Fix MAP metric for empty cases

CHANGELOG.md

@@ -35,7 +35,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
-- Fix empty predictions in MAP metric ([#594](https://github.com/PyTorchLightning/metrics/pull/594))
+- Fix empty predictions in MAP metric ([#594](https://github.com/PyTorchLightning/metrics/pull/594), [#624](https://github.com/PyTorchLightning/metrics/pull/624))
 
 
 - Fix edge case of AUROC with `average=weighted` on GPU ([#606](https://github.com/PyTorchLightning/metrics/pull/606))

tests/detection/test_map.py

@@ -181,7 +181,6 @@ class TestMAP(MetricTester):
     @pytest.mark.parametrize("ddp", [False, True])
     def test_map(self, ddp):
         """Test modular implementation for correctness."""
-
         self.run_class_metric_test(
             ddp=ddp,
             preds=_inputs.preds,
@@ -198,7 +197,6 @@ def test_map(self, ddp):
 @pytest.mark.skipif(_pytest_condition, reason="test requires that pycocotools and torchvision=>0.8.0 is installed")
 def test_error_on_wrong_init():
     """Test class raises the expected errors."""
-
     MAP()  # no error
 
     with pytest.raises(ValueError, match="Expected argument `class_metrics` to be a boolean"):
@@ -208,7 +206,6 @@ def test_error_on_wrong_init():
 @pytest.mark.skipif(_pytest_condition, reason="test requires that pycocotools and torchvision=>0.8.0 is installed")
 def test_empty_preds():
     """Test empty predictions."""
-
     metric = MAP()
 
     metric.update(
@@ -219,13 +216,28 @@ def test_empty_preds():
             dict(boxes=torch.Tensor([[214.1500, 41.2900, 562.4100, 285.0700]]), labels=torch.IntTensor([4])),
         ],
     )
+
+    metric.update(
+        [
+            dict(boxes=torch.Tensor([]), scores=torch.Tensor([]), labels=torch.IntTensor([])),
+        ],
+        [
+            dict(boxes=torch.Tensor([[214.1500, 41.2900, 562.4100, 285.0700]]), labels=torch.IntTensor([4])),
+        ],
+    )
+    metric.compute()
+
+
+@pytest.mark.skipif(_pytest_condition, reason="test requires that pycocotools and torchvision=>0.8.0 is installed")
+def test_empty_metric():
+    """Test empty metric."""
+    metric = MAP()
     metric.compute()
 
 
 @pytest.mark.skipif(_pytest_condition, reason="test requires that pycocotools and torchvision=>0.8.0 is installed")
 def test_error_on_wrong_input():
     """Test class input validation."""
-
     metric = MAP()
 
     metric.update([], [])  # no error

torchmetrics/detection/map.py

@@ -97,7 +97,6 @@ def __exit__(self, exc_type, exc_val, exc_tb) -> None:  # type: ignore
 
 def _input_validator(preds: List[Dict[str, torch.Tensor]], targets: List[Dict[str, torch.Tensor]]) -> None:
     """Ensure the correct input format of `preds` and `targets`"""
-
     if not isinstance(preds, Sequence):
         raise ValueError("Expected argument `preds` to be of type List")
     if not isinstance(targets, Sequence):
@@ -325,7 +324,7 @@ def compute(self) -> dict:
         if self.class_metrics:
             map_per_class_list = []
             mar_100_per_class_list = []
-            for class_id in torch.cat(self.detection_labels + self.groundtruth_labels).unique().cpu().tolist():
+            for class_id in self._get_classes():
                 coco_eval.params.catIds = [class_id]
                 with _hide_prints():
                     coco_eval.evaluate()
@@ -363,12 +362,14 @@ def _get_coco_format(
 
         Format is defined at https://cocodataset.org/#format-data
         """
-
         images = []
         annotations = []
         annotation_id = 1  # has to start with 1, otherwise COCOEval results are wrong
 
-        boxes = [box_convert(box, in_fmt="xyxy", out_fmt="xywh") if box.size(1) == 4 else box for box in boxes]
+        boxes = [
+            box_convert(box, in_fmt="xyxy", out_fmt="xywh") if box.size() == torch.Size([1, 4]) else box
+            for box in boxes
+        ]
         for image_id, (image_boxes, image_labels) in enumerate(zip(boxes, labels)):
             image_boxes = image_boxes.cpu().tolist()
             image_labels = image_labels.cpu().tolist()
@@ -405,8 +406,11 @@ def _get_coco_format(
                 annotations.append(annotation)
                 annotation_id += 1
 
-        classes = [
-            {"id": i, "name": str(i)}
-            for i in torch.cat(self.detection_labels + self.groundtruth_labels).unique().cpu().tolist()
-        ]
+        classes = [{"id": i, "name": str(i)} for i in self._get_classes()]
         return {"images": images, "annotations": annotations, "categories": classes}
+
+    def _get_classes(self) -> list:
+        """Get list of unique classes depending on groundtruth_labels and detection_labels."""
+        if len(self.detection_labels) > 0 or len(self.groundtruth_labels) > 0:
+            return torch.cat(self.detection_labels + self.groundtruth_labels).unique().cpu().tolist()
+        return []