LPR Events

Overview

LPR events have category === 'lpr'. They are generated when a Viewtron camera detects a license plate.

Unlike the Python SDK, the Node.js SDK uses a single ViewtronEvent object for both IPC and NVR events. The source property tells you which format the event came from, and NVR-only fields (plateColor, vehicle, carOwner) are empty or null on IPC events.

Properties

Common (IPC and NVR)

Property	Type	Description
plateNumber	string	Detected plate text (e.g., "ABC1234"). Empty string if not detected.
plateGroup	string	Plate list or group name. See IPC Plate Groups and NVR Plate Groups below.
source	string	"IPC" or "NVR"
category	string	Always "lpr" for these events
eventType	string	Raw alarm type: "VEHICE" or "VEHICLE" (IPC), "vehicle" (NVR)
eventDescription	string	"License Plate Detection"
timestamp	string	Event timestamp from the device
cameraName	string	Camera or NVR device name

NVR Only

These properties are populated only when source === 'NVR'. On IPC events they are empty strings or null.

Property	Type	Description	Example
plateColor	string	Color of the license plate	"blue", "yellow", "white"
vehicle	object or null	Vehicle attributes, or null if not detected	\{ type, color, brand, model \}
vehicle.type	string	Vehicle type	"sedan", "SUV", "truck"
vehicle.color	string	Vehicle body color	"white", "black", "red"
vehicle.brand	string	Vehicle manufacturer	"Toyota", "Ford", "Honda"
vehicle.model	string	Vehicle model	Model name string
carOwner	string	Owner name from NVR plate database	"John Smith"
cameraIp	string	Camera IP address	"192.168.1.108"
cameraMac	string	Camera MAC address	"00:12:34:56:78:9A"
channelId	string	NVR channel number	"1"

All NVR string properties return an empty string (not undefined) when the value is not detected or not available. The vehicle property is null when the NVR does not detect any vehicle attributes.

Images

Property	Type	Description
sourceImage	string	Overview/scene image as a base64 string. Empty string if not available.
targetImage	string	Cropped plate image as a base64 string. Empty string if not available.
sourceImageBytes	Buffer or null	Overview image decoded to a Buffer. Lazy -- decoded on first access.
targetImageBytes	Buffer or null	Cropped plate image decoded to a Buffer. Lazy -- decoded on first access.
hasImages	boolean	true if either sourceImage or targetImage contains data.

IPC vs NVR Comparison

Feature	IPC	NVR
Plate number	Yes	Yes
Plate group	Fixed: "whiteList", "blackList", "temporaryList"	User-defined group names from NVR database
Plate color	No	Yes
Vehicle type	No	Yes (sedan, SUV, truck, etc.)
Vehicle color	No	Yes
Vehicle brand	No	Yes (Toyota, Ford, etc.)
Vehicle model	No	Yes
Car owner	No	Yes (from NVR database)
Camera IP / MAC	No	Yes
Channel ID	No	Yes

Code Examples

Basic Plate Logging

const { ViewtronServer } = require('viewtron-sdk');

const server = new ViewtronServer({
  port: 5050,
  onEvent: (event, clientIP) => {
    if (event.category === 'lpr') {
      console.log(`Plate: ${event.plateNumber} | Group: ${event.plateGroup} | Camera: ${event.cameraName}`);
    }
  }
});

server.start();

Full Vehicle Details (NVR)

server.on('event', (event, clientIP) => {
  if (event.category === 'lpr') {
    console.log(`Plate: ${event.plateNumber}`);
    console.log(`  Source: ${event.source}`);
    console.log(`  Group: ${event.plateGroup}`);

    // NVR provides vehicle attributes
    if (event.source === 'NVR') {
      console.log(`  Plate color: ${event.plateColor}`);
      console.log(`  Owner: ${event.carOwner}`);

      if (event.vehicle) {
        console.log(`  Vehicle: ${event.vehicle.color} ${event.vehicle.brand} ${event.vehicle.type}`);
        console.log(`  Model: ${event.vehicle.model}`);
      }

      console.log(`  Channel: ${event.channelId}`);
    }
  }
});

Saving Plate Images

const fs = require('fs');
const path = require('path');

server.on('event', (event, clientIP) => {
  if (event.category === 'lpr' && event.hasImages) {
    const plate = event.plateNumber || 'unknown';
    const ts = Date.now();

    // Save the overview image
    if (event.sourceImageBytes) {
      fs.writeFileSync(
        path.join('plates', `${plate}_${ts}_overview.jpg`),
        event.sourceImageBytes
      );
    }

    // Save the cropped plate image
    if (event.targetImageBytes) {
      fs.writeFileSync(
        path.join('plates', `${plate}_${ts}_plate.jpg`),
        event.targetImageBytes
      );
    }
  }
});

IPC Plate Groups

IPC cameras use three fixed plate groups:

• "whiteList" -- Allow list
• "blackList" -- Block list
• "temporaryList" -- Temporary/visitor list

These are hardcoded in the camera firmware. The group is determined by which list the plate was added to via the camera's web interface or the plate management API.

If the detected plate is not in the camera's database, plateGroup is an empty string.

NVR Plate Groups

NVR plate groups are user-defined. You create custom group names in the NVR interface (e.g., "Residents", "Staff", "Visitors"). The plateGroup property returns whatever group name was assigned to the matched plate in the NVR database.

If the detected plate is not in the NVR database, plateGroup is an empty string.

Related Pages

• ViewtronEvent -- factory function and common properties
• Images -- working with plate and overview images
• API Reference: License Plate Recognition -- raw XML endpoint documentation