is None
+            and base_info.resolution_unit is not None):
+        info.resolution_unit = base_info.resolution_unit
+    if info.colormap is None and base_info.colormap is not None:
+        info.colormap = base_info.colormap
+    if info.extra_tags is None and base_info.extra_tags is not None:
+        info.extra_tags = base_info.extra_tags
+    if (info.image_description is None
+            and base_info.image_description is not None):
+        info.image_description = base_info.image_description
+    if (info.extra_samples is None
+            and base_info.extra_samples is not None):
+        info.extra_samples = base_info.extra_samples
+
+    # If the overview already has its own georef, the rest of the
+    # inheritance (transform + CRS-side fields) is unnecessary -- return
+    # now with just the per-IFD-tag inheritance applied above.
+    if info.has_georef:
+        return info
+
     if not base_info.has_georef:
         return info
 

xrspatial/geotiff/tests/test_cog_overview_nodata_1613.py

@@ -86,11 +86,15 @@ def test_cpu_cog_overview_mean_partial_block(tmp_path):
 
     ov = open_geotiff(p, overview_level=1)
     # Top-left 2x2 was all-NaN -> reduces to NaN -> rewritten to -9999
+    #   on disk, then read back as NaN once the overview-nodata
+    #   inheritance fix (#1739) restores attrs['nodata'] and re-masks
+    #   the sentinel.
     # Top-right 2x2 [3,4,7,8] -> mean 5.5
     # Bottom-left [10,20,10,20] -> 15
     # Bottom-right [30,40,30,40] -> 35
     data = np.asarray(ov.data)
-    assert data[0, 0] == -9999.0
+    assert ov.attrs.get('nodata') == -9999.0
+    assert np.isnan(data[0, 0])
     np.testing.assert_allclose(data[0, 1], 5.5)
     np.testing.assert_allclose(data[1, 0], 15.0)
     np.testing.assert_allclose(data[1, 1], 35.0)