Densify the geostationary bounding polygon before reprojecting in AreaSlicer

pyresample/slicer.py

@@ -175,7 +175,18 @@ def get_polygon_to_contain(self):
             x, y = self.area_to_contain.get_edge_lonlats(vertices_per_side=10)
         if self.area_to_crop.is_geostationary:
             x_geos, y_geos = get_geostationary_bounding_box_in_proj_coords(self.area_to_crop, 360)
-            x_geos, y_geos = self._source_transformer.transform(x_geos, y_geos, direction=TransformDirection.INVERSE)
+            geos_poly = Polygon(zip(x_geos, y_geos, strict=True))
+            # Densify the polygon before reprojecting it to the destination crs.
+            # Clipping a partial disk (for example rapid scan or region of interest
+            # data) to its area extent introduces straight chord edges. Those chords
+            # are not straight in the destination crs, so without intermediate
+            # vertices the reprojected polygon cuts the corner and the computed slice
+            # is too small, leaving part of the destination without data.
+            segment_length = np.max(np.abs(self.area_to_crop.area_extent)) / 100
+            geos_poly = geos_poly.segmentize(segment_length)
+            x_geos, y_geos = geos_poly.exterior.coords.xy
+            x_geos, y_geos = self._source_transformer.transform(
+                np.asarray(x_geos), np.asarray(y_geos), direction=TransformDirection.INVERSE)
             geos_poly = Polygon(zip(x_geos, y_geos, strict=True))
             poly = Polygon(zip(x, y, strict=True))
             poly = poly.intersection(geos_poly)

pyresample/test/test_slicer.py

@@ -64,6 +64,25 @@ def test_source_area_does_not_cover_dest_area_entirely(self):
         assert x_slice.start > 0 and x_slice.stop < 100
         assert y_slice.start > 0 and y_slice.stop >= 100
 
+    def test_partial_geostationary_disk_covering_dest_area_is_not_truncated(self):
+        """Test that a partial geostationary disk covering the destination is not truncated.
+
+        The southern boundary of a partial disk is a straight chord in the source
+        projection. It has to be densified before being reprojected to the
+        destination crs, otherwise the computed source slice stops short and part
+        of the destination is left without data.
+        """
+        src_area = AreaDefinition('rss', 'rss area', None,
+                                  {'ellps': 'WGS84', 'h': '35785831', 'proj': 'geos', 'lon_0': 9.5},
+                                  3712, 1392,
+                                  (5550000.0, 5550000.0, -5550000.0, 2000000.0))
+        slicer = create_slicer(src_area, self.dst_area)
+        x_slice, y_slice = slicer.get_slices()
+        # The whole destination falls inside the strip; its southern edge maps to
+        # source row 584, so the slice must reach well below the strip top rows.
+        assert y_slice.start < 700
+        assert y_slice.stop >= 1328
+
     def test_source_area_does_not_cover_dest_area_at_all(self):
         """Test source area does not cover dest area at all."""
         src_area = AreaDefinition('dst', 'dst area', None,