In this tutorial, you will learn how to draw a contour around an object.
- You have Python 3.7 or higher
Draw a Contour Around a T-Shirt
We’ll start with this t-shirt above. Save that image to some folder on your computer.
Now, in the same folder you saved that image above (we’ll call the file tshirt.jpg), open up a new Python program.
Name the program draw_contour.py.
Write the following code:
# Project: How To Draw Contours Around Objects Using OpenCV # Author: Addison Sears-Collins # Date created: October 7, 2020 # Reference: https://stackoverflow.com/questions/58405171/how-to-find-the-extreme-corner-point-in-image import cv2 # OpenCV library import numpy as np # NumPy scientific computing library # Read the image image = cv2.imread("tshirt.jpg") # Convert the image to grayscale gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY) # Convert the image to black and white. # Modify the threshold (e.g. 75 for tshirt.jpg) accordingly depending on how to output looks. # If you have a dark item on a light background, use cv2.THRESH_BINARY_INV and consider # changing the lower color threshold to 115. thresh = cv2.threshold(gray, 75, 255, cv2.THRESH_BINARY) #thresh = cv2.threshold(gray, 115, 255, cv2.THRESH_BINARY_INV) # Create a kernel (i.e. a small matrix) kernel = np.ones((5,5),np.uint8) # Use the kernel to perform morphological opening thresh = cv2.morphologyEx(thresh, cv2.MORPH_OPEN, kernel) # If you have a dark item on a light background, uncomment this line. #thresh = cv2.morphologyEx(thresh, cv2.MORPH_CLOSE, kernel) # Find the contours cnts = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE) cnts = cnts if len(cnts) == 2 else cnts # Create a blank image blank_image = np.ones((450,600,3), np.uint8) # Set the minimum area for a contour min_area = 5000 # Draw the contours on the original image and the blank image for c in cnts: area = cv2.contourArea(c) if area > min_area: cv2.drawContours(image,, 0, (36,255,12), 2) cv2.drawContours(blank_image,, 0, (255,255,255), 2) # Conver the blank image to grayscale for corner detection gray = cv2.cvtColor(blank_image, cv2.COLOR_BGR2GRAY) # Detect corners using the contours corners = cv2.goodFeaturesToTrack(image=gray,maxCorners=25,qualityLevel=0.20,minDistance=50) # Determines strong corners on an image # Draw the corners on the original image for corner in corners: x,y = corner.ravel() cv2.circle(image,(x,y),10,(0,0,255),-1) # Display the image image_copy = cv2.imread("tshirt.jpg") cv2.imshow('original image', image_copy) cv2.imshow('image with contours and corners', image) cv2.imshow('blank_image with contours', blank_image) # Save the image that has the contours and corners cv2.imwrite('contour_tshirt.jpg', image) # Save the image that has just the contours cv2.imwrite('contour_tshirt_blank_image.jpg', blank_image) # Exit OpenCV if cv2.waitKey(0) & 0xff == 27: cv2.destroyAllWindows()
Run the code. Here is what you should see:
Detecting Corners on Jeans
To detect corners on jeans, you’ll need to make the changes mentioned in the code. This is because the jeans are a dark object on a light background (in contrast to a light object on a dark background in the case of the t-shirt).
Let’s draw a contour around the pair of jeans.
Here is the input image (jeans.jpg):
Change the fileName variable in your code so that it is assigned the name of the image (‘jeans.jpg’).
Here is the output image:
That’s it. Keep building!