Fix imread so palette images are converted to grayscale or RGB images

scikits/image/io/pil_imread.py

@@ -34,6 +34,34 @@ def imread(fname, flatten=False, dtype=None):
                           " instructions.")
 
     im = Image.open(fname)
+    if im.mode == 'P':
+        if palette_is_grayscale(im):
+            im = im.convert('L')
+        else:
+            im = im.convert('RGB')
     if flatten and not im.mode in ('1', 'L', 'I', 'F', 'I;16', 'I;16L', 'I;16B'):
         im = im.convert('F')
     return np.array(im, dtype=dtype)
+
+
+def palette_is_grayscale(pil_image):
+    """Return True if PIL image is grayscale.
+    
+    Parameters
+    ----------
+    pil_image : PIL image
+        PIL Image that is in Palette mode.
+    
+    Returns
+    -------
+    is_grayscale : bool
+        True if all colors in image palette are gray.
+    """
+    assert pil_image.mode == 'P'
+    # get palette as an array with R, G, B columns
+    palette = np.asarray(pil_image.getpalette()).reshape((256, 3))
+    # Not all palette colors are used; unused colors have junk values.
+    start, stop = pil_image.getextrema()
+    valid_palette = palette[start:stop]
+    # Image is grayscale if channel differences (R - G and G - B) are all zero.
+    return np.allclose(np.diff(valid_palette), 0)

scikits/image/io/tests/test_imread.py

@@ -15,3 +15,9 @@ def test_imread_flatten():
 def test_imread_dtype():
     img = imread(os.path.join(data_dir, 'camera.png'), dtype=np.float64)
     assert img.dtype == np.float64
+
+def test_imread_palette():
+    img = imread(os.path.join(data_dir, 'palette_gray.png'))
+    assert img.ndim == 2
+    img = imread(os.path.join(data_dir, 'palette_color.png'))
+    assert img.ndim == 3