BUG: Fix background labelling for case when (0, 0) belongs to the background.

skimage/morphology/ccomp.pyx

@@ -155,6 +155,9 @@ def label(np.ndarray[DTYPE_t, ndim=2] input,
         raise ValueError('Neighbors must be either 4 or 8.')
 
     # Initialize the first row
+    if data[0, 0] == background:
+        link_bg(forest_p, 0, &background_node)
+
     for j in range(1, cols):
         if data[0, j] == background:
             link_bg(forest_p, j, &background_node)

skimage/morphology/tests/test_ccomp.py

@@ -61,5 +61,26 @@ class TestConnectedComponents:
                             [0,  0,  1],
                             [-1, -1, -1]])
 
+    def test_background_two_regions(self):
+        x = np.array([[0, 0, 6],
+                      [0, 0, 6],
+                      [5, 5, 5]])
+
+        assert_array_equal(label(x, background=0),
+                           [[-1, -1, 0],
+                            [-1, -1, 0],
+                            [ 1,  1, 1]])
+
+    def test_background_one_region_center(self):
+        x = np.array([[0, 0, 0],
+                      [0, 1, 0],
+                      [0, 0, 0]])
+
+        assert_array_equal(label(x, neighbors=4, background=0),
+                           [[-1, -1, -1],
+                            [-1,  0, -1],
+                            [-1, -1, -1]])
+
+
 if __name__ == "__main__":
     run_module_suite()