fix transformation union and add test case

skimage/transform/geometric.py

@@ -106,13 +106,17 @@ class GeometricTransformation(object):
         return geometric_transform(coords, self.inverse_matrix)
 
     def union(self, other):
-        return GeometricTransformation(self.matrix.dot(other.matrix))
+        if type(self) == type(other):
+            transformation = self.__class__
+        else:
+            transformation = GeometricTransformation
+        return transformation(self.matrix.dot(other.matrix))
 
     def __mul__(self, other):
-        return self.union(self, other)
+        return self.union(other)
 
     def __add__(self, other):
-        return self.union(self, other)
+        return self.union(other)
 
 
 class SimilarityTransformation(GeometricTransformation):

skimage/transform/tests/test_geometric.py

@@ -122,6 +122,27 @@ def test_polynomial():
     assert_array_almost_equal(tform.forward(SRC), DST, 6)
 
 
+def test_union():
+    tform1 = SimilarityTransformation()
+    scale1 = 0.1
+    rotation1 = 1
+    translation1 = (0, 0)
+    tform1.from_params(scale1, rotation1, translation1)
+
+    tform2 = SimilarityTransformation()
+    scale2 = 0.1
+    rotation2 = 1
+    translation2 = (0, 0)
+    tform2.from_params(scale2, rotation2, translation2)
+
+    tform = tform1.union(tform2)
+    tform = tform1 + tform2
+    tform = tform1 * tform2
+
+    assert_array_almost_equal(tform.scale, scale1 * scale2)
+    assert_array_almost_equal(tform.rotation, rotation1 + rotation2)
+
+
 def test_homography():
     x = np.zeros((5,5), dtype=np.uint8)
     x[1, 1] = 255