How to find individual numbers in an image with Python OpenCV?


I have an image similar to the following. I want to separate two numbers 7 and 4 as shown in the image, in that I want to have a bounding box for each of these two objects.

enter image description here

How could I do this with OpenCV? I have no idea, how could I do this and was thinking if there is some way by using Sobel operator. The only thing that I tired was getting the Sobel.

s = cv2.Sobel(img, cv2.CV_64F,1,0,ksize=5)

enter image description here

but have no idea on how to proceed from here.


To segment and detect figures in an image, the main idea is as follows:

  1. Convert image into grayscale using cv2.cvtColor()
  2. Blur image with cv2.GaussianBlur()
  3. Find edges with cv2.Canny()
  4. Find contours with cv2.findContours() and sort from left-to-right using
    imutils.contours.sort_contours() to ensure that when we iterate through contours, they are in the correct order
  5. Iterate through each contour
    • Obtain bounding rectangle using cv2.boundingRect()
    • Find ROI of each contour with Numpy slicing
    • Draw bounding box rectangle using cv2.rectangle()

Canny Edge Detection

enter image description here

Detected Contours

enter image description here

Cropped and saved ROIs

enter image description here


Contours Detected: 2


import numpy as np
import cv2
from imutils import contours

# Load image, grayscale, Gaussian blur, Canny edge detection
image = cv2.imread("1.png")
original = image.copy()
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blurred = cv2.GaussianBlur(gray, (3,3), 0)
canny = cv2.Canny(blurred, 120, 255, 1)

# Find contours and extract ROI
ROI_number = 0
cnts = cv2.findContours(canny, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
cnts = cnts[0] if len(cnts) == 2 else cnts[1]
cnts, _ = contours.sort_contours(cnts, method="left-to-right")
for c in cnts:
    x,y,w,h = cv2.boundingRect(c)
    ROI = original[y:y+h, x:x+w]
    cv2.rectangle(image, (x,y), (x+w,y+h),(36, 255, 12), 3)
    cv2.imwrite('ROI_{}.png'.format(ROI_number), ROI)
    ROI_number += 1

print('Contours Detected: {}'.format(ROI_number))
cv2.imshow("image", image) 
cv2.imshow("canny", canny)

Answered By – nathancy

Answer Checked By – Marie Seifert (AngularFixing Admin)

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