Add note on loss of precision.

scikits/image/color/colorconv.py

@@ -31,6 +31,15 @@ def rgb2hsv(rgb):
     ValueError
         If `rgb` is not a 3-D array of shape (.., .., 3).
 
+    Notes
+    -----
+    Conversion between RGB and HSV color spaces results in some loss of
+    precision, due to integer arithmetic and rounding [1]_.
+
+    References
+    ----------
+    .. [1] http://en.wikipedia.org/wiki/HSL_and_HSV
+
     Examples
     --------
     >>> import os
@@ -102,6 +111,15 @@ def hsv2rgb(hsv):
     ValueError
         If `hsv` is not a 3-D array of shape (.., .., 3).
 
+    Notes
+    -----
+    Conversion between RGB and HSV color spaces results in some loss of
+    precision, due to integer arithmetic and rounding [1]_.
+
+    References
+    ----------
+    .. [1] http://en.wikipedia.org/wiki/HSL_and_HSV
+
     Examples
     --------
     >>> import os

scikits/image/color/tests/test_colorconv.py

@@ -54,7 +54,7 @@ class TestColorconv(TestCase):
         hsv = np.array([colorsys.rgb_to_hsv(pt[0], pt[1], pt[2])
                         for pt in rgb.reshape(-1, 3)]).reshape(rgb.shape)
         # convert back to RGB and compare with original.
-        # float32 -> relative precision about 1e-6
+        # relative precision for RGB -> HSV roundtrip is about 1e-6
         assert_almost_equal(rgb, hsv2rgb(hsv), decimal=4)
 
     def test_hsv2rgb_error_grayscale(self):