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How to detect license plates using OpenCV Python?
We will use the Haar cascade classifier to detect the license number plate in the image. A haar cascade classifier is an effective object detection method. It is a machine learning based approach.
To train a number plate classifier, the algorithm initially needs a lot of positive images (images of number plates) and negative images (images without number plates). The classifier is trained from these positive and negative images. It is then used to detect objects (number plates) in other images. We can use already trained haar cascades for object detection.
How to Download Haarcascade?
You can find different haarcascades following the GitHub website address −
https://github.com/opencv/opencv/tree/master/data/haarcascadesTo download the Russian number plate haarcascade click on the haarcascade_russian_plate_number.xml file. Open it in raw format, right click and save.
Steps
To detect license number plate in an image, we could follow the below steps-
Import the required library. In all the following examples, the required Python library is OpenCV. Make sure you have already installed it.
Read the input image using cv2.imread(). Specify the full image path. Convert the image to grayscale image.
Initiate the Haarcascade classifier object plate_cascade = cv2.CascadeClassifier() for number plate detection. Pass the full path of the haar cascade xml files. You can use the haar cascade file haarcascade_russian_plate_number.xml to detect the number plate in the image.
Detect the number plate in the input image using plate_cascade.detectMultiScale(). It returns the coordinates of the detected number plate in (x,y,w,h) format.
Draw the bounding rectangles around the detected number plate in the original image using cv2.rectangle().
Display the image with the drawn bounding rectangles around the license number plate.
Let's have a look at some examples for more clear understanding.
Example
In this Python program, we detect license number plates in the input image using a haarcascade.
# import required libraries import cv2 import numpy as np # Read input image img = cv2.imread("audi.jpg") # convert input image to grayscale gray = cv2.cvtColor(img, cv2.COLOR_BGR2RGB) # read haarcascade for number plate detection cascade = cv2.CascadeClassifier('haarcascades\haarcascade_russian_plate_number.xml') # Detect license number plates plates = cascade.detectMultiScale(gray, 1.2, 5) print('Number of detected license plates:', len(plates)) # loop over all plates for (x,y,w,h) in plates: # draw bounding rectangle around the license number plate cv2.rectangle(img, (x,y), (x+w, y+h), (0,255,0), 2) gray_plates = gray[y:y+h, x:x+w] color_plates = img[y:y+h, x:x+w] # save number plate detected cv2.imwrite('Numberplate.jpg', gray_plates) cv2.imshow('Number Plate', gray_plates) cv2.imshow('Number Plate Image', img) cv2.waitKey(0) cv2.destroyAllWindows()
We will use this image as the Input File for this program −
Output
On execution, it will produce the following output −
Number of detected license plates: 1
And we get the following output window −
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