Fix CenterNetMaskTargetAssigner for images with no annotations.

PiperOrigin-RevId: 323059388

research/object_detection/core/target_assigner.py

@@ -1659,6 +1659,8 @@ class CenterNetMaskTargetAssigner(object):
       # Shape: [h, w, num_classes].
       segmentations_for_image = tf.reduce_max(
           gt_masks * gt_classes_reshaped, axis=0)
+      # Avoid the case where max of an empty array is -inf.
+      segmentations_for_image = tf.maximum(segmentations_for_image, 0.0)
       segmentation_targets_list.append(segmentations_for_image)
 
     segmentation_target = tf.stack(segmentation_targets_list, axis=0)

research/object_detection/core/target_assigner_test.py

@@ -1905,6 +1905,22 @@ class CenterNetMaskTargetAssignerTest(test_case.TestCase):
     np.testing.assert_array_almost_equal(
         expected_seg_target, segmentation_target)
 
+  def test_assign_segmentation_targets_no_objects(self):
+    def graph_fn():
+      gt_masks_list = [tf.zeros((0, 5, 5))]
+      gt_classes_list = [tf.zeros((0, 10))]
+      cn_assigner = targetassigner.CenterNetMaskTargetAssigner(stride=1)
+      segmentation_target = cn_assigner.assign_segmentation_targets(
+          gt_masks_list=gt_masks_list,
+          gt_classes_list=gt_classes_list,
+          mask_resize_method=targetassigner.ResizeMethod.NEAREST_NEIGHBOR)
+      return segmentation_target
+
+    segmentation_target = self.execute(graph_fn, [])
+    expected_seg_target = np.zeros((1, 5, 5, 10))
+    np.testing.assert_array_almost_equal(
+        expected_seg_target, segmentation_target)
+
 
 class CenterNetDensePoseTargetAssignerTest(test_case.TestCase):