License Plate Recognition

License Plate Recognition (LPR/ANPR)

Meridian VMS includes built-in License Plate Recognition (LPR), also known as Automatic Number Plate Recognition (ANPR). The system reads license plates from camera feeds, logs every read with a thumbnail and confidence score, and can trigger alarms or automations based on whitelist/blacklist rules.

Dual Source Architecture

LPR operates from two distinct camera sources, which can be used independently or together:

Dedicated LPR Cameras

Purpose-built LPR cameras positioned at vehicle entry/exit points with optimal angle, lighting, and resolution for plate capture. These cameras are flagged as LPR-enabled in camera settings, and the system applies plate-specific preprocessing (contrast enhancement, region cropping) before OCR.

Setting	Description
LPR Region	Crop region within the frame where plates are expected (reduces processing)
Plate Format	Expected plate format regex for validation (e.g., `[A-Z]{2,3}\s?\d{3}\s?[A-Z]{0,2}`)
Min Confidence	Minimum OCR confidence to accept a plate read (default: 70%)

General Detection Cameras

Any camera running AI detection can also contribute plate reads. When the detection engine identifies a vehicle (car, truck, bus, motorcycle), the vehicle crop is passed through the LPR pipeline as a secondary analysis step. This provides opportunistic plate reads from existing cameras without dedicated hardware.

Detection-sourced reads typically have lower confidence than dedicated LPR cameras but provide broader coverage.

LPR Pipeline

Camera Frame
  --> Vehicle Detection (AI model)
  --> Plate Region Detection (plate localizer)
  --> OCR (character recognition)
  --> Format Validation
  --> Whitelist/Blacklist Check
  --> Event Creation + Notification

Each plate read produces an LPR event containing:

Field	Description
`plate`	Recognized plate text (normalized)
`confidence`	OCR confidence (0-100%)
`det_confidence`	Vehicle detection confidence
`vehicle_class`	Detected vehicle type (car, truck, bus, motorcycle)
`camera_id`	Source camera
`thumbnail`	Cropped image of the plate region
`full_frame`	Full frame at time of detection
`timestamp`	Time of the read
`direction`	Entry or exit (if camera is configured with a direction)

Whitelist and Blacklist

Plate lists provide automated responses to known vehicles.

Whitelist

Plates on the whitelist are flagged as authorized. Whitelist matches can trigger automations such as gate opening, barrier lifting, or silent logging without alarm. Each whitelist entry includes:

Plate number (exact match or pattern)
Label (e.g., owner name, department, vehicle description)
Expiry date (optional, for temporary access)
Active hours (optional, restrict to specific time windows)

Blacklist

Plates on the blacklist trigger immediate alarm creation. Use cases include stolen vehicles, banned individuals, or vehicles of interest. Blacklist matches:

Create a high-priority alarm automatically
Send notifications via all configured channels
Highlight the event in the LPR dashboard with a red indicator

List Management

Lists are managed from Settings > LPR > Plate Lists. Plates can be:

Added individually through the UI
Imported in bulk via CSV upload (columns: plate, label, list, expiry)
Exported for backup or transfer between sites
Synced from an external system via the REST API

Plate Search

The plate search interface at Operations > LPR > Search allows querying all historical plate reads:

Filter	Description
Plate	Full or partial plate number (wildcard search)
Camera	Filter by specific camera or location
Date range	Start and end timestamps
List match	Show only whitelist hits, blacklist hits, or unknown plates
Vehicle class	Filter by vehicle type
Min confidence	Only show reads above a confidence threshold

Results display in a sortable table with thumbnail, plate text, camera, timestamp, confidence, and list status. Click any result to view the full frame and jump to recorded playback at that timestamp.

Event Notifications

LPR events integrate with the standard notification system:

Trigger	Notification Content
Any plate read	Plate number, camera, timestamp, thumbnail
Whitelist match	Plate + label + “Authorized” status
Blacklist match	Plate + label + “ALERT” status + full frame
Unknown plate	Plate number flagged for review

Notification channels (Telegram, Discord, email, webhook) are configured per alarm definition. Create an LPR alarm definition with the Detection trigger type and class filter set to license_plate.

Home Assistant Integration

LPR events are exposed in Home Assistant as dedicated event entities:

Entity: event.{camera_name}_license_plate
Event type: plate_detected
Event data: plate, confidence, vehicle_class, camera, timestamp

See Home Assistant Integration for automation examples including automatic gate control based on plate whitelist matching.

API Reference

Method	Endpoint	Description
`GET`	`/api/lpr/events`	Query plate read events with filters
`GET`	`/api/lpr/events/{id}`	Get a single LPR event with full details
`GET`	`/api/lpr/search?plate={query}`	Search plates by partial match
`GET`	`/api/lpr/lists`	Get all whitelist/blacklist entries
`POST`	`/api/lpr/lists`	Add a plate to a list
`DELETE`	`/api/lpr/lists/{id}`	Remove a plate from a list
`POST`	`/api/lpr/lists/import`	Bulk import plates from CSV
`GET`	`/api/lpr/lists/export`	Export all plate lists as CSV
`GET`	`/api/lpr/stats`	LPR statistics (reads today, unique plates, list hit rates)

Configuration

Enable LPR from Settings > AI Detection > LPR:

Setting	Default	Description
Enable LPR	Off	Master toggle for LPR processing
OCR Engine	Built-in	OCR backend (built-in or custom endpoint)
Min Plate Confidence	70%	Minimum OCR confidence to record a plate read
Deduplicate Window	60s	Suppress duplicate reads of the same plate within this window
Store Full Frames	On	Save full-resolution frames for plate reads (uses more storage)
Direction Detection	Off	Attempt to determine vehicle entry/exit direction