feat: Support Zarr Conventions

src/rasterix/lib.py

@@ -1,6 +1,7 @@
 """Shared library utilities for rasterix."""
 
 import logging
+from typing import NotRequired, TypedDict
 
 from affine import Affine
 
@@ -104,3 +105,70 @@ def affine_from_stac_proj_metadata(metadata: dict) -> Affine | None:
 
     a, b, c, d, e, f = transform[:6]
     return Affine(a, b, c, d, e, f)
+
+
+_ZarrConventionRegistration = TypedDict("_ZarrConventionRegistration", {"spatial:": str})
+
+_ZarrSpatialMetadata = TypedDict(
+    "_ZarrSpatialMetadata",
+    {
+        "zarr_conventions": NotRequired[list[_ZarrConventionRegistration | dict]],
+        "spatial:transform": NotRequired[list[float]],
+        "spatial:transform_type": NotRequired[str],
+        "spatial:registration": NotRequired[str],
+    },
+)
+
+
+def _has_spatial_zarr_convention(metadata: _ZarrSpatialMetadata) -> bool:
+    zarr_conventions = metadata.get("zarr_conventions")
+    if not zarr_conventions:
+        return False
+    for entry in zarr_conventions:
+        if isinstance(entry, dict) and entry.get("name") == "spatial:":
+            return True
+    return False
+
+
+def affine_from_spatial_zarr_convention(metadata: dict) -> Affine | None:
+    """Extract Affine transform from Zarr spatial convention metadata.
+
+    See https://github.com/zarr-conventions/spatial for the full specification.
+
+    Parameters
+    ----------
+    metadata : dict
+        Dictionary containing Zarr spatial convention metadata.
+
+    Returns
+    -------
+    Affine or None
+        Affine transformation matrix if minimal Zarr spatial metadata is found, None otherwise.
+
+    Examples
+    --------
+    >>> ds: xr.Dataset = ...
+    >>> affine = affine_from_spatial_zarr_convention(ds.attrs)
+    """
+    possibly_spatial_metadata: _ZarrSpatialMetadata = metadata  # type: ignore[assignment]
+
+    if _has_spatial_zarr_convention(possibly_spatial_metadata):
+        if transform := possibly_spatial_metadata.get("spatial:transform"):
+            if len(transform) < 6:
+                raise ValueError(f"spatial:transform must have at least 6 elements, got {len(transform)}")
+
+            transform_type = possibly_spatial_metadata.get("spatial:transform_type", "affine")
+            if transform_type != "affine":
+                raise NotImplementedError(
+                    f"Unsupported spatial:transform_type {transform_type!r}; only 'affine' is supported."
+                )
+
+            registration = possibly_spatial_metadata.get("spatial:registration", "pixel")
+            if registration != "pixel":
+                raise NotImplementedError(
+                    f"Unsupported spatial:registration {registration!r}; only 'pixel' is supported."
+                )
+
+            return Affine(*map(float, transform[:6]))
+
+    return None

src/rasterix/raster_index.py

@@ -21,7 +21,7 @@
 
 from rasterix.odc_compat import BoundingBox, bbox_intersection, bbox_union, maybe_int, snap_grid
 from rasterix.rioxarray_compat import guess_dims
-from rasterix.utils import get_affine
+from rasterix.utils import get_affine, get_crs_from_proj_zarr_convention
 
 T_Xarray = TypeVar("T_Xarray", "DataArray", "Dataset")
 
@@ -87,13 +87,17 @@ def assign_index(
 
     affine = get_affine(obj, x_dim=x_dim, y_dim=y_dim, clear_transform=True)
 
+    detected_crs = obj.proj.crs if crs else None
+    if detected_crs is None:
+        detected_crs = get_crs_from_proj_zarr_convention(obj)
+
     index = RasterIndex.from_transform(
         affine,
         width=obj.sizes[x_dim],
         height=obj.sizes[y_dim],
         x_dim=x_dim,
         y_dim=y_dim,
-        crs=obj.proj.crs if crs else None,
+        crs=detected_crs,
     )
     coords = Coordinates.from_xindex(index)
     return obj.assign_coords(coords)

src/rasterix/utils.py

@@ -1,7 +1,15 @@
+from typing import NotRequired, TypedDict
+
 import xarray as xr
 from affine import Affine
+from pyproj import CRS
 
