parking_solution/wiki/concepts/lane-presence-and-anpr-entry.md

type, tags, sources, updated, status

type	tags	sources	updated	status
concept	parking camera anpr subscription lane vision		2026-06-22	open

Lane Presence & ANPR Subscriber Entry

Two related things a camera's vehicle detection feeds, worked out over a long field session (2026-06-22, see lpr-camera for the camera-side saga + the corrected "it was the undrawn detection area" conclusion):

1. Lane busy/free — BUILT. An advisory barrier light on the booth.
2. ANPR subscriber entry — PLANNED. A subscriber's plate, read from the lane camera, drives their entry/exit through the EXISTING subscription flow. The "bridge" below.

The camera (Hik DS-2CD1043G2-LIU) only emits VMD events with eventState=active and a targetType of vehicle/human — a coarse presence signal, never an identity. Everything here is built on that, and on the camera's hard limits.

What the camera actually gives us (measured)

• Push only, no poll. No ISAPI endpoint reports "is a car in the zone now"; the only live source is the event push. We tried to force a steadier signal by flipping notificationRecurrence beginning → recurring via ISAPI — the firmware accepts the PUT but silently reverts (locked to beginning on this value line). So: notify-once-at-motion-start is all we get.
• No leave signal. The camera never sends an inactive/end event. Confirmed by config (notificationRecurrence: beginning) AND a controlled in/out test.
• Re-fire is MOVEMENT-driven, not steady. Controlled test (call out enter/leave, correlate to the event log): while a car MOVES, active repeats ~1–3 s apart; while it sits MOTIONLESS, gaps stretch to ~15–25 s. Crucially the camera has ~no dwell lag — it goes silent within ~1 s of the car leaving (last event 16:15:17 vs car-left ~16:15:30).

1. Lane busy/free (BUILT)

apps/server/src/lane-status.ts (LaneStatus) + the hik-alarm handler + the booth WS. A vehicle active event on a camera bound to entry/exit marks THAT lane busy and arms an auto-clear timer; the booth shows two barrier lights beside the scan input (green=free, red=busy). Advisory only — gates nothing (never blocks a ticket or opens a barrier; the standing rule).

• "Free" is timeout-driven (no leave signal). The TTL must exceed the still-car gap (~25 s) or a parked car flickers free — so LANE_BUSY_TTL_MS default is 30 s (started at a guessed 90 s, briefly 5 s, then set to 30 s from the measured data). The camera's lack of dwell lag means 30 s also clears promptly after departure.
• A both-direction camera marks both lanes. Pushed over the existing /api/ws (kind lane-status).

2. ANPR subscriber entry — THE BRIDGE (PLANNED, not built)

"Bridge" = a HANDLER FUNCTION in apps/server (≈40 lines, e.g. anpr-entry.ts). NOT a new service / container / app. It is in-process glue that calls things that ALREADY exist.

The goal (narrowed deliberately — see Rejected below): a subscriber's plate, read by the lane camera, admits them through the same gated flow a QR/card scan uses. Scope was cut to subscribers ONLY — no queue segmentation, no per-car tracking, no make/model, no ticket-button gating.

Almost everything already exists; the bridge is the one missing wire:

Piece	Status
Camera vehicle event	✅ lpr-camera (hik-alarm)
Pull a snapshot	✅ snapshot.ts (captureSnapshot)
Read the plate	✅ opencv-anpr-service /analyze (~50 ms on the DEV PC; appliance TBD)
Match a plate → subscriber	✅ subscription-flow.ts match() + subscription_plates (via:"plate")
Plate read → gated entry/exit	✅ read-dispatch.ts + SubscriptionFlow (active/window/blocklist/car-count)
Emit the plate onto the read bus	❌ the bridge — today the snapshot ANPR only RECORDS the plate as advisory telemetry; it does NOT emitRead. hik-alarm.ts literally says "NOT a DeviceReadEvent yet".

The bridge logic: on a camera vehicle/active event from an opt-in camera (config.anpr), snapshot → vision.analyze → if a plate clears a HIGH confidence floor → debounce → emit DeviceReadEvent{kind:"plate", value, deviceId}. The existing dispatcher + flow do the rest.

Decisions settled with the user (2026-06-22)

• Both directions. Entry- and exit-bound cameras both work; the dispatcher infers the verb from the camera's bound relay direction (an entry camera → entry, exit → exit). No per-read inference.
• High confidence required. A barrier-driving read needs a stricter bar than the advisory-record floor — a NEW VISION_ENTRY_MIN_CONFIDENCE (≈0.85) distinct from VISION_MIN_CONFIDENCE. (The subscriber still holds their card/QR, so a near-miss read just falls back to that.)
• Opt-in per camera (config.anpr), so a site that didn't ask for plate-entry is unaffected.
• Debounce is REQUIRED — for correctness, NOT CPU. The camera re-fires ~1 Hz while a car is present; emitting a read every second would drive REPEAT entries (a fleet sub opens a 2nd occurrence; a single-car sub spams "already inside") or, on an exit camera, repeat exits (sub.refused.noSession after the first, and FIFO could phantom-close another occurrence). So the same plate on the same camera within ~10–15 s = ONE credential presentation.
  • NB: a direction-bound camera does NOT flip entry↔exit on repeat reads (the barrier's direction fixes the verb), so the earlier "flip-flop" fear was wrong — but repeat-same-direction is still bad. Debounce stands.
• Threat-model rule preserved by construction. A plate is trivially spoofable (print it on paper); it must NEVER be the sole reason a barrier opens. Routing through the existing SubscriptionFlow means the plate is just another credential through the same gate (active / window / blocklist / car-count) — not a bypass. See opencv-anpr-service, append-only-event-chain.

To validate (booth-PC test day, ~2026-06-23)

The dev-PC ANPR is ~50 ms/frame, but that says little about the hardened booth appliance (likely a low-power CPU, possibly 5–15× slower). Real-hardware latency is the open number. The user is bringing the actual booth PC to test on.

Rejected / out of scope (and why)

• Vision monitors the RTSP livestream continuously for presence — rejected: rebuilds (worse) what the camera already does (presence), pegs the appliance CPU 24/7, and the leave-detection heuristic is no cleaner than a 30 s timeout.
• Vision polls every ~1 s to track THE car, segment a queue, count cars, read make/model — a genuinely harder need (bumper-to-bumper cars the camera can't tell apart). Set ASIDE, not dismissed: it needs a general vehicle DETECTOR (opencv-anpr-service is plate-only today) + "same-car-vs-new" logic + make/model (a third, weak, heavy model), and its viability is gated on appliance inference budget we cannot measure on the dev PC. Revisit only if real traffic + hardware justify it. The vision feasibility was checked: fast_alpr live + ready; plate-only; no vehicle/presence detector exists yet.
• Gate the entry ticket button on lane-busy (don't print when no car) — deferred. The lane-busy signal supports it, but it gates a PHYSICAL action so it must FAIL-OPEN (allow when presence is unknown). Parked pending the user's real goal (anti-spam print vs one-ticket-per-car).