interactive plotting with lonboard

ci/environment.yml

@@ -13,6 +13,9 @@ dependencies:
   - hypothesis
   - ruff
   - typing-extensions
+  - geoarrow-pyarrow
+  - lonboard
   - pip
   - pip:
+      - arro3-core
       - h3ronpy

pyproject.toml

@@ -40,6 +40,14 @@ dependencies = [
     "typing-extensions",
 ]
 
+[project.optional-dependencies]
+explore = [
+  "lonboard>=0.9.3",
+  "pyproj>=3.3",
+  "matplotlib",
+  "arro3-core>=0.4.0"
+]
+
 [project.urls]
 # Home = "https://xdggs.readthedocs.io"
 Repository = "https://github.com/xarray-contrib/xdggs"

xdggs/accessor.py

@@ -3,6 +3,7 @@
 
 from xdggs.grid import DGGSInfo
 from xdggs.index import DGGSIndex
+from xdggs.plotting import explore
 
 
 @xr.register_dataset_accessor("dggs")
@@ -115,3 +116,38 @@ def cell_boundaries(self):
         return xr.DataArray(
             boundaries, coords={self._name: self.cell_ids}, dims=self.cell_ids.dims
         )
+
+    def explore(self, *, cmap="viridis", center=None, alpha=None):
+        """interactively explore the data using `lonboard`
+
+        Requires `lonboard`, `matplotlib`, and `arro3.core` to be installed.
+
+        Parameters
+        ----------
+        cmap : str
+            The name of the color map to use
+        center : int or float, optional
+            If set, will use this as the center value of a diverging color map.
+        alpha : float, optional
+            If set, controls the transparency of the polygons.
+
+        Returns
+        -------
+        map : lonboard.Map
+            The rendered map.
+
+        Notes
+        -----
+        Plotting currently is restricted to 1D `DataArray` objects.
+        """
+        if isinstance(self._obj, xr.Dataset):
+            raise ValueError("does not work with Dataset objects, yet")
+
+        cell_dim = self._obj[self._name].dims[0]
+        return explore(
+            self._obj,
+            cell_dim=cell_dim,
+            cmap=cmap,
+            center=center,
+            alpha=alpha,
+        )

xdggs/grid.py

@@ -34,5 +34,5 @@ def cell_ids2geographic(self, cell_ids):
     def geographic2cell_ids(self, lon, lat):
         raise NotImplementedError()
 
-    def cell_boundaries(self, cell_ids):
+    def cell_boundaries(self, cell_ids, backend="shapely"):
         raise NotImplementedError()

xdggs/h3.py

@@ -1,3 +1,4 @@
+import json
 from collections.abc import Mapping
 from dataclasses import dataclass
 from typing import Any, ClassVar
@@ -8,7 +9,6 @@
     from typing_extensions import Self
 
 import numpy as np
-import shapely
 import xarray as xr
 from h3ronpy.arrow.vector import (
     cells_to_coordinates,
@@ -22,6 +22,42 @@
 from xdggs.utils import _extract_cell_id_variable, register_dggs
 
 
+def polygons_shapely(wkb):
+    import shapely
+
+    return shapely.from_wkb(wkb)
+
+
+def polygons_geoarrow(wkb):
+    import pyproj
+    import shapely
+    from arro3.core import list_array
+
+    polygons = shapely.from_wkb(wkb)
+    crs = pyproj.CRS.from_epsg(4326)
+
+    geometry_type, coords, (ring_offsets, geom_offsets) = shapely.to_ragged_array(
+        polygons
+    )
+
+    if geometry_type != shapely.GeometryType.POLYGON:
+        raise ValueError(f"unsupported geometry type found: {geometry_type}")
+
+    polygon_array = list_array(
+        geom_offsets.astype("int32"), list_array(ring_offsets.astype("int32"), coords)
+    )
+    polygon_array_with_geo_meta = polygon_array.cast(
+        polygon_array.field.with_metadata(
+            {
+                "ARROW:extension:name": "geoarrow.polygon",
+                "ARROW:extension:metadata": json.dumps({"crs": crs.to_json_dict()}),
+            }
+        )
+    )
+
+    return polygon_array_with_geo_meta
+
+
 @dataclass(frozen=True)
 class H3Info(DGGSInfo):
     resolution: int
@@ -50,10 +86,18 @@ def cell_ids2geographic(
     def geographic2cell_ids(self, lon, lat):
         return coordinates_to_cells(lat, lon, self.resolution, radians=False)
 
-    def cell_boundaries(self, cell_ids):
+    def cell_boundaries(self, cell_ids, backend="shapely"):
+        # TODO: convert cell ids directly to geoarrow once h3ronpy supports it
         wkb = cells_to_wkb_polygons(cell_ids, radians=False, link_cells=False)
 
