Address review findings: fix _composite alpha, tighten comment (#3640)

- animated_hillshade.py: _composite had a dead alpha branch (line 120
  reassigned base to RGB, so the base.shape[-1]==4 guard on line 121
  was always false and output alpha was hard-coded to 255). Rewrite to
  keep rgb separate and rebuild base with np.concatenate, matching the
  _stack helper used in the notebooks. Also add xrspatial to the
  requires line in the docstring.
- pharmacy-deserts.ipynb: sharpen the merge='last' comment to state the
  non-overlap assumption (each pixel covered by exactly one polygon)
  that makes it equivalent to ds.mean.

Author: Melissari1997

examples/animated_hillshade.py

@@ -3,7 +3,7 @@
 Generates a synthetic terrain, places trees and water on it, and renders a
 rotating-hillshade GIF plus a composited build-up GIF.
 
-Requires: numpy, xarray, matplotlib, pillow.
+Requires: numpy, xarray, matplotlib, pillow, xrspatial.
 """
 from functools import partial
 
@@ -117,10 +117,9 @@ def _composite(layers):
     base = layers[0].astype(np.float64)
     for top in layers[1:]:
         a = (top[..., 3:4] / 255.0)
-        base = top[..., :3] * a + base[..., :3] * (1 - a)
-        base_a = np.maximum(top[..., 3:4], base[..., 3:4] if base.shape[-1] == 4
-                            else np.full_like(a, 255))
-        base = np.concatenate([base, base_a * 255], axis=-1)
+        rgb = top[..., :3] * a + base[..., :3] * (1 - a)
+        out_a = np.maximum(top[..., 3:4], base[..., 3:4])
+        base = np.concatenate([rgb, out_a], axis=-1)
     return np.clip(base, 0, 255).astype(np.uint8)
 
 

examples/pharmacy-deserts.ipynb

@@ -320,7 +320,7 @@
    "outputs": [],
    "source": [
     "# Rasterize the block-group polygons with their PCT_ABOVE_65 attribute.\n",
-    "# Block groups tile the plane (non-overlapping), so merge='last' is\n",
+    "# Block groups tile the plane (non-overlapping: each pixel is\n    # covered by exactly one polygon), so merge='last' is\n",
     "# equivalent to datashader's ds.mean('PCT_ABOVE_65') here.\n",
     "age_raster = template.xrs.rasterize(\n",
     "    blockgroup_df, column='PCT_ABOVE_65', merge='last',\n",