Do not acquire GIL for hough_circle

skimage/transform/_hough_transform.pyx

@@ -79,22 +79,24 @@ def _hough_circle(cnp.ndarray img,
 
         num_circle_pixels = circle_x.size
 
-        if normalize:
-            incr = 1.0 / num_circle_pixels
-        else:
-            incr = 1
+        with nogil:
 
-        # For each non zero pixel
-        for p in range(num_pixels):
-            # Plug the circle at (px, py),
-            # its coordinates are (tx, ty)
-            for c in range(num_circle_pixels):
-                tx = circle_x[c] + x[p]
-                ty = circle_y[c] + y[p]
-                if offset:
-                    acc[i, tx, ty] += incr
-                elif 0 <= tx < xmax and 0 <= ty < ymax:
-                    acc[i, tx, ty] += incr
+            if normalize:
+                incr = 1.0 / num_circle_pixels
+            else:
+                incr = 1
+
+            # For each non zero pixel
+            for p in range(num_pixels):
+                # Plug the circle at (px, py),
+                # its coordinates are (tx, ty)
+                for c in range(num_circle_pixels):
+                    tx = circle_x[c] + x[p]
+                    ty = circle_y[c] + y[p]
+                    if offset:
+                        acc[i, tx, ty] += incr
+                    elif 0 <= tx < xmax and 0 <= ty < ymax:
+                        acc[i, tx, ty] += incr
 
     return acc
 

skimage/transform/tests/test_hough_transform.py

@@ -4,6 +4,7 @@ from numpy.testing import assert_almost_equal, assert_equal
 import skimage.transform as tf
 from skimage.draw import line, circle_perimeter, ellipse_perimeter
 from skimage._shared._warnings import expected_warnings
+from skimage._shared.testing import test_parallel
 
 
 def append_desc(func, description):
@@ -129,6 +130,7 @@ def test_hough_line_peaks_num():
                                        min_angle=0, num_peaks=1)[0]) == 1
 
 
+@test_parallel()
 def test_hough_circle():
     # Prepare picture
     img = np.zeros((120, 100), dtype=int)