-        return shapely.from_wkb(wkb)
+        backends = {
+            "shapely": polygons_shapely,
+            "geoarrow": polygons_geoarrow,
+        }
+        backend_func = backends.get(backend)
+        if backend_func is None:
+            raise ValueError("invalid backend: {backend!r}")
+        return backend_func(wkb)
 
 
 @register_dggs("h3")

xdggs/healpix.py

@@ -1,3 +1,4 @@
+import json
 import operator
 from collections.abc import Mapping
 from dataclasses import dataclass, field
@@ -26,6 +27,72 @@
     from exceptiongroup import ExceptionGroup
 
 
+def polygons_shapely(vertices):
+    import shapely
+
+    return shapely.polygons(vertices)
+
+
+def polygons_geoarrow(vertices):
+    import pyproj
+    from arro3.core import list_array
+
+    polygon_vertices = np.concatenate([vertices, vertices[:, :1, :]], axis=1)
+    crs = pyproj.CRS.from_epsg(4326)
+
+    # construct geoarrow arrays
+    coords = np.reshape(polygon_vertices, (-1, 2))
+    coords_per_pixel = polygon_vertices.shape[1]
+    geom_offsets = np.arange(vertices.shape[0] + 1, dtype="int32")
+    ring_offsets = geom_offsets * coords_per_pixel
+
+    polygon_array = list_array(geom_offsets, list_array(ring_offsets, coords))
+
+    # We need to tag the array with extension metadata (`geoarrow.polygon`) so that Lonboard knows that this is a geospatial column.
+    polygon_array_with_geo_meta = polygon_array.cast(
+        polygon_array.field.with_metadata(
+            {
+                "ARROW:extension:name": "geoarrow.polygon",
+                "ARROW:extension:metadata": json.dumps(
+                    {"crs": crs.to_json_dict(), "edges": "spherical"}
+                ),
+            }
+        )
+    )
+    return polygon_array_with_geo_meta
+
+
+def center_around_prime_meridian(lon, lat):
+    # three tasks:
+    # - center around the prime meridian (map to a range of [-180, 180])
+    # - replace the longitude of points at the poles with the median
+    #   of longitude of the other vertices
+    # - cells that cross the dateline should have longitudes around 180
+
+    # center around prime meridian
+    recentered = (lon + 180) % 360 - 180
+
+    # replace lon of pole with the median of the remaining vertices
+    contains_poles = np.isin(lat, np.array([-90, 90]))
+    pole_cells = np.any(contains_poles, axis=-1)
+    recentered[contains_poles] = np.median(
+        np.reshape(
+            recentered[pole_cells[:, None] & np.logical_not(contains_poles)], (-1, 3)
+        ),
+        axis=-1,
+    )
+
+    # keep cells that cross the dateline centered around 180
+    polygons_to_fix = np.any(recentered < -100, axis=-1) & np.any(
+        recentered > 100, axis=-1
+    )
+    result = np.where(
+        polygons_to_fix[:, None] & (recentered < 0), recentered + 360, recentered
+    )
+
+    return result
+
+
 @dataclass(frozen=True)
 class HealpixInfo(DGGSInfo):
     resolution: int
@@ -135,23 +202,29 @@ def cell_ids2geographic(self, cell_ids):
     def geographic2cell_ids(self, lon, lat):
         return healpy.ang2pix(self.nside, lon, lat, lonlat=True, nest=self.nest)
 
-    def cell_boundaries(self, cell_ids: Any) -> np.ndarray:
-        import shapely
-
+    def cell_boundaries(self, cell_ids: Any, backend="shapely") -> np.ndarray:
         boundary_vectors = healpy.boundaries(
             self.nside, cell_ids, step=1, nest=self.nest
         )
 
         lon, lat = healpy.vec2ang(np.moveaxis(boundary_vectors, 1, -1), lonlat=True)
-        boundaries = np.reshape(np.stack((lon, lat), axis=-1), (-1, 4, 2))
+        lon_reshaped = np.reshape(lon, (-1, 4))
+        lat_reshaped = np.reshape(lat, (-1, 4))
+
+        lon_ = center_around_prime_meridian(lon_reshaped, lat_reshaped)
+
+        vertices = np.stack((lon_, lat_reshaped), axis=-1)
+
+        backends = {
+            "shapely": polygons_shapely,
+            "geoarrow": polygons_geoarrow,
+        }
 
