add iminvert (#58)

* add iminvert

* fix for flake8

* minor improvement

* add test for iminvert

* fix for flake8

* fix for flake8

* revise __init__

* revise __init__

mmcv/image/__init__.py

 from .io import imread, imwrite, imfrombytes
 from .transforms import (bgr2gray, gray2bgr, bgr2rgb, rgb2bgr, bgr2hsv,
-                         hsv2bgr, imflip, imrotate, imcrop, impad,
+                         hsv2bgr, iminvert, imflip, imrotate, imcrop, impad,
                          impad_to_multiple, imnormalize, imdenormalize,
                          imresize, imresize_like, imrescale)
 
 __all__ = [
     'imread', 'imwrite', 'imfrombytes', 'bgr2gray', 'gray2bgr', 'bgr2rgb',
-    'rgb2bgr', 'bgr2hsv', 'hsv2bgr', 'imflip', 'imrotate', 'imcrop', 'impad',
-    'impad_to_multiple', 'imnormalize', 'imdenormalize', 'imresize',
-    'imresize_like', 'imrescale'
+    'rgb2bgr', 'bgr2hsv', 'hsv2bgr', 'iminvert', 'imflip', 'imrotate',
+    'imcrop', 'impad', 'impad_to_multiple', 'imnormalize', 'imdenormalize',
+    'imresize', 'imresize_like', 'imrescale'
 ]

mmcv/image/transforms/__init__.py

-from .colorspace import bgr2gray, gray2bgr, bgr2rgb, rgb2bgr, bgr2hsv, hsv2bgr
+from .colorspace import (bgr2gray, gray2bgr, bgr2rgb, rgb2bgr, bgr2hsv,
+                         hsv2bgr, iminvert)
 from .geometry import imflip, imrotate, imcrop, impad, impad_to_multiple
 from .normalize import imnormalize, imdenormalize
 from .resize import imresize, imresize_like, imrescale
 
 __all__ = [
     'bgr2gray', 'gray2bgr', 'bgr2rgb', 'rgb2bgr', 'bgr2hsv', 'hsv2bgr',
-    'imflip', 'imrotate', 'imcrop', 'impad', 'impad_to_multiple',
+    'iminvert', 'imflip', 'imrotate', 'imcrop', 'impad', 'impad_to_multiple',
     'imnormalize', 'imdenormalize', 'imresize', 'imresize_like', 'imrescale'
 ]

mmcv/image/transforms/colorspace.py

 import cv2
+import numpy as np
+
+
+def iminvert(img):
+    """Invert (negate) an image
+    Args:
+        img (ndarray): Image to be inverted.
+
+    Returns:
+        ndarray: The inverted image.
+    """
+    return np.full_like(img, 255) - img
 
 
 def bgr2gray(img, keepdim=False):

tests/test_image.py

@@ -285,3 +285,10 @@ class TestImage(object):
 
         with pytest.raises(ValueError):
             mmcv.imrotate(img, 90, center=(0, 0), auto_bound=True)
+
+    def test_iminvert(self):
+        img = np.array([[0, 128, 255], [1, 127, 254], [2, 129, 253]],
+                       dtype=np.uint8)
+        img_r = np.array([[255, 127, 0], [254, 128, 1], [253, 126, 2]],
+                         dtype=np.uint8)
+        assert_array_equal(mmcv.iminvert(img), img_r)