Add RGB to XYZ bidi conversion.

scikits/image/color/colorconv.py

@@ -1,17 +1,31 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
-"""
-:author: Nicolas Pinto, 2009
+"""Functions for converting between color spaces.
+
+Supported color spaces
+----------------------
+- RGB
+- HSV
+- XYZ
+
+Authors
+-------
+- rgb2hsv was written by Nicolas Pinto
+- hsv2rgb was written by Ralf Gommers
+- other functions were originally written by Travis Oliphant and adapted by
+  Ralf Gommers
+
 :license: modified BSD
 """
 
 from __future__ import division
 
-__all__ = ['rgb2hsv', 'hsv2rgb']
+__all__ = ['rgb2hsv', 'hsv2rgb', 'rgb2xyz', 'xyz2rgb']
 __docformat__ = "restructuredtext en"
 
 import numpy as np
+from scipy import linalg
 
 
 def _prepare_colorarray(arr, dtype="float32"):
@@ -155,3 +169,59 @@ def hsv2rgb(hsv):
     out[np.isnan(out)] = 0
 
     return out
+
+
+#---------------------------------------------------------------
+# Primaries for the coordinate systems
+#---------------------------------------------------------------
+cie_primaries = [700, 546.1, 435.8]
+sb_primaries = [1./155 * 1e5, 1./190 * 1e5, 1./225 * 1e5]
+
+#---------------------------------------------------------------
+# Matrices that define conversion between different color spaces
+#---------------------------------------------------------------
+
+# From sRGB specification
+xyz_from_rgb =  [[0.412453, 0.357580, 0.180423],
+                 [0.212671, 0.715160, 0.072169],
+                 [0.019334, 0.119193, 0.950227]]
+
+rgb_from_xyz = linalg.inv(xyz_from_rgb)
+
+#-------------------------------------------------------------
+# The conversion functions that make use of the matrices above
+#-------------------------------------------------------------
+
+def _convert(matrix, arr):
+    """Do the color space conversion.
+
+    Parameters
+    ----------
+    matrix : array_like
+        The 3x3 matrix to use.
+    arr : ndarray
+        The input array.
+
+    Returns
+    -------
+    out : ndarray
+        The converted array.
+    """
+    arr = _prepare_colorarray(arr)
+    arr = np.swapaxes(arr, 0, 2)
+    oldshape = arr.shape
+    arr = np.reshape(arr, (3, -1))
+    out = np.dot(matrix, arr)
+    out.shape = oldshape
+    out = np.swapaxes(out, 2, 0)
+
+    return out
+
+
+def xyz2rgb(xyz):
+    return _convert(rgb_from_xyz, xyz)
+
+def rgb2xyz(rgb):
+    return _convert(xyz_from_rgb, rgb)
+
+

scikits/image/color/tests/test_colorconv.py

@@ -1,8 +1,13 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
-"""
-:author: Nicolas Pinto, 2009
+"""Tests for color conversion functions.
+
+Authors
+-------
+- the rgb2hsv test was written by Nicolas Pinto, 2009
+- other tests written by Ralf Gommers, 2009
+
 :license: modified BSD
 """
 
@@ -15,6 +20,8 @@ from scikits.image.io import imread
 from scikits.image.color import (
     rgb2hsv,
     hsv2rgb,
+    rgb2xyz,
+    xyz2rgb
     )
 
 from scikits.image import data_dir
@@ -27,6 +34,13 @@ class TestColorconv(TestCase):
     img_rgb = imread(os.path.join(data_dir, 'color.png'))
     img_grayscale = imread(os.path.join(data_dir, 'camera.png'))
 
+    colbars = np.array([[1, 1, 0, 0, 1, 1, 0, 0],
+                        [1, 1, 1, 1, 0, 0, 0, 0],
+                        [1, 0, 1, 0, 1, 0, 1, 0]])
+    colbars_array = np.swapaxes(colbars.reshape(3, 4, 2), 0, 2)
+    colbars_point75 = colbars * 0.75
+    colbars_point75_array = np.swapaxes(colbars_point75.reshape(3, 4, 2), 0, 2)
+
     # RGB to HSV
     def test_rgb2hsv_conversion(self):
         rgb = self.img_rgb.astype("float32")[::16, ::16]
@@ -67,6 +81,38 @@ class TestColorconv(TestCase):
         self.assertRaises(TypeError, hsv2rgb, [self.img_rgb[0,0]])
 
 
+    # RGB to XYZ
+    def test_rgb2xyz_conversion(self):
+        gt = np.array([[[ 0.950456,  1.      ,  1.088754],
+                        [ 0.538003,  0.787329,  1.06942 ],
+                        [ 0.592876,  0.28484 ,  0.969561],
+                        [ 0.180423,  0.072169,  0.950227]],
+                       [[ 0.770033,  0.927831,  0.138527],
+                        [ 0.35758 ,  0.71516 ,  0.119193],
+                        [ 0.412453,  0.212671,  0.019334],
+                        [ 0.      ,  0.      ,  0.      ]]])
+
+        assert_almost_equal(rgb2xyz(self.colbars_array), gt)
+
+    # stop repeating the "raises" checks for all other functions that are
+    # implemented with color._convert()
+    def test_rgb2xyz_error_grayscale(self):
+        self.assertRaises(ValueError, rgb2xyz, self.img_grayscale)
+
+    def test_rgb2xyz_error_one_element(self):
+        self.assertRaises(ValueError, rgb2xyz, self.img_rgb[0,0])
+
+    def test_rgb2xyz_error_list(self):
+        self.assertRaises(TypeError, rgb2xyz, [self.img_rgb[0,0]])
+
+
+    # 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)
+
+
 if __name__ == "__main__":
     run_module_suite()