Do not acquire GIL for corner detectors

skimage/feature/corner_cy.pyx

@@ -5,7 +5,7 @@
 import numpy as np
 cimport numpy as cnp
 from libc.float cimport DBL_MAX
-from libc.math cimport atan2
+from libc.math cimport atan2, fabs
 
 from ..util import img_as_float, pad
 from ..color import rgb2grey
@@ -67,6 +67,8 @@ def corner_moravec(image, Py_ssize_t window_size=1):
 
     cdef double msum, min_msum
     cdef Py_ssize_t r, c, br, bc, mr, mc, a, b
+
+    with nogil:
         for r in range(2 * window_size, rows - 2 * window_size):
             for c in range(2 * window_size, cols - 2 * window_size):
                 min_msum = DBL_MAX
@@ -87,7 +89,8 @@ def corner_moravec(image, Py_ssize_t window_size=1):
 
 cdef inline double _corner_fast_response(double curr_pixel,
                                          double* circle_intensities,
-                                         signed char* bins, signed char state, char n):
+                                         signed char* bins, signed char state,
+                                         char n) nogil:
     cdef char consecutive_count = 0
     cdef double curr_response
     cdef Py_ssize_t l, m
@@ -97,7 +100,7 @@ cdef inline double _corner_fast_response(double curr_pixel,
             if consecutive_count == n:
                 curr_response = 0
                 for m in range(16):
-                    curr_response += abs(circle_intensities[m] - curr_pixel)
+                    curr_response += fabs(circle_intensities[m] - curr_pixel)
                 return curr_response
         else:
             consecutive_count = 0
@@ -124,6 +127,7 @@ def _corner_fast(double[:, ::1] image, signed char n, double threshold):
 
     cdef double curr_response
 
+    with nogil:
         for i in range(3, rows - 3):
             for j in range(3, cols - 3):
 
@@ -254,6 +258,7 @@ def corner_orientations(image, Py_ssize_t[:, :] corners, mask):
     cdef double curr_pixel
     cdef double m01, m10, m01_tmp
 
+    with nogil:
         for i in range(corners.shape[0]):
             r0 = corners[i, 0]
             c0 = corners[i, 1]

skimage/feature/tests/test_corner.py

@@ -6,6 +6,7 @@ from skimage import data
 from skimage import img_as_float
 from skimage.color import rgb2gray
 from skimage.morphology import octagon
+from skimage._shared.testing import test_parallel
 
 from skimage.feature import (corner_moravec, corner_harris, corner_shi_tomasi,
                              corner_subpix, peak_local_max, corner_peaks,
@@ -99,6 +100,7 @@ def test_hessian_matrix_det():
     assert_almost_equal(det, 0, decimal = 3)
 
 
+@test_parallel()
 def test_square_image():
     im = np.zeros((50, 50)).astype(float)
     im[:25, :25] = 1.
@@ -280,6 +282,7 @@ def test_corner_fast_image_unsupported_error():
     assert_raises(ValueError, corner_fast, img)
 
 
+@test_parallel()
 def test_corner_fast_lena():
     img = rgb2gray(data.astronaut())
     expected = np.array([[101, 198],
@@ -335,6 +338,7 @@ def test_corner_orientations_even_shape_error():
                   np.asarray([[7, 7]]), np.ones((4, 4)))
 
 
+@test_parallel()
 def test_corner_orientations_lena():
     img = rgb2gray(data.lena())
     corners = corner_peaks(corner_fast(img, 11, 0.35))