Number Plate Detection in OpenCV | ANPR Basics with Python

Chapter 5.3: Number Plate Detection (ANPR Basics)

Automatic Number Plate Recognition (ANPR) is a computer vision application that detects and reads vehicle license plates. Using OpenCV, we can identify plates with techniques such as edge detection, contour detection, and character segmentation.

📌 Step 1: Preprocessing

Convert the image to grayscale, reduce noise, and apply edge detection to highlight potential number plate regions.

import cv2

# Load image
img = cv2.imread("car.jpg")
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

# Noise reduction
blur = cv2.bilateralFilter(gray, 11, 17, 17)

# Edge detection
edges = cv2.Canny(blur, 30, 200)

cv2.imshow("Edges", edges)
cv2.waitKey(0)
cv2.destroyAllWindows()

📌 Step 2: Finding Contours

Detect contours and filter by shape to locate rectangular areas that could be number plates.

contours, _ = cv2.findContours(edges.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
contours = sorted(contours, key=cv2.contourArea, reverse=True)[:10]

for c in contours:
    approx = cv2.approxPolyDP(c, 0.018 * cv2.arcLength(c, True), True)
    if len(approx) == 4:  # Plate is usually rectangular
        x, y, w, h = cv2.boundingRect(approx)
        plate = img[y:y+h, x:x+w]
        cv2.imshow("Number Plate", plate)
        break

cv2.waitKey(0)
cv2.destroyAllWindows()

📌 Step 3: Character Segmentation (Basics)

Once the plate is detected, Optical Character Recognition (OCR) tools like Tesseract can extract characters. This step goes beyond OpenCV and integrates OCR.

import pytesseract

# Convert plate to grayscale & threshold
plate_gray = cv2.cvtColor(plate, cv2.COLOR_BGR2GRAY)
_, thresh = cv2.threshold(plate_gray, 150, 255, cv2.THRESH_BINARY)

# OCR
text = pytesseract.image_to_string(thresh, config="--psm 8")
print("Detected Number Plate:", text)

🚀 Applications

• 🔹 Traffic surveillance systems
• 🔹 Automated toll collection
• 🔹 Parking management
• 🔹 Vehicle security tracking

✅ In this lesson, you learned the basics of Automatic Number Plate Recognition (ANPR) using OpenCV. We applied preprocessing, contour detection, and OCR to detect and read vehicle license plates.
👉 Next, we will explore Face Mask Detection using Deep Learning & OpenCV.