Merge pull request #130 from stefanv/grey2rgb

ENH: Add grey2rgb.

skimage/color/colorconv.py

@@ -44,7 +44,7 @@ References
 from __future__ import division
 
 __all__ = ['convert_colorspace', 'rgb2hsv', 'hsv2rgb', 'rgb2xyz', 'xyz2rgb',
-           'rgb2rgbcie', 'rgbcie2rgb', 'rgb2grey', 'rgb2gray']
+           'rgb2rgbcie', 'rgbcie2rgb', 'rgb2grey', 'rgb2gray', 'gray2rgb']
 
 __docformat__ = "restructuredtext en"
 
@@ -103,15 +103,18 @@ def convert_colorspace(arr, fromspace, tospace):
     return todict[tospace](fromdict[fromspace](arr))
 
 
-def _prepare_colorarray(arr, dtype=np.float32):
-    """Check the shape of the array, and give it the requested type."""
+def _prepare_colorarray(arr):
+    """Check the shape of the array and convert it to
+    floating point representation.
+
+    """
     arr = np.asanyarray(arr)
 
     if arr.ndim != 3 or arr.shape[2] != 3:
         msg = "the input array must be have a shape == (.,.,3))"
         raise ValueError(msg)
 
-    return arr.astype(dtype)
+    return dtype.img_as_float(arr)
 
 
 def rgb2hsv(rgb):
@@ -303,7 +306,6 @@ def _convert(matrix, arr):
     out : ndarray, dtype=float
         The converted array.
     """
-    arr = dtype.img_as_float(arr)
     arr = _prepare_colorarray(arr)
     arr = np.swapaxes(arr, 0, 2)
     oldshape = arr.shape
@@ -496,7 +498,7 @@ def rgb2grey(rgb):
     CRT phosphors::
 
         Y = 0.2125 R + 0.7154 G + 0.0721 B
-        
+
     If there is an alpha channel present, it is ignored.
 
     Examples
@@ -512,3 +514,29 @@ def rgb2grey(rgb):
     return _convert(grey_from_rgb, rgb[:, :, :3])[..., 0]
 
 rgb2gray = rgb2grey
+
+
+def gray2rgb(image):
+    """Create an RGB representation of a grey-level image.
+
+    Parameters
+    ----------
+    image : array_like
+        Input image of shape ``(M, N)``.
+
+    Returns
+    -------
+    rgb : ndarray
+        RGB image of shape ``(M, N, 3)``.
+
+    Raises
+    ------
+    ValueError
+        If the input is not 2-dimensional.
+
+    """
+    if image.ndim != 2:
+        raise ValueError('Gray-level image should be two-dimensional.')
+
+    M, N = image.shape
+    return np.dstack((image, image, image))

skimage/color/tests/test_colorconv.py

@@ -16,13 +16,14 @@ import os.path
 import numpy as np
 from numpy.testing import *
 
+from skimage import img_as_float
 from skimage.io import imread
 from skimage.color import (
     rgb2hsv, hsv2rgb,
     rgb2xyz, xyz2rgb,
     rgb2rgbcie, rgbcie2rgb,
     convert_colorspace,
-    rgb2grey
+    rgb2grey, gray2rgb
     )
 
 from skimage import data_dir
@@ -44,7 +45,7 @@ class TestColorconv(TestCase):
 
     # RGB to HSV
     def test_rgb2hsv_conversion(self):
-        rgb = self.img_rgb.astype("float32")[::16, ::16]
+        rgb = img_as_float(self.img_rgb)[::16, ::16]
         hsv = rgb2hsv(rgb).reshape(-1, 3)
         # ground truth from colorsys
         gt = np.array([colorsys.rgb_to_hsv(pt[0], pt[1], pt[2])
@@ -150,6 +151,23 @@ class TestColorconv(TestCase):
 
         assert_equal(g.shape, (1, 1))
 
+def test_gray2rgb():
+    x = np.array([0, 0.5, 1])
+    assert_raises(ValueError, gray2rgb, x)
+
+    x = x.reshape((3, 1))
+    y = gray2rgb(x)
+
+    assert_equal(y.shape, (3, 1, 3))
+    assert_equal(y.dtype, x.dtype)
+
+    x = np.array([[0, 128, 255]], dtype=np.uint8)
+    z = gray2rgb(x)
+
+    assert_equal(z.shape, (1, 3, 3))
+    assert_equal(z[..., 0], x)
+    assert_equal(z[0, 1, :], [128, 128, 128])
+
 if __name__ == "__main__":
     run_module_suite()