Prevent infinite loop in adapthist for low clip_limit

Prevent infinite loop in adapthist

Add a test that exposed the infinite loop

Use better check to prevent endless loop

skimage/exposure/_adapthist.py

@@ -244,6 +244,8 @@ def clip_histogram(hist, clip_limit):
     n_excess -= mid.size * clip_limit - mid.sum()
     hist[mid_mask] = clip_limit
 
+    prev_n_excess = n_excess
+
     while n_excess > 0:  # Redistribute remaining excess
         index = 0
         while n_excess > 0 and index < hist.size:
@@ -256,6 +258,10 @@ def clip_histogram(hist, clip_limit):
             hist[under_mask] += 1
             n_excess -= under_mask.sum()
             index += 1
+        # bail if we have not distributed any excess
+        if prev_n_excess == n_excess:
+            break
+        prev_n_excess = n_excess
 
     return hist
 

skimage/exposure/tests/test_exposure.py

@@ -213,7 +213,7 @@ def test_adapthist_grayscale():
     img = np.dstack((img, img, img))
     with expected_warnings(['precision loss|non-contiguous input', 
                             'deprecated']):
-        adapted_old = exposure.equalize_adapthist(img, 10, 9, clip_limit=0.01,
+        adapted_old = exposure.equalize_adapthist(img, 10, 9, clip_limit=0.001,
                                                   nbins=128)
         adapted = exposure.equalize_adapthist(img, kernel_size=(57, 51), clip_limit=0.01, nbins=128)
     assert img.shape == adapted.shape