Python | Detecting polygons in an image using OpenCV

# Python code to detect an arrow (seven-sided) in an image.

import numpy as np

import cv2

 
# Read image

img2 = cv2.imread ( ’arrow.jpg’ , cv2.IMREAD_COLOR)

 
# Read the same image in a different variable and
# converted not gray scale.

img = cv2.imread ( ’arrow.jpg’ , cv2.IMREAD_GRAYSCALE)

 
# Convert image to binary
# (black only -white image).

_, threshold = cv2.threshold (img, 110 , 255 , 

cv2.THRESH_BINARY)

 
# Detect shapes in the image by region selection
# with the same colors or intensities.

contours, _ = cv2.findContours (threshold, cv2.RETR_TREE,

cv2.CHAIN_APPROX_SIMPLE)

  
# Search in each region selected for
# find the desired polygon.

for cnt in contours:

area = cv2.contourArea (cnt)

# List regions based on their region.

  if area"  400 : 

approx = cv2.approxPolyDP (cnt, 

0.009 * cv2. arcLength (cnt, True ), True )

  # Check if not. sides of the selected region is 7.

if ( len (approx) = = 7 ): 

cv2.drawContours (img2, [approx], 0 , ( 0 , 0 , 255 ), 5 )

 
# Show the image along with the arrowhead .

cv2.imshow ( ’image2’ , img2) 

  
# Exit the window if "q" is pressed on the keyboard.

if cv2.waitKey ( 0 ) & amp ;  0xFF = = ord ( ’q’ ): 

cv2.destroyAllWindows ()

Note . The 110 in the threshold can be adjusted as needed if the object has a different color and is based on trial and error.

Result :

Image with an arrow

Binary Image

Outline arrow