Python | Detect Polygons in an Image using OpenCV Last Updated : 12 Jul, 2025 Comments Improve Suggest changes 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 Comment More infoAdvertise with us Next Article Must Do Coding Questions - Topic-wise V vyom_garg Follow Improve Article Tags : Python Image-Processing Practice Tags : python Similar Reads Interview PreparationInterview Preparation For Software DevelopersMust Coding Questions - Company-wise Must Do Coding Questions - Topic-wiseCompany-wise Practice ProblemsCompany PreparationCompetitive ProgrammingSoftware Design-PatternsCompany-wise Interview ExperienceExperienced - Interview ExperiencesInternship - Interview ExperiencesPractice @GeeksforgeeksProblem of the DayTopic-wise PracticeDifficulty Level - SchoolDifficulty Level - BasicDifficulty Level - EasyDifficulty Level - MediumDifficulty Level - HardLeaderboard !!Explore More...Data StructuresArraysLinked ListStackQueueBinary TreeBinary Search TreeHeapHashingGraphAdvance Data StructuresMatrixStringAll Data StructuresAlgorithmsAnalysis of AlgorithmsSearching AlgorithmsSorting AlgorithmsPattern SearchingGeometric AlgorithmsMathematical AlgorithmsRandomized AlgorithmsGreedy AlgorithmsDynamic ProgrammingDivide & ConquerBacktrackingBranch & BoundAll AlgorithmsProgramming LanguagesCC++JavaPythonC#Go LangSQLPHPScalaPerlKotlinWeb TechnologiesHTMLCSSJavaScriptBootstrapTailwind CSSAngularJSReactJSjQueryNodeJSPHPWeb DesignWeb BrowserFile FormatsComputer Science SubjectsOperating SystemsDBMSComputer NetworkComputer Organization & ArchitectureTOCCompiler DesignDigital Elec. & Logic DesignSoftware EngineeringEngineering MathematicsData Science & MLComplete Data Science CourseData Science TutorialMachine Learning TutorialDeep Learning TutorialNLP TutorialMachine Learning ProjectsData Analysis TutorialTutorial LibraryPython TutorialDjango TutorialPandas TutorialKivy TutorialTkinter TutorialOpenCV TutorialSelenium TutorialGATE CSGATE CS NotesGate CornerPrevious Year GATE PapersLast Minute Notes (LMNs)Important Topic For GATE CSGATE CoursePrevious Year Paper: CS exams Like