Merge 8a3ff74 into ffdf4d4

regions/shapes/polygon.py

@@ -59,7 +59,17 @@ def __init__(self, vertices, meta=None, visual=None):
     @property
     def area(self):
         """Region area (float)."""
-        raise NotImplementedError
+        # See https://stackoverflow.com/questions/24467972
+
+        # Use offsets to improve numerical precision
+        x_ = self.vertices.x - self.vertices.x.mean()
+        y_ = self.vertices.y - self.vertices.y.mean()
+
+        # Shoelace formula, for our case where the start vertex
+        # isn't duplicated at the end, written to avoid an array copy
+        area_main = np.dot(x_[:-1], y_[1:]) - np.dot(y_[:-1], x_[1:])
+        area_last = x_[-1] * y_[0] - y_[-1] * x_[0]
+        return 0.5 * np.abs(area_main + area_last)
 
     def contains(self, pixcoord):
         pixcoord = PixCoord._validate(pixcoord, 'pixcoord')

regions/shapes/tests/test_common.py

@@ -7,7 +7,6 @@
 from numpy.testing import assert_equal, assert_allclose
 import pytest
 
-from ..polygon import PolygonPixelRegion
 from ...core import PixCoord, BoundingBox
 
 class BaseTestRegion(object):
@@ -22,10 +21,6 @@ def test_str(self):
 class BaseTestPixelRegion(BaseTestRegion):
 
     def test_area(self):
-        # TODO: remove the pytest.skip once polygon area is implemented
-        if isinstance(self.reg, PolygonPixelRegion):
-            pytest.skip()
-
         assert_allclose(self.reg.area, self.expected_area)
 
     def test_mask_area(self):