Add handling for images with negative integers

skimage/thresholding/tests/test_thresholding.py

@@ -16,10 +16,9 @@ class TestSimpleImage():
     def test_otsu(self):
         assert threshold_otsu(self.image) == 2
 
-    @np.testing.raises(NotImplementedError)
-    def test_otsu_raises_error(self):
+    def test_otsu_negative_int(self):
         image = self.image - 2
-        threshold_otsu(image)
+        assert threshold_otsu(image) == 0
 
     def test_otsu_float_image(self):
         image = np.float64(self.image)

skimage/thresholding/thresholding.py

@@ -89,11 +89,11 @@ def histogram(image, bins):
         The values at the center of the bins.
     """
     if np.issubdtype(image.dtype, np.integer):
+        offset = 0
         if np.min(image) < 0:
-            msg = "Images with negative values not allowed"
-            raise NotImplementedError(msg)
-        hist = np.bincount(image.flat)
-        bin_centers = np.arange(len(hist))
+            offset = np.min(image)
+        hist = np.bincount(image.ravel() - offset)
+        bin_centers = np.arange(len(hist)) + offset
 
         # clip histogram to return only non-zero bins
         idx = np.nonzero(hist)[0][0]