Add method to_split in SimpleTilePainter for checking whether to spli…

…t a tile

hips/draw/simple.py

@@ -172,6 +172,29 @@ def plot_mpl_hips_tile_grid(self) -> None:
         ax.imshow(self.image, origin='lower')
 
 
+def _measure_tile_shape(corners: tuple) -> List[list]:
+    """Compute length of tile edges and diagonals."""
+    x, y = corners
+
+    def compute_distance(i: int, j: int) -> float:
+        """Compute distance between two points."""
+        return np.sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2)
+
+    edges = [compute_distance((i + 1) % 4, i) for i in range(4)]
+    diagonals = [compute_distance(0, 2), compute_distance(1, 3)]
+    ratio = float(np.min(diagonals) / np.max(diagonals))
+
+    return [edges, diagonals, ratio]
+
+def _is_tile_distorted(corners: tuple) -> bool:
+    """Implement tile splitting criteria as mentioned in :ref:`drawing_algo` page."""
+    edges, diagonals, ratio = _measure_tile_shape(corners)
+
+    return max(edges) > 300 or \
+           max(diagonals) > 150 or \
+           ratio < 0.7
+
+
 def tile_corner_pixel_coordinates(width, file_format) -> np.ndarray:
     """Tile corner pixel coordinates for projective transform.
 

hips/draw/tests/test_simple.py

@@ -5,7 +5,7 @@
 from astropy.coordinates import SkyCoord
 from astropy.tests.helper import remote_data
 from ...tiles import HipsSurveyProperties
-from ..simple import make_sky_image, SimpleTilePainter, plot_mpl_single_tile
+from ..simple import make_sky_image, SimpleTilePainter, plot_mpl_single_tile, _is_tile_distorted, _measure_tile_shape
 from ...utils.wcs import WCSGeometry
 from ...utils.testing import make_test_wcs_geometry, requires_hips_extra
 
@@ -86,6 +86,7 @@ def test_compute_matching_hips_order(self, pars):
             width=2000, height=1000, fov=pars['fov'],
             coordsys='icrs', projection='AIT',
         )
+
         simple_tile_painter = SimpleTilePainter(geometry, self.hips_survey, 'fits')
         assert simple_tile_painter.draw_hips_order == pars['order']
 
@@ -101,3 +102,28 @@ def test_draw_debug_image(self):
         tile = self.painter.tiles[3]
         image = self.painter.image
         plot_mpl_single_tile(self.geometry, tile, image)
+
+    def test_corners(self):
+        tile = self.painter.tiles[3]
+        x, y = tile.meta.skycoord_corners.to_pixel(self.geometry.wcs)
+
+        assert_allclose(x, [764.627476, 999., 764.646551, 530.26981])
+        assert_allclose(y, [300.055412, 101.107245, -97.849955, 101.105373])
+
+    def test_is_tile_distorted(self):
+        tile = self.painter.tiles[3]
+        corners = tile.meta.skycoord_corners.to_pixel(self.geometry.wcs)
+        assert _is_tile_distorted(corners) == True
+
+    def test_measure_tile_shape(self):
+        tile = self.painter.tiles[3]
+        corners = tile.meta.skycoord_corners.to_pixel(self.geometry.wcs)
+        edges, diagonals, ratio = _measure_tile_shape(corners)
+
+        edges_precomp = [307.426175, 307.417479, 307.434024, 307.41606]
+        diagonals_precomp = [397.905367, 468.73019]
+        ratio_precomp = 0.848900658905216
+
+        assert_allclose(edges_precomp, edges)
+        assert_allclose(diagonals_precomp, diagonals)
+        assert_allclose(ratio_precomp, ratio)