Added vertical flipping in bbox_flip (#1115)

* Added vertical flipping in bbox_flip

* Fixed linting errors

* Fixed linting errors

* Added a blank line in docstrings

* Fixed linting errors.

mmdet/datasets/transforms.py

@@ -49,18 +49,23 @@ class ImageTransform(object):
         return img, img_shape, pad_shape, scale_factor
 
 
-def bbox_flip(bboxes, img_shape):
-    """Flip bboxes horizontally.
+def bbox_flip(bboxes, img_shape, direction='horizontal'):
+    """Flip bboxes horizontally or vertically.
 
     Args:
         bboxes(ndarray): shape (..., 4*k)
         img_shape(tuple): (height, width)
     """
     assert bboxes.shape[-1] % 4 == 0
-    w = img_shape[1]
     flipped = bboxes.copy()
+    if direction == 'horizontal':
+        w = img_shape[1]
         flipped[..., 0::4] = w - bboxes[..., 2::4] - 1
         flipped[..., 2::4] = w - bboxes[..., 0::4] - 1
+    else:
+        h = img_shape[0]
+        flipped[..., 1::4] = h - bboxes[..., 3::4] - 1
+        flipped[..., 3::4] = h - bboxes[..., 1::4] - 1
     return flipped