Merge pull request #1687 from connellybarnes/master

fix scikit-image bug #1680 (grayscale png round trip fails) and add u…

skimage/io/_plugins/pil_plugin.py

@@ -87,6 +87,9 @@ def pil_to_ndarray(im, dtype=None, img_num=None):
             i += 1
             continue
 
+        if im.format == 'PNG' and im.mode == 'I' and dtype is None:
+            dtype = 'uint16'
+
         if im.mode == 'P':
             if grayscale is None:
                 grayscale = _palette_is_grayscale(im)

skimage/io/tests/test_pil.py

@@ -6,7 +6,7 @@ from numpy.testing import (
 
 from tempfile import NamedTemporaryFile
 
-from ... import data_dir
+from ... import data_dir, img_as_float
 from .. import imread, imsave, use_plugin, reset_plugins
 from ..._shared.testing import mono_check, color_check
 from ..._shared._warnings import expected_warnings
@@ -39,6 +39,15 @@ def setup_module(self):
     except ImportError:
         pass
 
+def test_png_round_trip():
+    f = NamedTemporaryFile(suffix='.png')
+    fname = f.name
+    f.close()
+    I = np.eye(3)
+    imsave(fname, I)
+    Ip = img_as_float(imread(fname))
+    os.remove(fname)
+    assert np.sum(np.abs(Ip-I)) < 1e-3
 
 def test_imread_flatten():
     # a color image is flattened