[Fix] Fix cd transform (open-mmlab#3598)

## Motivation

Fix the bug that data augmentation only takes effect on one image in the
change detection task.

## Modification

configs/base/datasets/levir_256x256.py
configs/swin/swin-tiny-patch4-window7_upernet_1xb8-20k_levir-256x256.py
mmseg/datasets/transforms/transforms.py

Author: Zoulinx

configs/_base_/datasets/levir_256x256.py

@@ -11,7 +11,16 @@
 train_pipeline = [
     dict(type='LoadMultipleRSImageFromFile'),
     dict(type='LoadAnnotations'),
-    dict(type='Albu', transforms=albu_train_transforms),
+    dict(
+        type='Albu',
+        keymap={
+            'img': 'image',
+            'img2': 'image2',
+            'gt_seg_map': 'mask'
+        },
+        transforms=albu_train_transforms,
+        additional_targets={'image2': 'image'},
+        bgr_to_rgb=False),
     dict(type='ConcatCDInput'),
     dict(type='PackSegInputs')
 ]

configs/swin/swin-tiny-patch4-window7_upernet_1xb8-20k_levir-256x256.py

@@ -8,7 +8,8 @@
     size=crop_size,
     type='SegDataPreProcessor',
     mean=[123.675, 116.28, 103.53, 123.675, 116.28, 103.53],
-    std=[58.395, 57.12, 57.375, 58.395, 57.12, 57.375])
+    std=[58.395, 57.12, 57.375, 58.395, 57.12, 57.375],
+    bgr_to_rgb=False)
 
 model = dict(
     data_preprocessor=data_preprocessor,

mmseg/datasets/transforms/transforms.py

@@ -2329,14 +2329,19 @@ class Albu(BaseTransform):
     Args:
         transforms (list[dict]): A list of albu transformations
         keymap (dict): Contains {'input key':'albumentation-style key'}
+        additional_targets(dict):  Allows applying same augmentations to \
+        multiple objects of same type.
         update_pad_shape (bool): Whether to update padding shape according to \
             the output shape of the last transform
+        bgr_to_rgb (bool): Whether to convert the band order to RGB
     """
 
     def __init__(self,
                  transforms: List[dict],
                  keymap: Optional[dict] = None,
-                 update_pad_shape: bool = False):
+                 additional_targets: Optional[dict] = None,
+                 update_pad_shape: bool = False,
+                 bgr_to_rgb: bool = True):
         if not ALBU_INSTALLED:
             raise ImportError(
                 'albumentations is not installed, '
@@ -2349,9 +2354,12 @@ def __init__(self,
 
         self.transforms = transforms
         self.keymap = keymap
+        self.additional_targets = additional_targets
         self.update_pad_shape = update_pad_shape
+        self.bgr_to_rgb = bgr_to_rgb
 
-        self.aug = Compose([self.albu_builder(t) for t in self.transforms])
+        self.aug = Compose([self.albu_builder(t) for t in self.transforms],
+                           additional_targets=self.additional_targets)
 
         if not keymap:
             self.keymap_to_albu = {'img': 'image', 'gt_seg_map': 'mask'}
@@ -2417,12 +2425,27 @@ def transform(self, results):
         results = self.mapper(results, self.keymap_to_albu)
 
         # Convert to RGB since Albumentations works with RGB images
-        results['image'] = cv2.cvtColor(results['image'], cv2.COLOR_BGR2RGB)
-
+        if self.bgr_to_rgb:
+            results['image'] = cv2.cvtColor(results['image'],
+                                            cv2.COLOR_BGR2RGB)
+            if self.additional_targets:
+                for key, value in self.additional_targets.items():
+                    if value == 'image':
+                        results[key] = cv2.cvtColor(results[key],
+                                                    cv2.COLOR_BGR2RGB)
+
+        # Apply Transform
         results = self.aug(**results)
 
         # Convert back to BGR
-        results['image'] = cv2.cvtColor(results['image'], cv2.COLOR_RGB2BGR)
+        if self.bgr_to_rgb:
+            results['image'] = cv2.cvtColor(results['image'],
+                                            cv2.COLOR_RGB2BGR)
+            if self.additional_targets:
+                for key, value in self.additional_targets.items():
+                    if value == 'image':
+                        results[key] = cv2.cvtColor(results['image2'],
+                                                    cv2.COLOR_RGB2BGR)
 
         # back to the original format
         results = self.mapper(results, self.keymap_back)