Fixed RGB<->XYZ conversion.

Now correctly follows the algorithm from EasyRGB.
Also fixed a XYZ<->Lab conversion issue by adding
a .copy().

skimage/color/colorconv.py

@@ -379,8 +379,14 @@ 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?X=MATH&H=01#text1
+    # except we don't multiply/divide by 100 in the conversion
+    arr = _prepare_colorarray(xyz)
+    arr = _convert(rgb_from_xyz, arr)
+    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):
     """RGB to XYZ color space conversion.
@@ -415,7 +421,13 @@ 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?X=MATH&H=02#text2
+    # 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):
@@ -594,7 +606,7 @@ def xyz2lab(xyz):
     >>> lena_xyz = rgb2xyz(lena)
     >>> lena_lab = xyz2lab(lena_xyz)
     """
-    arr = _prepare_colorarray(xyz)
+    arr = _prepare_colorarray(xyz).copy()
 
     # scale by CIE XYZ tristimulus values of the reference white point
     arr = arr / lab_ref_white

skimage/color/tests/test_colorconv.py

@@ -113,10 +113,14 @@ class TestColorconv(TestCase):
 
     # 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],