Corrected bug in the computation of the Euler characteristic.

skimage/measure/_regionprops.py

@@ -125,7 +125,7 @@ class _RegionProperties(object):
     def euler_number(self):
         euler_array = self.filled_image != self.image
         _, num = label(euler_array, neighbors=8, return_num=True,
-                       background=-1)
+                       background=0)
         return -num + 1
 
     def extent(self):

skimage/measure/tests/test_regionprops.py

@@ -129,12 +129,12 @@ def test_equiv_diameter():
 
 def test_euler_number():
     en = regionprops(SAMPLE)[0].euler_number
-    assert en == 0
+    assert en == 1
 
     SAMPLE_mod = SAMPLE.copy()
     SAMPLE_mod[7, -3] = 0
     en = regionprops(SAMPLE_mod)[0].euler_number
-    assert en == -1
+    assert en == 0
 
 
 def test_extent():