✨ Introduce pyiem.grid.util.grid_smear

Fill in masked data by shifting grid four times
refs akrherz/iem#923

CHANGELOG.md

@@ -15,6 +15,7 @@ All notable changes to this library are documented in this file.
 
 - Add `_check_dueling_tropics` VTEC check looking for TR+HU overlap (#930).
 - Add IEMRE DOMAINS to support upcoming expansion.
+- Add `pyiem.grid.util.grid_smear` to shift masked data to fill in missing.
 - Allow `iemapp(memcachexpire)` to be a callable, called back with environ.
 - Cross check WCNs against watch database storage for PDS status (#925).
 - For `iemapp` if the `memcachekey=` callable returns `None`, the check of

src/pyiem/grid/util.py

@@ -0,0 +1,34 @@
+"""pyIEM grid utilities."""
+
+import numpy as np
+
+
+def grid_smear(grid: np.ndarray, pad: int = 4) -> np.ndarray:
+    """Smear data around to fill in masked values (likely near coastlines).
+
+    Args:
+        grid: 2D numpy array
+        pad: number of pixels to smear the data around by in each direction
+
+    Returns:
+        2D numpy array with smeared data
+    """
+    # Pad grid
+    padded = np.ma.masked_all(
+        (grid.shape[0] + pad * 2, grid.shape[1] + pad * 2)
+    )
+    # set values from inbound grid
+    padded[pad:-pad, pad:-pad] = grid
+
+    # shift the grid by 4 pixels in each direction to fill in the padded region
+    for xorigin in [0, pad * 2]:
+        for yorigin in [0, pad * 2]:
+            xslice = slice(xorigin, xorigin + grid.shape[0])
+            yslice = slice(yorigin, yorigin + grid.shape[1])
+            padded[xslice, yslice] = np.ma.where(
+                np.logical_and(padded[xslice, yslice].mask, ~grid.mask),
+                grid,
+                padded[xslice, yslice],
+            )
+
+    return padded[pad:-pad, pad:-pad]

tests/grid/test_util.py

@@ -0,0 +1,14 @@
+"""Test pyiem.grid.util"""
+
+import numpy as np
+
+from pyiem.grid.util import grid_smear
+
+
+def test_grid_smear():
+    """Test the smearing."""
+    grid = np.ma.ones((10, 10)) * np.arange(10)
+    # set value at 8,8 to missing
+    grid[8, 8] = np.ma.masked
+    grid2 = grid_smear(grid)
+    assert grid2[8, 8] == 4