-from rasterix.lib import affine_from_stac_proj_metadata, affine_from_tiepoint_and_scale, logger
+from rasterix.lib import (
+    affine_from_spatial_zarr_convention,
+    affine_from_stac_proj_metadata,
+    affine_from_tiepoint_and_scale,
+    logger,
+)
 
 
 def get_grid_mapping_var(obj: xr.Dataset | xr.DataArray) -> xr.DataArray | None:
@@ -58,7 +66,7 @@ def get_affine(
             del grid_mapping_var.attrs["GeoTransform"]
         return Affine.from_gdal(*map(float, transform.split(" ")))
 
-    # Check for STAC and GeoTIFF metadata in DataArray attrs
+    # Check for STAC, GeoTIFF, or spatial zarr convention metadata in DataArray attrs
     attrs = obj.attrs if isinstance(obj, xr.DataArray) else {}
 
     # Try to extract affine from STAC proj:transform
@@ -80,6 +88,13 @@ def get_affine(
 
         return affine
 
+    # Try to extract from spatial zarr convention attributes
+    if affine := affine_from_spatial_zarr_convention(attrs):
+        logger.trace("Creating affine from spatial zarr convention attributes")
+        if clear_transform:
+            del attrs["spatial:transform"]
+        return affine
+
     # Fall back to computing from coordinate arrays
     logger.trace(f"Creating affine from coordinate arrays {x_dim=!r} and {y_dim=!r}")
     if x_dim not in obj.coords or y_dim not in obj.coords:
@@ -106,3 +121,55 @@ def get_affine(
     return Affine.translation(
         x[0].item() - dx / 2, (y[0] if dy < 0 else y[-1]).item() - dy / 2
     ) * Affine.scale(dx, dy)
+
+
+_ZarrConventionRegistration = TypedDict("_ZarrConventionRegistration", {"proj:": str})
+
+_ZarrProjMetadata = TypedDict(
+    "_ZarrProjMetadata",
+    {
+        "zarr_conventions": NotRequired[list[_ZarrConventionRegistration | dict]],
+        "proj:code": NotRequired[str],
+        "proj:wkt2": NotRequired[str],
+        "proj:projjson": NotRequired[object],
+    },
+)
+
+
+def _has_proj_zarr_convention(metadata: _ZarrProjMetadata) -> bool:
+    zarr_conventions = metadata.get("zarr_conventions")
+    if not zarr_conventions:
+        return False
+    for entry in zarr_conventions:
+        if isinstance(entry, dict) and entry.get("name") == "proj:":
+            return True
+    return False
+
+
+def get_crs_from_proj_zarr_convention(obj: xr.Dataset | xr.DataArray) -> CRS | None:
+    """Extract CRS from Zarr proj: convention metadata if present.
+
+    See https://github.com/zarr-conventions/geo-proj for more details.
+
+    Parameters
+    ----------
+    obj: xr.Dataset or xr.DataArray
+        The Xarray object to extract CRS from.
+
+    Returns
+    -------
+    CRS or None
+        The extracted CRS object, or None if not found.
+    """
+    metadata: _ZarrProjMetadata = obj.attrs  # type: ignore[assignment]
+
+    if not _has_proj_zarr_convention(metadata):
+        return None
+
+    if code := metadata.get("proj:code"):
+        return CRS.from_string(code)
+    if wkt2 := metadata.get("proj:wkt2"):
+        return CRS.from_wkt(wkt2)
+    if projjson := metadata.get("proj:projjson"):
+        return CRS.from_user_input(projjson)
+    return None

tests/test_raster_index.py

@@ -566,6 +566,75 @@ def test_assign_index_with_stac_proj_transform_9_elements():
     assert actual_affine == expected_affine
 
