Implement polygon approx for annulus

glue/core/roi.py

@@ -597,14 +597,21 @@ def defined(self):
             status = False
         return status
 
-    # FIXME: We need 2 polygons, with the inner one inverted, just like subset state.
     def to_polygon(self):
         if not self.defined():
             return [], []
-        raise NotImplementedError("Cannot transform annulus to simple polygon")
+        theta = np.linspace(0, 2 * np.pi, num=20)
+        x_inner = self.xc + self.inner_radius * np.cos(theta)
+        y_inner = self.yc + self.inner_radius * np.sin(theta)
+        x_outer = self.xc + self.outer_radius * np.cos(theta)
+        y_outer = self.yc + self.outer_radius * np.sin(theta)
+        # theta=2pi=360deg=0deg --> x=r, y=0
+        x = np.concatenate((x_inner, x_outer[::-1], x_inner[0]), axis=None)
+        y = np.concatenate((y_inner, y_outer[::-1], y_inner[0]), axis=None)
+        return x, y
 
     def transformed(self, xfunc=None, yfunc=None):
-        raise NotImplementedError("Cannot transform annulus to simple polygon")
+        return PolygonalROI(*self.to_polygon()).transformed(xfunc=xfunc, yfunc=yfunc)
 
     def center(self):
         return self.xc, self.yc

glue/core/tests/test_roi.py

@@ -371,8 +371,11 @@ def test_circular_annulus_defined():
     assert_almost_equal(new_roi.outer_radius, roi.outer_radius)
 
     # test_poly
-    with pytest.raises(NotImplementedError, match="Cannot transform annulus to simple polygon"):
-        roi.to_polygon()
+    x, y = roi.to_polygon()
+    poly = PolygonalROI(vx=x, vy=y)
+    assert not poly.contains(0, 0)
+    assert poly.contains(0, 2)
+    assert not poly.contains(2, 0)  # We have to cut it at theta=0
 
     # test_reset
     assert roi.defined()