Test 90deg rotation for similarity transform

skimage/transform/_geometric.py

@@ -620,9 +620,9 @@ class SimilarityTransform(ProjectiveTransform):
 
     @property
     def scale(self):
-        if math.cos(self.rotation) == 0:
+        if abs(math.cos(self.rotation)) < np.spacing(1):
             # sin(self.rotation) == 1
-            scale = self.params[0, 1]
+            scale = self.params[1, 0]
         else:
             scale = self.params[0, 0] / math.cos(self.rotation)
         return scale

skimage/transform/tests/test_geometric.py

@@ -99,6 +99,17 @@ def test_similarity_init():
     assert_array_almost_equal(tform.translation, translation)
 
 
+    # test special case for scale if rotation=90deg
+    scale = 0.1
+    rotation = np.pi / 2
+    translation = (1, 1)
+    tform = SimilarityTransform(scale=scale, rotation=rotation,
+                                translation=translation)
+    assert_array_almost_equal(tform.scale, scale)
+    assert_array_almost_equal(tform.rotation, rotation)
+    assert_array_almost_equal(tform.translation, translation)
+
+
 def test_affine_estimation():
     # exact solution
     tform = estimate_transform('affine', SRC[:3, :], DST[:3, :])