 
+def test_assign_index_with_spatial_zarr_convention():
+    da = xr.DataArray(
+        np.ones((100, 100)),
+        dims=("y", "x"),
+        attrs={
+            "zarr_conventions": [{"name": "spatial:"}],
+            "spatial:transform": [30.0, 0.0, 323400.0, 0.0, 30.0, 4268400.0],
+        },
+    )
+
+    result = assign_index(da)
+
+    # Check that the index was created
+    assert isinstance(result.xindexes["x"], RasterIndex)
+    assert isinstance(result.xindexes["y"], RasterIndex)
+
+    # Verify the affine transform
+    expected_affine = Affine(30.0, 0.0, 323400.0, 0.0, 30.0, 4268400.0)
+    actual_affine = result.xindexes["x"].transform()
+    assert actual_affine == expected_affine
+
+    # Verify spatial:transform attribute is removed
+    assert "spatial:transform" not in result.attrs
+
+
+def test_assign_index_with_spatual_zarr_convention_too_few_raises():
+    da = xr.DataArray(
+        np.ones((100, 100)),
+        dims=("y", "x"),
+        attrs={
+            "zarr_conventions": [{"name": "spatial:"}],
+            "spatial:transform": [30.0, 0.0, 323400.0, 0.0, 30.0],
+        },
+    )
+
+    with pytest.raises(ValueError, match="spatial:transform must have at least 6 elements"):
+        assign_index(da)
+
+
+def test_assign_index_with_spatual_zarr_convention_transform_type_not_implemented():
+    da = xr.DataArray(
+        np.ones((100, 100)),
+        dims=("y", "x"),
+        attrs={
+            "zarr_conventions": [{"name": "spatial:"}],
+            "spatial:transform_type": "not_affine",
+            "spatial:transform": [30.0, 0.0, 323400.0, 0.0, 30.0, 4268400.0],
+        },
+    )
+
+    with pytest.raises(NotImplementedError, match="Unsupported spatial:transform_type"):
+        assign_index(da)
+
+
+def test_assign_index_with_spatual_zarr_convention_registration_not_implemented():
+    da = xr.DataArray(
+        np.ones((100, 100)),
+        dims=("y", "x"),
+        attrs={
+            "zarr_conventions": [{"name": "spatial:"}],
+            "spatial:registration": "not_pixel",
+            "spatial:transform": [30.0, 0.0, 323400.0, 0.0, 30.0, 4268400.0],
+        },
+    )
+
+    with pytest.raises(NotImplementedError, match="Unsupported spatial:registration"):
+        assign_index(da)
+
+
 def test_assign_index_no_coords_no_metadata():
     """Test that assign_index raises error when coords are missing and no transform metadata."""
     da = xr.DataArray(np.ones((10, 10)), dims=("y", "x"))
@@ -706,3 +775,50 @@ def test_raster_index_from_stac_proj_metadata_with_crs():
     # Verify CRS was set
     assert index.crs is not None
     assert index.crs.to_epsg() == 32610
+
+
+def test_assign_index_proj_zarr_convention_code():
+    ds = xr.DataArray(
+        np.ones((3, 4)),
+        dims=("y", "x"),
+        attrs={
+            "zarr_conventions": [{"name": "proj:"}, {"name": "spatial:"}],
+            "proj:code": "EPSG:4326",
+            "spatial:transform": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0],
+        },
+    )
+    indexed = assign_index(ds)
+    assert indexed.xindexes["x"].crs is not None
+    assert indexed.xindexes["x"].crs.to_epsg() == 4326
+
+
+def test_assign_index_proj_zarr_convention_wkt2():
+    crs = pyproj.CRS.from_epsg(3857)
+    ds = xr.DataArray(
+        np.ones((3, 4)),
+        dims=("y", "x"),
+        attrs={
+            "zarr_conventions": [{"name": "proj:"}, {"name": "spatial:"}],
+            "proj:wkt2": crs.to_wkt(),
+            "spatial:transform": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0],
+        },
+    )
+    indexed = assign_index(ds)
+    assert indexed.xindexes["x"].crs is not None
+    assert indexed.xindexes["x"].crs.to_epsg() == 3857
+
+
+def test_assign_index_proj_zarr_convention_projjson():
+    crs = pyproj.CRS.from_epsg(32610)
+    ds = xr.DataArray(
+        np.ones((3, 4)),
+        dims=("y", "x"),
+        attrs={
+            "zarr_conventions": [{"name": "proj:"}, {"name": "spatial:"}],
+            "proj:projjson": crs.to_json_dict(),
+            "spatial:transform": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0],
+        },
+    )
+    indexed = assign_index(ds)
+    assert indexed.xindexes["x"].crs is not None
+    assert indexed.xindexes["x"].crs.to_epsg() == 32610