How to Detect Contours in Images using OpenCV in Python

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How to Detect Contours in Images using OpenCV in Python

Vinay Khatri
Last updated on September 26, 2022

    The contours are the boundaries of the object, they are similar to the edges, but we cannot use them as edges. The counter can be seen as the boundaries of the continuous lines around an object. They come in very handy when we want to perform shape analysis and object detection.

    Here, in this Python tutorial, we will learn how to find and identifies contours in OpenCV.

    Install Python OpenCV library

    So let's begin with installing the dependency or OpenCV library.

    pip install opencv-python

    When you install the opencv-python module, it will also install the Python numpy library along. So you do not have to worry about installing the numpy library separately. for this tutorial, we will be using the following birds.jpg image.

    Now open your favorite Python IDE or Text Editor and start coding. Let's start with importing the OpenCV module and load the image with cv.imread() method.

    #import module
    import cv2 as cv
    
    #load image
    image = cv.imread("birds.jpg")

    Now convert the image into the grayscale image, because we want to set the color intensity to binary black and white, so finding the edges around the object becomes easy. To convert the image into a greyscale image in OpenCV, we use the cvtColor(image, cv.COLOR_BGR2GRAY) method.

    #convert to gray scale
    gray_image = cv.cvtColor(image, cv.COLOR_BGR2GRAY)
    
    #show grayscale image
    cv.imshow("Gray Image",gray_image )
    
    cv.waitKey(0)

    you will see the following grayscale image.

    As you can see that now we have a grayscale image of our birds image. Converting the RGB color image to a black and white image is very important because it helps in finding the proper edges of individual objects.

    After converting the image into grayscale, now let's detect edges and the contours of the grayscale image using OpenCV cv.Canny() , and cv.findContours() method.

    #detect edges
    canny = cv.Canny(gray_image, 215, 275)
    
    #identify contours
    contours, hierarchies = cv.findContours(canny,cv.RETR_LIST, cv.CHAIN_APPROX_NONE)

    The cv.Canny() method accepts an image and two thresholds, intensities 215 and 275. The cv.findContours() method finds the number of contours present in the image and returns a tuple of two values contours list and hierarchies .

    After finding the contours, let's draw the Contours on the original image with the help of drawContours() method.

    #draw contours on image
    cv.drawContours(image, contours, -1, (0,0,255), 2)

    The drawContours() method accepts the image on which you want to draw the contour, the contours list itself, the number of counters (-1 represent all contours), BGR code (0,0,255) (Red), the intensity of contours ( 2 ). Now let's display the image with cv.imshow() method

    #show contours outlining on image
    cv.imshow("Contours",image)
    
    #wait till infinity
    cv.waitKey(0)

    The output will be:

    In the above example, we draw the Contours on the original image. Now, let's create a black blank image and draw the same Contours on that blank Image.

    Draw Contours on the Blank Image

    To create a black blank image, we will use the Python NumPy library, so make sure Numpy is installed in your system.

    import cv2 as cv
    import numpy as np
    
    image = cv.imread("birds.jpg")
    
    #blank matrix
    blank = np.zeros(image.shape, dtype='uint8')
    
    #convert to gray scale
    gray_image = cv.cvtColor(image, cv.COLOR_BGR2GRAY)
    
    #detect edges
    canny = cv.Canny(gray_image, 215, 275)
    
    #identify contours
    contours, hierarchies = cv.findContours(canny,cv.RETR_LIST, cv.CHAIN_APPROX_NONE)
    
    #draw contours on blank image
    cv.drawContours(blank, contours, -1, (0,  255,0), 1)
    
    #show contours outlining on image
    cv.imshow("Contours on Blank",blank)
    
    #wait till infinity
    cv.waitKey(0)

    Output

    Conclusion

    In this Python tutorial, we learned how to detect Contours in an Image using the Python OpenCV library. You will be often detecting Contours for image segmentation, shape analysis, and object detection.

    Here, in this tutorial, we have used the OpenCV edge detecting technique to detect the Contours of an image. However, there are also other methods to find the Contours in an image, such as hough transform and K-Means segmentation.

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