Merge pull request #367 from stefanv/gray2rgb_for_rgb

BUG: Allow RGB images to be passed to grey2rgb.

skimage/color/colorconv.py

@@ -45,7 +45,7 @@ from __future__ import division
 
 __all__ = ['convert_colorspace', 'rgb2hsv', 'hsv2rgb', 'rgb2xyz', 'xyz2rgb',
            'rgb2rgbcie', 'rgbcie2rgb', 'rgb2grey', 'rgb2gray', 'gray2rgb',
-           'xyz2lab', 'lab2xyz', 'lab2rgb', 'rgb2lab'
+           'xyz2lab', 'lab2xyz', 'lab2rgb', 'rgb2lab', 'is_rgb', 'is_gray'
            ]
 
 __docformat__ = "restructuredtext en"
@@ -55,6 +55,31 @@ from scipy import linalg
 from ..util import dtype
 
 
+def is_rgb(image):
+    """Test whether the image is RGB or RGBA.
+
+    Parameters
+    ----------
+    image : ndarray
+        Input image.
+
+    """
+    return (image.ndim == 3 and image.shape[2] in (3, 4))
+
+
+def is_gray(image):
+    """Test whether the image is gray (i.e. has only one color band).
+
+    Parameters
+    ----------
+    image : ndarray
+        Input image.
+
+    """
+    return image.ndim == 2
+
+
+
 def convert_colorspace(arr, fromspace, tospace):
     """Convert an image array to a new color space.
 
@@ -281,7 +306,7 @@ rgbcie_from_rgb = np.dot(rgbcie_from_xyz, xyz_from_rgb)
 rgb_from_rgbcie = np.dot(rgb_from_xyz, xyz_from_rgbcie)
 
 
-grey_from_rgb = np.array([[0.2125, 0.7154, 0.0721],
+gray_from_rgb = np.array([[0.2125, 0.7154, 0.0721],
                           [0, 0, 0],
                           [0, 0, 0]])
 
@@ -458,7 +483,7 @@ def rgbcie2rgb(rgbcie):
     return _convert(rgb_from_rgbcie, rgbcie)
 
 
-def rgb2grey(rgb):
+def rgb2gray(rgb):
     """Compute luminance of an RGB image.
 
     Parameters
@@ -475,7 +500,7 @@ def rgb2grey(rgb):
     Raises
     ------
     ValueError
-        If `rgb2grey` is not a 3-D array of shape (.., .., 3) or
+        If `rgb2gray` is not a 3-D array of shape (.., .., 3) or
         (.., .., 4).
 
     References
@@ -493,21 +518,21 @@ def rgb2grey(rgb):
 
     Examples
     --------
-    >>> from skimage.color import rgb2grey
+    >>> from skimage.color import rgb2gray
     >>> from skimage import data
     >>> lena = data.lena()
-    >>> lena_grey = rgb2grey(lena)
+    >>> lena_gray = rgb2gray(lena)
     """
     if rgb.ndim == 2:
         return rgb
 
-    return _convert(grey_from_rgb, rgb[:, :, :3])[..., 0]
+    return _convert(gray_from_rgb, rgb[:, :, :3])[..., 0]
 
-rgb2gray = rgb2grey
+rgb2grey = rgb2gray
 
 
 def gray2rgb(image):
-    """Create an RGB representation of a grey-level image.
+    """Create an RGB representation of a gray-level image.
 
     Parameters
     ----------
@@ -525,11 +550,12 @@ def gray2rgb(image):
         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))
+    if is_rgb(image):
+        return image
+    elif is_gray(image):
+        return np.dstack((image, image, image))
+    else:
+        raise ValueError("Input image expected to be RGB, RGBA or gray.")
 
 
 def xyz2lab(xyz):

skimage/color/tests/test_colorconv.py

@@ -25,10 +25,11 @@ from skimage.color import (
     convert_colorspace,
     rgb2grey, gray2rgb,
     xyz2lab, lab2xyz,
-    lab2rgb, rgb2lab
+    lab2rgb, rgb2lab,
+    is_rgb, is_gray
     )
 
-from skimage import data_dir
+from skimage import data_dir, data
 
 import colorsys
 
@@ -196,5 +197,23 @@ def test_gray2rgb():
     assert_equal(z[..., 0], x)
     assert_equal(z[0, 1, :], [128, 128, 128])
 
+
+def test_gray2rgb_rgb():
+    x = np.random.random((5, 5, 4))
+    y = gray2rgb(x)
+    assert_equal(x, y)
+
+
+def test_is_rgb():
+    color = data.lena()
+    gray = data.camera()
+
+    assert is_rgb(color)
+    assert not is_gray(color)
+
+    assert is_gray(gray)
+    assert not is_gray(color)
+
+
 if __name__ == "__main__":
     run_module_suite()