Merge pull request scikit-image#378 from sergeyk/master

Fixed RGB<->XYZ conversion (was incorrect before).

skimage/color/colorconv.py

@@ -79,7 +79,6 @@ def is_gray(image):
     return image.ndim == 2
 
 
-
 def convert_colorspace(arr, fromspace, tospace):
     """Convert an image array to a new color space.
 
@@ -288,8 +287,8 @@ def hsv2rgb(hsv):
 
 # From sRGB specification
 xyz_from_rgb = np.array([[0.412453, 0.357580, 0.180423],
-                          [0.212671, 0.715160, 0.072169],
-                          [0.019334, 0.119193, 0.950227]])
+                        [0.212671, 0.715160, 0.072169],
+                        [0.019334, 0.119193, 0.950227]])
 
 rgb_from_xyz = linalg.inv(xyz_from_rgb)
 
@@ -379,7 +378,13 @@ def xyz2rgb(xyz):
     >>> lena_xyz = rgb2xyz(lena)
     >>> lena_rgb = xyz2rgb(lena_xyz)
     """
-    return _convert(rgb_from_xyz, xyz)
+    # Follow the algorithm from http://www.easyrgb.com/index.php
+    # except we don't multiply/divide by 100 in the conversion
+    arr = _convert(rgb_from_xyz, xyz)
+    mask = arr > 0.0031308
+    arr[mask] = 1.055 * np.power(arr[mask], 1 / 2.4) - 0.055
+    arr[~mask] *= 12.92
+    return arr
 
 
 def rgb2xyz(rgb):
@@ -415,7 +420,13 @@ def rgb2xyz(rgb):
     >>> lena = data.lena()
     >>> lena_xyz = rgb2xyz(lena)
     """
-    return _convert(xyz_from_rgb, rgb)
+    # Follow the algorithm from http://www.easyrgb.com/index.php
+    # except we don't multiply/divide by 100 in the conversion
+    arr = _prepare_colorarray(rgb).copy()
+    mask = arr > 0.04045
+    arr[mask] = np.power((arr[mask] + 0.055) / 1.055, 2.4)
+    arr[~mask] /= 12.92
+    return _convert(xyz_from_rgb, arr)
 
 
 def rgb2rgbcie(rgb):

skimage/color/tests/test_colorconv.py

@@ -113,10 +113,14 @@ def test_rgb2xyz_error_one_element(self):
 
     # XYZ to RGB
     def test_xyz2rgb_conversion(self):
-        # only roundtrip test, we checked rgb2xyz above already
         assert_almost_equal(xyz2rgb(rgb2xyz(self.colbars_array)),
                             self.colbars_array)
 
+    # RGB<->XYZ roundtrip on another image
+    def test_xyz_rgb_roundtrip(self):
+        img_rgb = img_as_float(self.img_rgb)
+        assert_array_almost_equal(xyz2rgb(rgb2xyz(img_rgb)), img_rgb)
+
     # RGB to RGB CIE
     def test_rgb2rgbcie_conversion(self):
         gt = np.array([[[ 0.1488856 ,  0.18288098,  0.19277574],
@@ -175,11 +179,29 @@ def test_lab2xyz(self):
         assert_array_almost_equal(lab2xyz(self.lab_array),
                                   self.xyz_array, decimal=3)
 
+    def test_rgb2lab_brucelindbloom(self):
+        """
+        Test the RGB->Lab conversion by comparing to the calculator on the
+        authoritative Bruce Lindbloom
+        [website](http://brucelindbloom.com/index.html?ColorCalculator.html).
+        """
+        # Obtained with D65 white point, sRGB model and gamma
+        gt_for_colbars = np.array([
+            [100,0,0],
+            [97.1393, -21.5537, 94.4780],
+            [91.1132, -48.0875, -14.1312],
+            [87.7347, -86.1827, 83.1793],
+            [60.3242, 98.2343, -60.8249],
+            [53.2408, 80.0925, 67.2032],
+            [32.2970, 79.1875, -107.8602],
+            [0,0,0]]).T
+        gt_array = np.swapaxes(gt_for_colbars.reshape(3, 4, 2), 0, 2)
+        assert_array_almost_equal(rgb2lab(self.colbars_array), gt_array, decimal=2)
+
     def test_lab_rgb_roundtrip(self):
         img_rgb = img_as_float(self.img_rgb)
         assert_array_almost_equal(lab2rgb(rgb2lab(img_rgb)), img_rgb)
 
-
 def test_gray2rgb():
     x = np.array([0, 0.5, 1])
     assert_raises(ValueError, gray2rgb, x)

skimage/segmentation/tests/test_quickshift.py

@@ -34,10 +34,10 @@ def test_color():
     seg = quickshift(img, random_seed=0, max_dist=30, kernel_size=10, sigma=0)
     # we expect 4 segments:
     assert_equal(len(np.unique(seg)), 4)
-    assert_array_equal(seg[:10, :10], 0)
-    assert_array_equal(seg[10:, :10], 3)
-    assert_array_equal(seg[:10, 10:], 1)
-    assert_array_equal(seg[10:, 10:], 2)
+    assert_array_equal(seg[:10, :10], 1)
+    assert_array_equal(seg[10:, :10], 2)
+    assert_array_equal(seg[:10, 10:], 0)
+    assert_array_equal(seg[10:, 10:], 3)
 
     seg2 = quickshift(img, kernel_size=1, max_dist=2, random_seed=0,
             convert2lab=False, sigma=0)