Python | Detect Polygons in an Image using OpenCV Last Updated : 13 Oct, 2019 Summarize Suggest changes Share Like Article Like Report Approach : The approach we would be used to detect the shape of a given polygon will be based on classifying the detected shape on the basis of a number of sides it has. For example, if the detected polynomial has 3 sides, then it could be considered as a triangle, if the polynomial has 4 sides then it could be classified as a square or a rectangle. Prerequisite : Make sure you have Python3, OpenCV, numpy already installed on your computer. Basic knowledge about OpenCV would be helpful - Basics of OpenCV Make sure to save the image in which shapes is to be detected in your local directory Implementation : In the following code, we will be detecting an arrow-shaped object from the image 'arrow.jpg'. The shape will be detected on the basis of the number of sides it has Code: Python program to detect polygons in an image Python3 # Python code to detect an arrow (seven-sided shape) from an image. import numpy as np import cv2 # Reading image img2 = cv2.imread('arrow.jpg', cv2.IMREAD_COLOR) # Reading same image in another variable and # converting to gray scale. img = cv2.imread('arrow.jpg', cv2.IMREAD_GRAYSCALE) # Converting image to a binary image # (black and white only image). _,threshold = cv2.threshold(img, 110, 255, cv2.THRESH_BINARY) # Detecting shapes in image by selecting region # with same colors or intensity. contours,_=cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE) # Searching through every region selected to # find the required polygon. for cnt in contours : area = cv2.contourArea(cnt) # Shortlisting the regions based on there area. if area > 400: approx = cv2.approxPolyDP(cnt, 0.009 * cv2.arcLength(cnt, True), True) # Checking if the no. of sides of the selected region is 7. if(len(approx) == 7): cv2.drawContours(img2, [approx], 0, (0, 0, 255), 5) # Showing the image along with outlined arrow. cv2.imshow('image2', img2) # Exiting the window if 'q' is pressed on the keyboard. if cv2.waitKey(0) & 0xFF == ord('q'): cv2.destroyAllWindows() Note : The parameter '110' in threshold could be adjusted as per need if the object is of different colour and is based on trial and error. Result : Image with arrow Binary Image Outlined Arrow Advertise with us Next Article Detect Cat Faces in Real-Time using Python-OpenCV V vyom_garg Follow Similar Reads Python | Detect corner of an image using OpenCV OpenCV (Open Source Computer Vision) is a computer vision library that contains various functions to perform operations on Images or videos. OpenCV library can be used to perform multiple operations on videos. Let's see how to detect the corner in the image. cv2.goodFeaturesToTrack() method finds N 2 min read How to Detect Shapes in Images in Python using OpenCV? OpenCV is an open source library used mainly for processing images and videos to identify shapes, objects, text etc. It is mostly used with python. In this article we are going to see how to detect shapes in image. For this we need cv2.findContours() function of OpenCV, and also we are going to use 3 min read Reading an image in OpenCV using Python Prerequisite: Basics of OpenCVIn this article, we'll try to open an image by using OpenCV (Open Source Computer Vision) library.  Following types of files are supported in OpenCV library:Windows bitmaps - *.bmp, *.dibJPEG files - *.jpeg, *.jpgPortable Network Graphics - *.png WebP - *.webp Sun raste 6 min read Color Identification in Images using Python - OpenCV An open-source library in Python, OpenCV is basically used for image and video processing. Not only supported by any system, such as Windows, Linux, Mac, etc. but also it can be run in any programming language like Python, C++, Java, etc. OpenCV also allows you to identify color in images. Don’t you 3 min read Detect Cat Faces in Real-Time using Python-OpenCV Face Detection is a technology used to identify faces in images. We can implement it using Python's OpenCV which provides pre-trained haar classifiers for detecting faces in animals such as the haarcascade_frontalcatface.xml and haarcascade_frontalcatface_extended.xml files in the haar cascades dire 2 min read Python OpenCV - Getting and Setting Pixels In this article, we will discuss Getting and Setting Pixels through OpenCV in Python. Image is made up of pixels. A pixel will be denoted as an array. The 3 integers represent the intensity of red, green, blue in the same order. Eg. [0,0,0] in RGB mode represent black color. There are other modes a 3 min read Article Tags : Python Image-Processing Practice Tags : python Like