parking_solution/wiki/entities/lpr-camera.md

---
type: entity
tags: [parking, hardware, readers, offline-first]
sources: [parking-system-architecture]
updated: 2026-06-15
---

# LPR Camera

License-plate-recognition camera. For **casual/transient** vehicles, the **plate acts as ticket +
an independent record**. (See [[parking-system-architecture]] §8, §9.)

> **Superseded direction (2026-06-15):** recognition now runs **host-side** on snapshots from
> ordinary Hikvision/Dahua cameras via the [[opencv-anpr-service]], **not** on a dedicated edge-AI
> LPR camera — see [[vision-service]]. The edge-AI camera below is kept as the original assumption /
> a fallback option, but is no longer the planned path. The host-side service also does **vehicle
> verification** (anti-plate-spoofing), which an edge-LPR camera does not.

- **Edge AI (original assumption)**: recognition runs **on-device**, so it keeps working with no
  internet — fits [[offline-first]].
- It's a **host-side** identity source: only the host sees the read; the host decides and
  commands the relay open (the [[uhppote-controller]] is demoted to a commanded relay for that
  lane). See [[entry-exit-readers]].
- Being host-in-the-loop is **good for fraud detection** — you get two independent records (the
  host's signed [[append-only-event-chain]] entry + the controller's remote-open event) that
  should reconcile one-to-one; any mismatch is an anomaly.
- Mounting: within ~15° of vehicle travel at a controlled chokepoint for best reads.

## Snapshot driver (entry/exit fraud-control record)

Separate from edge-AI LPR: the camera driver (`packages/devices/src/drivers/camera.ts`) does
**snapshot-on-event** — the host pulls a still over HTTP when an entry/exit fires and stores it,
referenced from the signed [[append-only-event-chain]] entry as an independent record. The camera
**pulls, it does not push** — so it is NOT `pushesToBackend` and the setup wizard correctly hides
the "Backend push IP" field for it (gated on the driver's `pushesToBackend` flag; only
[[dingtian-relay]] sets it).

- **Hikvision** uses **ISAPI**: `GET /ISAPI/Streaming/channels/<id>/picture` (`101` = ch1 main
  stream) with **HTTP Digest** auth. The "Enable Hikvision-CGI" toggle (Network → Advanced →
  Integration Protocol) is a *different* legacy CGI surface — **not** needed for ISAPI.
- **Dahua** uses CGI: `GET /cgi-bin/snapshot.cgi?channel=<n>` (0-based channel; the wizard's
  1-based channel is decremented).

**Driver / storage boundary:** the driver FETCHES the image bytes (client-side HTTP Digest in
`drivers/http-digest.ts`) and returns them on `Snapshot.bytes`; **storage is the caller's job**
(the future entry/exit flow stores the bytes + mints a durable `imageRef`). This keeps the device
adapter free of any filesystem/blob-store dependency. `healthCheck()` is honest — it actually pulls
a frame (exercising reachability + auth + path/channel in one shot), not a fake `ready/stub`.

### Verified on hardware (2026-06-15)

A **Hikvision** unit ("Camera 20", MAC `94:e1:ac:…`, Hikvision OUI) at `10.0.10.121`, creds
`admin` / `admin123` (Digest), TCP 80:

- Initial `curl` test confirmed the ISAPI path returns a 2688×1520 JPEG (~306 KB).
- The **real driver** (no longer a stub) was then run end to end against it:
  `healthCheck()` → `ready` (pulled a frame), `captureSnapshot()` → valid `image/jpeg`, ~322 KB,
  correct JPEG magic. Digest handshake works through `HttpCamera`.
- Reaching it from the WSL dev box required forcing the source address (`config.localAddress`,
  threaded into the driver) — see [[wsl-dev-networking]] (multi-subnet source-selection trap).

## Camera PUSH — "Alarm Server" event notifications (2026-06-22)

Separate from the **pull** snapshot path above: newer Hikvision firmware can **push** an event to
us. Under **Event → Smart/VCA** (e.g. line crossing / intrusion / "Vehicle Detection") the unit
exposes **Detection Target: Human / Vehicle** — selecting **Vehicle** + **Notify Surveillance
Center**, then **Alarm Settings → Alarm Server**, makes the camera **HTTP-POST an
`EventNotificationAlert`** to a URL we host on each detection. Same machine-call shape as the
[[dingtian-relay]] Input Link push — no polling.

