Fix for incorrect mean AP

src/torchmetrics/detection/mean_ap.py

@@ -794,21 +794,25 @@ def __calculate_recall_precision_scores(
             return recall, precision, scores
 
         det_scores = torch.cat([e["dtScores"][:max_det] for e in img_eval_cls_bbox])
-
         # different sorting method generates slightly different results.
         # mergesort is used to be consistent as Matlab implementation.
         # Sort in PyTorch does not support bool types on CUDA (yet, 1.11.0)
         dtype = torch.uint8 if det_scores.is_cuda and det_scores.dtype is torch.bool else det_scores.dtype
         # Explicitly cast to uint8 to avoid error for bool inputs on CUDA to argsort
         inds = torch.argsort(det_scores.to(dtype), descending=True)
-        det_scores_sorted = det_scores[inds]
-
-        det_matches = torch.cat([e["dtMatches"][:, :max_det] for e in img_eval_cls_bbox], axis=1)[:, inds]
         det_ignore = torch.cat([e["dtIgnore"][:, :max_det] for e in img_eval_cls_bbox], axis=1)[:, inds]
         gt_ignore = torch.cat([e["gtIgnore"] for e in img_eval_cls_bbox])
+
         npig = torch.count_nonzero(gt_ignore == False)  # noqa: E712
         if npig == 0:
+            # If there are any predictions, make Precision 0; otherwise, -1.
+            npreds = torch.count_nonzero(det_ignore == False)  # noqa: E712
+            if npreds != 0:
+                precision[:, :, idx_cls, idx_bbox_area, idx_max_det_thrs] = 0.0
             return recall, precision, scores
+
+        det_scores_sorted = det_scores[inds]
+        det_matches = torch.cat([e["dtMatches"][:, :max_det] for e in img_eval_cls_bbox], axis=1)[:, inds]
         tps = torch.logical_and(det_matches, torch.logical_not(det_ignore))
         fps = torch.logical_and(torch.logical_not(det_matches), torch.logical_not(det_ignore))
 

tests/unittests/detection/test_map.py

@@ -578,6 +578,52 @@ def test_missing_gt():
     assert result["map"] < 1, "MAP cannot be 1, as there is an image with no ground truth, but some predictions."
 
 
+@pytest.mark.skipif(_pytest_condition, reason="test requires that pycocotools and torchvision=>0.8.0 is installed")
+def test_class_metrics_with_missing_gt():
+    """Checks MAP for each class when there are 4 detections, each for a different class.
+
+    But there are targets for only 2 classes. Hence, MAP should be lower than 1. MAP for classes with targets should be
+    1 and 0 for the others.
+    """
+    # Example source: Issue https://github.com/Lightning-AI/metrics/issues/1184
+    preds = [
+        {
+            "boxes": torch.Tensor(
+                [
+                    [0, 0, 20, 20],
+                    [30, 30, 50, 50],
+                    [70, 70, 90, 90],  # FP
+                    [100, 100, 120, 120],  # FP
+                ]
+            ),
+            "scores": torch.Tensor([0.6, 0.6, 0.6, 0.6]),
+            "labels": torch.IntTensor([0, 1, 2, 3]),
+        }
+    ]
+
+    targets = [
+        {
+            "boxes": torch.Tensor([[0, 0, 20, 20], [30, 30, 50, 50]]),
+            "labels": torch.IntTensor([0, 1]),
+        }
+    ]
+
+    metric = MeanAveragePrecision(class_metrics=True)
+    metric.update(preds, targets)
+    result = metric.compute()
+
+    assert result["map"] < 1, "MAP cannot be 1, as for class 2 and 3, there are some predictions, but not targets."
+
+    result_map_per_class = result.get("map_per_class", None)
+    assert result_map_per_class is not None, "map_per_class must be present in results."
+    assert isinstance(result_map_per_class, Tensor), "map_per_class must be a tensor"
+    assert len(result_map_per_class) == 4, "map_per_class must be of length 4, same as the number of classes."
+    assert result_map_per_class[0].item() == 1.0, "map for class 0 must be 1."
+    assert result_map_per_class[1].item() == 1.0, "map for class 1 must be 1."
+    assert result_map_per_class[2].item() == 0.0, "map for class 2 must be 0."
+    assert result_map_per_class[3].item() == 0.0, "map for class 3 must be 0."
+
+
 @pytest.mark.skipif(_pytest_condition, reason="test requires that torchvision=>0.8.0 is installed")
 def test_segm_iou_empty_gt_mask():
     """Test empty ground truths."""