How can I draw only the largest rectangle contour to detect Vehicle License Plate (specifically for motorbike license plate)

Question

I’ve been working on detecting motorbike license plate using only OpenCV. My question is:

Can I find and draw only the largest contour (which is around the license plate)?
If not then what should I do for this problem?
Are there any other methods for detecting motorbike license plate?

Here is the source code

import cv2

image = cv2.imread('image_0002.jpg')
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blurred = cv2.GaussianBlur(gray, (3, 3), 0)
canny = cv2.Canny(blurred, 120, 255, 1)

# Find contours
cnts = cv2.findContours(canny, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]

# Iterate thorugh contours and draw rectangles around contours
for c in cnts:
    x,y,w,h = cv2.boundingRect(c)
    cv2.rectangle(image, (x, y), (x + w, y + h), (36,255,12), 2)

cv2.imshow('canny', canny)
cv2.imshow('image', image)
cv2.imwrite('canny.png', canny)
cv2.imwrite('image.png', image)
cv2.waitKey(0)

Asked By: Michael Nguyen

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Source

Answer 1

1- For the largest rectangle – calculate areas of all rectangles

largest_area = 0
for c in cnts:
    x,y,w,h = cv2.boundingRect(c)
    area = 4*w*h
    if area > largest_area:
        largest_area = area
        largest_area_contour = c

Now, we have got the largest rectangle. Lets draw it (outside of for loop).

x,y,w,h = cv2.boundingRect(largest_area_contour)
cv2.rectangle(image, (x, y), (x + w, y + h), (36,255,12), 2)

2- Your model is giving you too many false positives. Ensure that largest rectangle method is always working.

3- License plate (car or bike doesn’t matter) reading is a well studied toy problem. You will find many discussion on internet.

Answered By: Abhi25t

How can I draw only the largest rectangle contour to detect Vehicle License Plate (specifically for motorbike license plate)

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