- **Ingress:** `POST /api/devices/hikvision/:deviceId/event` (`apps/server/src/routes/hikvision-alarm.ts`).
  **Source-IP guarded** (must come from the device's configured `host`) + **optional HTTP Digest**
  (some firmware can't authenticate the Alarm Server call → source-IP only). NOT behind the SPA
  cookie/CSRF (it's a device call), exactly like the Dingtian push.
- **Config:** added to the `hikvision` driver — `alarmPushEnabled` (bool), `pushUser`/`pushPassword`
  (optional Digest). The driver is now `pushesToBackend: true`, so first-run setup offers the backend
  push IP. Point the camera's Alarm Server at `http://<backend-ip>:<port>/api/devices/hikvision/<deviceId>/event`.
- **Discovery-first:** the endpoint is **permissive** — accepts ANY content-type as raw bytes (event
  XML, multipart-with-JPEG, or JSON; Hik's format varies by model/firmware), records the **verbatim
  body** as a `kind:"alarm"` device_event, and best-effort extracts `eventType` / `target` / `plate`
  / `dateTime` / `channelID`. The point of this first cut is to **see exactly what a given camera
  sends** (inspect via `GET /api/events` or the server log) before wiring it to the read bus.
- **Not yet a barrier trigger.** It records + breadcrumbs only; it does NOT emit a `DeviceReadEvent`
  or open anything. A plate read is **advisory, never the sole reason** a barrier opens
  ([[append-only-event-chain]], [[opencv-anpr-service]]) — the entry/exit wiring is a deliberate
  next step once the real payload is known. If the camera emits its own plate (`<plateNumber>`), we
  can use it as an advisory read directly; otherwise the server hands the attached/pulled frame to
  the [[opencv-anpr-service|vision service]] for ANPR.

### Gotchas learned the hard way (2026-06-22 field session)

Several traps surfaced trying to get a real camera to push. In order of how long each cost:

- **WSL rewrites the inbound source IP.** On the dev host (WSL mirrored mode), an inbound LAN packet
  arrives at our server with its **source rewritten to the host's own IP** (`10.0.10.203`), not the
  camera's. The source-IP guard then rejects every push as a mismatch. Fix: a per-device
  **`skipSourceIpCheck`** config flag (a Setup checkbox) that bypasses the IP guard — the signed
  ledger + optional Digest remain the real guards. Leave OFF on a normal LAN.
- **The setup checkbox saved booleans as the STRING `"true"`.** The generic config-field form had no
  boolean renderer, so a `type:"boolean"` field fell through to a text input. Fixed (checkbox
  renderer); the server also coerces `"true"`/`1`/`yes`/`on` defensively.
- **The camera's "Test" button proves almost nothing.** It does a TCP/connectivity probe and reports
  "service available" on ANY HTTP reply (even our 404) — it does **not** POST a real event to your
  URL. Only a real detection (or the ISAPI `httpHosts/<id>/test`) actually exercises the path.
- **`httpBroken` latches.** Once the camera marks the host broken (from earlier failed deliveries),
  it stays `true` across reboots and won't retry. Clear it by **re-PUTting** the httpHost config
  (`PUT /ISAPI/Event/notification/httpHosts/1` with `<httpBroken>false</httpBroken>`).
- **"Notify Surveillance Center" ≠ the HTTP Alarm Server** on some firmware (separate upload
  channels); and **basic Motion Detection may not POST to the httpHost** while smart/VCA events do.
  Always confirm the **Arming Schedule** covers the test time, too (a silent killer).

### Diagnosing a non-pushing camera — read its OWN state, don't guess

Status APIs lie (a `diskfull` error on a camera with **no disk** is a misleading label). The decisive
checks, in order — they need no cooperation from our server:

1. **`GET /ISAPI/Event/notification/alertStream`** (Digest) — the camera's live event bus. A HEALTHY
   camera streams periodic multipart heartbeat frames even when idle, and an event line when one
   fires. **Total silence (no heartbeat) for 20s = the event subsystem is producing nothing** — the
   fault is internal, upstream of any linkage (HTTP host, FTP, alarm stream all die together).
2. **SSH into the camera** (BusyBox "psh" restricted shell; default creds work): `showStatus` →
   look for **`EventScribe: except`** (event subsystem in an exception state); `dmesg` → UBIFS
   "recovery needed" on every volume (unclean flash) and **`fh rtc get time error ffffffff`** (dead
   RTC — a hardware fault; the clock reads 1970, which wedges time-keyed event logic). `ps` shows the
   `davinci` app. `df`/`printPart` are NOT available; partition sizes via the UBIFS lines in `dmesg`.
3. **`netstat` on the camera** while triggering — watch for an OUTBOUND line `cam:port → server:3000`.
   None appearing = the camera never even attempts the POST.

> **Verified-dead unit (2026-06-22):** a `DS-2CD1043G2-LIU` (V5.8.10, 8 months old) failed all three:
> silent alertStream, `EventScribe: except`, `diskfull` on `Event/triggers`, dead RTC, and **zero
> outbound to `:3000`** across a 3-minute netstat watch — through reboot, basic reset, AND full
> factory reset. Conclusion: **defective event engine (RMA)**, not our integration. Our endpoint was
> verified correct throughout (it accepts + records a POST the instant one actually arrives). When a
> camera's event push is dead, fall back to the **pull + [[opencv-anpr-service|vision]]** path, which
> works with the same camera (it still serves snapshots fine).