-        # fix the dateline / prime meridian issue
-        lon_ = boundaries[..., 0]
-        to_fix = abs(np.max(lon_, axis=-1) - np.min(lon_, axis=-1)) > 300
-        fixed_lon = (lon_[to_fix, :] + 180) % 360 - 180
-        boundaries[to_fix, :, 0] = fixed_lon
+        backend_func = backends.get(backend)
+        if backend_func is None:
+            raise ValueError("invalid backend: {backend!r}")
 
-        return shapely.polygons(boundaries)
+        return backend_func(vertices)
 
 
 @register_dggs("healpix")

xdggs/plotting.py

@@ -0,0 +1,72 @@
+import numpy as np
+
+
+def create_arrow_table(polygons, arr, coords=None):
+    from arro3.core import Array, ChunkedArray, Schema, Table
+
+    if coords is None:
+        coords = ["latitude", "longitude"]
+
+    array = Array.from_arrow(polygons)
+    name = arr.name or "data"
+    arrow_arrays = {
+        "geometry": array,
+        "cell_ids": ChunkedArray([Array.from_numpy(arr.coords["cell_ids"])]),
+        name: ChunkedArray([Array.from_numpy(arr.data)]),
+    } | {
+        coord: ChunkedArray([Array.from_numpy(arr.coords[coord].data)])
+        for coord in coords
+        if coord in arr.coords
+    }
+
+    fields = [array.field.with_name(name) for name, array in arrow_arrays.items()]
+    schema = Schema(fields)
+
+    return Table.from_arrays(list(arrow_arrays.values()), schema=schema)
+
+
+def normalize(var, center=None):
+    from matplotlib.colors import CenteredNorm, Normalize
+
+    if center is None:
+        vmin = var.min(skipna=True)
+        vmax = var.max(skipna=True)
+        normalizer = Normalize(vmin=vmin, vmax=vmax)
+    else:
+        halfrange = np.abs(var - center).max(skipna=True)
+        normalizer = CenteredNorm(vcenter=center, halfrange=halfrange)
+
+    return normalizer(var.data)
+
+
+def explore(
+    arr,
+    cell_dim="cells",
+    cmap="viridis",
+    center=None,
+    alpha=None,
+):
+    import lonboard
+    from lonboard import SolidPolygonLayer
+    from lonboard.colormap import apply_continuous_cmap
+    from matplotlib import colormaps
+
+    if len(arr.dims) != 1 or cell_dim not in arr.dims:
+        raise ValueError(
+            f"exploration only works with a single dimension ('{cell_dim}')"
+        )
+
+    cell_ids = arr.dggs.coord.data
+    grid_info = arr.dggs.grid_info
+
+    polygons = grid_info.cell_boundaries(cell_ids, backend="geoarrow")
+
+    normalized_data = normalize(arr.variable, center=center)
+
+    colormap = colormaps[cmap]
+    colors = apply_continuous_cmap(normalized_data, colormap, alpha=alpha)
+
+    table = create_arrow_table(polygons, arr)
+    layer = SolidPolygonLayer(table=table, filled=True, get_fill_color=colors)
+
+    return lonboard.Map(layer)

xdggs/tests/__init__.py

@@ -1,5 +1,12 @@
+import geoarrow.pyarrow as ga
+import shapely
+
 from xdggs.tests.matchers import (  # noqa: F401
     Match,
     MatchResult,
     assert_exceptions_equal,
 )
+
+
+def geoarrow_to_shapely(arr):
+    return shapely.from_wkb(ga.as_wkb(arr))

xdggs/tests/test_h3.py

@@ -8,6 +8,7 @@
 from xarray.core.indexes import PandasIndex
 
 from xdggs import h3
+from xdggs.tests import geoarrow_to_shapely
 
 # from the h3 gallery, at resolution 3
 cell_ids = [
@@ -202,14 +203,18 @@ def test_geographic2cell_ids(self, cell_centers, cell_ids):
             ),
         ),
     )
-    def test_cell_boundaries(self, resolution, cell_ids, expected_coords):
+    @pytest.mark.parametrize("backend", ["shapely", "geoarrow"])
+    def test_cell_boundaries(self, resolution, cell_ids, backend, expected_coords):
         expected = shapely.polygons(expected_coords)
 
         grid = h3.H3Info(resolution=resolution)
 
-        actual = grid.cell_boundaries(cell_ids)
+        backends = {"shapely": lambda arr: arr, "geoarrow": geoarrow_to_shapely}
+        converter = backends[backend]
 
-        shapely.testing.assert_geometries_equal(actual, expected)
+        actual = grid.cell_boundaries(cell_ids, backend=backend)
+
+        shapely.testing.assert_geometries_equal(converter(actual), expected)
 
 
 @pytest.mark.parametrize("resolution", resolutions)