Merge pull request #1919 from ahojnnes/rgb2gray

Speedup rgb2gray and return congiguous array

skimage/color/colorconv.py

@@ -707,10 +707,18 @@ def rgb2gray(rgb):
     >>> img = data.astronaut()
     >>> img_gray = rgb2gray(img)
     """
-    if rgb.ndim == 2:
-        return rgb
 
-    return _convert(gray_from_rgb, rgb[:, :, :3])[..., 0]
+    if rgb.ndim == 2:
+        return np.ascontiguousarray(rgb)
+
+    rgb = _prepare_colorarray(rgb[..., :3])
+
+    gray = 0.2125 * rgb[..., 0]
+    gray[:] += 0.7154 * rgb[..., 1]
+    gray[:] += 0.0721 * rgb[..., 2]
+
+    return gray
+
 
 rgb2grey = rgb2gray
 

skimage/color/tests/test_colorconv.py

@@ -226,6 +226,15 @@ class TestColorconv(TestCase):
 
         assert_equal(g.shape, (1, 1))
 
+    def test_rgb2grey_contiguous(self):
+        x = np.random.rand(10, 10, 3)
+        assert rgb2grey(x).flags["C_CONTIGUOUS"]
+        assert rgb2grey(x[:5, :5]).flags["C_CONTIGUOUS"]
+
+    def test_rgb2grey_alpha(self):
+        x = np.random.rand(10, 10, 4)
+        assert rgb2grey(x).ndim == 2
+
     def test_rgb2grey_on_grey(self):
         rgb2grey(np.random.rand(5, 5))