How many alerts match one crossing?
Open F01 and step to the gray sedan in the approaching lane. The rule in this example is a downward crossing of y=270 within the left half of the 640-pixel image. The front bumper first touches the segment at frame 27. The image origin is the top left. A crossing is an instant for this rule. A car remaining in view for the next second does not cross it again. L01 · Clip · Reference marks
Pairing rule for the field pilot
Use the system's source-event qualification timestamp. Sort eligible references by qualifying instant, then by stable reference ID. For each, consider unpaired outputs from the same camera, rule and class with a qualification time within one second either side of the reference. Choose the smallest absolute time difference. Break a tie by the earlier source timestamp, then by stable output ID. An output can be used once. Restrict both sides to continuously monitored evaluation intervals. Record exclusions before matching.
An event timestamp can mean first appearance, threshold completion or dispatch — establish which one the export supplies. If it cannot be converted to the declared qualification clock, keep the observation on file and mark the matching metric unavailable. Record detector events separately from notifications. Deduplicate retries only when they carry the same output ID. A fresh output ID is a separate alert, even when it shows the same bumper.
Worked arithmetic
Suppose a practice log has outputs at 3.0 and 3.4 seconds for the one reference at 2.7. Both are inside the gate. Pair 3.0 first and leave 3.4 unmatched. On these invented inputs recall is 1/1 and precision is 1/2. No vendor produced this example. Calling the second output a duplicate in the notes leaves the precision denominator at two.
Take marks at 2.7 and 3.1 with a single output at 3.0. The earlier eligible reference is processed first under this rule, so it takes the output even though the later mark sits closer. Record both reference IDs. A different matching policy can change which crossing appears to have been missed, and a reviewer can then see exactly which pair moves when someone proposes a different tolerance. Activity evaluation depends on the event definition; the one-second gate here belongs to CTR-1. N01 · NIST activity evaluation
Record identifiers · L01 · L02 · N01