Make sure padding size is not negative (#1792)

* Make sure padding size is not negative

If Resize maintains aspect ratio, could be the case that image shape is bigger than resize value.

* Test pad shape smaller than image shape

* Fix lint

* Remove whitespaces

mmcv/image/geometric.py

@@ -460,7 +460,6 @@ def impad(img,
             areas when padding_mode is 'constant'. Default: 0.
         padding_mode (str): Type of padding. Should be: constant, edge,
             reflect or symmetric. Default: constant.
-
             - constant: pads with a constant value, this value is specified
               with pad_val.
             - edge: pads with the last value at the edge of the image.
@@ -479,7 +478,9 @@ def impad(img,
 
     assert (shape is not None) ^ (padding is not None)
     if shape is not None:
-        padding = (0, 0, shape[1] - img.shape[1], shape[0] - img.shape[0])
+        width = max(shape[1] - img.shape[1], 0)
+        height = max(shape[0] - img.shape[0], 0)
+        padding = (0, 0, width, height)
 
     # check pad_val
     if isinstance(pad_val, tuple):

tests/test_image/test_geometric.py

@@ -461,6 +461,10 @@ def test_impad(self):
         with pytest.raises(AssertionError):
             mmcv.impad(img, shape=(12, 15), padding=(0, 0, 5, 2))
 
+        # Pad shape smaller than image shape
+        padded_img = mmcv.impad(img, shape=(8, 8))
+        assert padded_img.shape == (10, 10, 3)
+
     def test_impad_to_multiple(self):
         img = np.random.rand(11, 14, 3).astype(np.float32)
         padded_img = mmcv.impad_to_multiple(img, 4)