[Fix] FilterAnnotations bug (#8136)

* Bug fix: #8131

* The better solution from chhluo

* #8136

since we changed the filtering type from mask to indexes, we can no longer use ```keep.any()``` to check, because it does not work at index zero.
But we can use

```
if keep.size == 0:
             if self.keep_empty:
                 return None
```

* add a unit test for filtering PolygonMasks

* W293 blank line contains whitespace

Co-authored-by: Wenwei Zhang <40779233+ZwwWayne@users.noreply.github.com>
Co-authored-by: m.stepanov <m.stepanov@redmadrobot.com>

mmdet/datasets/pipelines/loading.py

@@ -407,7 +407,7 @@ def __repr__(self):
         repr_str += f'with_mask={self.with_mask}, '
         repr_str += f'with_seg={self.with_seg}, '
         repr_str += f'poly2mask={self.poly2mask}, '
-        repr_str += f'poly2mask={self.file_client_args})'
+        repr_str += f'file_client_args={self.file_client_args})'
         return repr_str
 
 
@@ -564,7 +564,7 @@ def __call__(self, results):
 
     def __repr__(self):
         return self.__class__.__name__ + \
-               f'(num_max_proposals={self.num_max_proposals})'
+            f'(num_max_proposals={self.num_max_proposals})'
 
 
 @PIPELINES.register_module()
@@ -625,19 +625,21 @@ def __call__(self, results):
         for t in tests[1:]:
             keep = keep & t
 
+        keep = keep.nonzero()[0]
+
         keys = ('gt_bboxes', 'gt_labels', 'gt_masks')
         for key in keys:
             if key in results:
                 results[key] = results[key][keep]
-        if not keep.any():
+        if keep.size == 0:
             if self.keep_empty:
                 return None
         return results
 
     def __repr__(self):
         return self.__class__.__name__ + \
-               f'(min_gt_bbox_wh={self.min_gt_bbox_wh},' \
-               f'(min_gt_mask_area={self.min_gt_mask_area},' \
-               f'(by_box={self.by_box},' \
-               f'(by_mask={self.by_mask},' \
-               f'always_keep={self.always_keep})'
+            f'(min_gt_bbox_wh={self.min_gt_bbox_wh},' \
+            f'min_gt_mask_area={self.min_gt_mask_area},' \
+            f'by_box={self.by_box},' \
+            f'by_mask={self.by_mask},' \
+            f'always_keep={self.always_keep})'

tests/test_data/test_pipelines/test_loading.py

@@ -6,7 +6,7 @@
 import numpy as np
 import pytest
 
-from mmdet.core.mask import BitmapMasks
+from mmdet.core.mask import BitmapMasks, PolygonMasks
 from mmdet.datasets.pipelines import (FilterAnnotations, LoadImageFromFile,
                                       LoadImageFromWebcam,
                                       LoadMultiChannelImageFromFiles)
@@ -118,3 +118,15 @@ def test_filter_annotations(target, kwargs):
     if results is not None:
         results = results['gt_bboxes'].shape[0]
     assert results == target
+
+    polygons = [[np.array([2.0, 10.0, 4.0, 10.0, 4.0, 14.0, 2.0, 14.0])],
+                [np.array([2.0, 10.0, 2.1, 10.0, 2.1, 10.1, 2.0, 10.1])]]
+    polygon_masks = PolygonMasks(polygons, 24, 24)
+
+    results = dict(gt_bboxes=bboxes, gt_masks=polygon_masks)
+    results = filter_ann(results)
+
+    if results is not None:
+        results = len(results.get('gt_masks').masks)
+
+    assert results == target