Detect Polygons in an Image using OpenCV in Python

In this tutorial, we will be learning to identify different shapes present in an image on the basis of the number of edges detected in Python. We will be detecting shapes in an image by evaluating contours(edges) in an image and calculating the number of sides of the shape eg-if there are four sides then it is a square or rectangle. We will be using OpenCV and NumPy for image processing.

Algorithm:

1. Read an image.
2. Convert the image into grayscale for better processing.
3. Detect shapes in the image by selecting a region on the basis of the same colors or intensity levels.
4. Search every region in the image for the desired polygon i.e – 3 for Triangle,4-for square or Rectangle,5 for Pentagon, and so on.
5. Select region based on area and check no of sides for the desired polygon.
6. Show the outlined image.

Library used:

OpenCV

pip install opencv-python

Numpy

pip install numpy

Implementation:

The given code will detect types of polygons found in an image and will print the name along with the polygon found.

import cv2
import numpy as np 
img = cv2.imread('image.jpg')  #read image from system
cv2.imshow('original', img)    #Displaying original image
cv2.waitKey(0) 
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)  #Convert to grayscale image
edged = cv2.Canny(gray, 170, 255)            #Determine edges of objects in an image

ret,thresh = cv2.threshold(gray,240,255,cv2.THRESH_BINARY)  

(contours,_) = cv2.findContours(edged,cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE) #Find contours in an image
def detectShape(c):          #Function to determine type of polygon on basis of number of sides
       shape = 'unknown' 
       peri=cv2.arcLength(cnt,True) 
       vertices = cv2.approxPolyDP(cnt, 0.02 * peri, True)
       sides = len(vertices) 
       if (sides == 3): 
            shape='triangle' 
       elif(sides==4): 
             x,y,w,h=cv2.boundingRect(cnt)
             aspectratio=float(w)/h 
             if (aspectratio==1):
                   shape='square'
             else:
                   shape="rectangle" 
       elif(sides==5):
            shape='pentagon' 
       elif(sides==6):
            shape='hexagon' 
       elif(sides==8): 
            shape='octagon' 
       elif(sides==10): 
            shape='star'
       else:
           shape='circle' 
       return shape 
for cnt in contours:
    moment=cv2.moments(cnt) 
    cx = int(moment['m10'] / moment['m00']) 
    cy = int(moment['m01'] / moment['m00']) 
    shape=detectShape(cnt) 
    cv2.drawContours(img,[cnt],-1,(0,255,0),2)
    cv2.putText(img,shape,(cx,cy),cv2.FONT_HERSHEY_SIMPLEX,0.5,(0,0,0),2)  #Putting name of polygon along with the shape 
    cv2.imshow('polygons_detected',img) 
cv2.waitKey(0) 
cv2.destroyAllWindows()

Input Image:

Output:

This Code will give an image with outlined shapes detected along with name of the shape.

This code works well in Python 3 with all the libraries installed. The code will perfectly detect the shapes and identify them. This tutorial aims at the basic understanding of edge detection and how to identify regions in an image. Learners are advised to implement code in their machines and modify the code according to their requirements. This code is a basic example of polygon detection.

4 responses to “Detect Polygons in an Image using OpenCV in Python”

1. gopal says:

   September 9, 2020 at 7:15 pm

   the opencv and python should be installed inside python and
   where should the above programme be dumpped in python promt/pyton idle/ command promt ..?

   Reply
2. Harshil Gupta says:

   September 11, 2020 at 10:53 pm

   Open CV is a library installed in Python.
   You can run program anywhere like in pycharm or jupyter notebook

   Reply
   • kevin says:

     November 12, 2020 at 7:10 pm

     Hi Harshil,

     Nice code. But Im getting this error

     cx = int(moment[‘m10’] / moment[‘m00’])
     ZeroDivisionError: float division by zero

     Reply
     • Ahmet Narmanlı says:

       July 5, 2021 at 1:00 pm

       You can comment out the relevant labeling ares such as cx, cy and cv.puttext. Since in my opinio these parts are extra for labelling. Then the zero division exception will fade.

       Reply