[Fix] Switch order of `reduce_zero_label` and applying `label_map` (#2500)

## Motivation

I want to fix a bug through this PR. The bug occurs when two options --
`reduce_zero_label=True`, and custom classes are used.
`reduce_zero_label` remaps the GT seg labels by remapping the zero-class
to 255 which is ignored. Conceptually, this should occur *before* the
`label_map` is applied, which maps *already reduced labels*. However,
currently, the `label_map` is applied before the zero label is reduced.

## Modification

The modification is simple:
- I've just interchanged the order of the two operations by moving 4
lines from bottom to top.
- I've added a test that passes when the fix is introduced, and fails on
the original `master` branch.

## BC-breaking (Optional)

I do not anticipate this change braking any backward-compatibility.

## Checklist

- [x] Pre-commit or other linting tools are used to fix the potential
lint issues.
  - _I've fixed all linting/pre-commit errors._
- [x] The modification is covered by complete unit tests. If not, please
add more unit test to ensure the correctness.
  - _I've added a unit test._ 
- [x] If the modification has potential influence on downstream
projects, this PR should be tested with downstream projects, like MMDet
or MMDet3D.
  - _I don't think this change affects MMDet or MMDet3D._
- [x] The documentation has been modified accordingly, like docstring or
example tutorials.
- _This change fixes an existing bug and doesn't require modifying any
documentation/docstring._

mmseg/core/evaluation/metrics.py

@@ -63,14 +63,14 @@ def intersect_and_union(pred_label,
     else:
         label = torch.from_numpy(label)
 
-    if label_map is not None:
-        label_copy = label.clone()
-        for old_id, new_id in label_map.items():
-            label[label_copy == old_id] = new_id
     if reduce_zero_label:
         label[label == 0] = 255
         label = label - 1
         label[label == 254] = 255
+    if label_map is not None:
+        label_copy = label.clone()
+        for old_id, new_id in label_map.items():
+            label[label_copy == old_id] = new_id
 
     mask = (label != ignore_index)
     pred_label = pred_label[mask]

mmseg/datasets/pipelines/loading.py

@@ -133,6 +133,12 @@ class LoadAnnotations(object):
         gt_semantic_seg = mmcv.imfrombytes(
             img_bytes, flag='unchanged',
             backend=self.imdecode_backend).squeeze().astype(np.uint8)
+        # reduce zero_label
+        if self.reduce_zero_label:
+            # avoid using underflow conversion
+            gt_semantic_seg[gt_semantic_seg == 0] = 255
+            gt_semantic_seg = gt_semantic_seg - 1
+            gt_semantic_seg[gt_semantic_seg == 254] = 255
         # modify if custom classes
         if results.get('label_map', None) is not None:
             # Add deep copy to solve bug of repeatedly
@@ -141,12 +147,6 @@ class LoadAnnotations(object):
             gt_semantic_seg_copy = gt_semantic_seg.copy()
             for old_id, new_id in results['label_map'].items():
                 gt_semantic_seg[gt_semantic_seg_copy == old_id] = new_id
-        # reduce zero_label
-        if self.reduce_zero_label:
-            # avoid using underflow conversion
-            gt_semantic_seg[gt_semantic_seg == 0] = 255
-            gt_semantic_seg = gt_semantic_seg - 1
-            gt_semantic_seg[gt_semantic_seg == 254] = 255
         results['gt_semantic_seg'] = gt_semantic_seg
         results['seg_fields'].append('gt_semantic_seg')
         return results

tests/test_data/test_loading.py

@@ -177,6 +177,61 @@ class TestLoading(object):
         assert gt_array.dtype == np.uint8
         np.testing.assert_array_equal(gt_array, true_mask)
 
+        # test with removing a class and reducing zero label simultaneously
+        results = dict(
+            img_info=dict(filename=img_path),
+            ann_info=dict(seg_map=gt_path),
+            # since reduce_zero_label is True, there are only 4 real classes.
+            # if the full set of classes is ["A", "B", "C", "D"], the
+            # following label map simulates the dataset option
+            # classes=["A", "C", "D"] which removes class "B".
+            label_map={
+                0: 0,
+                1: -1,  # simulate removing class 1
+                2: 1,
+                3: 2
+            },
+            seg_fields=[])
+
+        load_imgs = LoadImageFromFile()
+        results = load_imgs(copy.deepcopy(results))
+
+        # reduce zero label
+        load_anns = LoadAnnotations(reduce_zero_label=True)
+        results = load_anns(copy.deepcopy(results))
+
+        gt_array = results['gt_semantic_seg']
+
+        true_mask = np.ones_like(gt_array) * 255  # all zeros get mapped to 255
+        true_mask[2:4, 2:4] = 0  # 1s are reduced to class 0 mapped to class 0
+        true_mask[2:4, 6:8] = -1  # 2s are reduced to class 1 which is removed
+        true_mask[6:8, 2:4] = 1  # 3s are reduced to class 2 mapped to class 1
+        true_mask[6:8, 6:8] = 2  # 4s are reduced to class 3 mapped to class 2
+
+        assert results['seg_fields'] == ['gt_semantic_seg']
+        assert gt_array.shape == (10, 10)
+        assert gt_array.dtype == np.uint8
+        np.testing.assert_array_equal(gt_array, true_mask)
+
+        # test no custom classes
+        results = dict(
+            img_info=dict(filename=img_path),
+            ann_info=dict(seg_map=gt_path),
+            seg_fields=[])
+
+        load_imgs = LoadImageFromFile()
+        results = load_imgs(copy.deepcopy(results))
+
+        load_anns = LoadAnnotations()
+        results = load_anns(copy.deepcopy(results))
+
+        gt_array = results['gt_semantic_seg']
+
+        assert results['seg_fields'] == ['gt_semantic_seg']
+        assert gt_array.shape == (10, 10)
+        assert gt_array.dtype == np.uint8
+        np.testing.assert_array_equal(gt_array, test_gt)
+
         # test no custom classes
         results = dict(
             img_info=dict(filename=img_path),