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Spotting Of Passing Objects Using OpenCV In Python

March 9, 2022

Tracking and spotting moving objects has become crucial in our lives today. Talking of security, cameras help us a great deal to detect and track moving objects.

In this tutorial, we will learn how to spot a passing object and return the spotted object in real-time using computer vision.

Table of contents

Pre-requisites

  • You should have a basic understanding of Python.
  • A brief overview and prior knowledge of computer vision would help. You can learn a little about computer vision here.
  • You should have a webcam on your laptop.

With those at hand, we will be able to tackle our project. Let’s dive in.

Installation

Install OpenCV

First, we have to install OpenCV to manipulate and work with the webcam images or videos. It also helps with computer-vision related solutions to process images and live videos.

To learn more about OpenCV, it is recommended to go through this documentation.

In your working environment, open the terminal and paste the command below to have the OpenCV downloaded and installed simultaneously.

pip install OpenCV-Python

This library allows for modules such as cv2 to be installed. Since it is a cross-platform library, it contains a variety of functions to read and manipulate images.

Install NumPy

NumPy is used to express data as multi-dimensional arrays. In our case, we represent the image pixel values as arrays.

You can read more about NumPy in this documentation.

pip install numpy

Install imutils

imutils library helps to rotate, resize, and skeletonize the images alongside OpenCV.

You can learn more about imutils here.

pip install imutils

After successful installations, we will now jump into the code area.

Implementation

Import external libraries

As we had discussed earlier, we will import all the above-mentioned libraries to our Python code as shown:

import cv2
import numpy as np
import datetime
import imutils

datetime library returns the current date and time. In our case, it helps keep track of the live or real-time data.

Initialize variables

We will have to initialize objects that helps us capture and record video data as shown:

rec = cv2.VideoCapture(0)
sto, mapping1 = rec.read()
sto, mapping2 = rec.read()

From the above code:

  • The first mapping is to store the initial frame.
  • The second mapping is to store the subsequent frames.
  • In the function VideoCapture, we use 0 since it denotes the access for the webcam.

Detect and convert frames to grayscale

A video can be defined as a series of continuous images with the difference in time. So, we will have to initially identify and recognize the frames then have them processed.

Image frames are converted to grayscale to increase the accuracy to detect the key features and eliminate possible misses.

To learn more about what Grayscale is, it is recommended to read this article.

while rec.isOpened():
    sub = cv2.absdiff(mapping1, mapping2)
    convclr = cv2.cvtColor(sub, cv2.COLOR_BGR2GRAY)
    blur = cv2.GaussianBlur(convclr, (3, 3), 1)
    _, thresh = cv2.threshold(blur, 20, 255, cv2.THRESH_BINARY)
    enlarged = cv2.dilate(thresh, None, iterations=1)
    contours, _ = cv2.findContours(enlarged, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

In the above code:

  • We will assign sub to the difference between the first and second mapping.
  • The cvtColor method is called to convert the sub to grayscale on specifying cv2.COLOR_BGR2GRAY.
  • We then call the GaussianBlur function to the blur on sub.
  • After the above operations, we call the dilate function to enlarge the sub, thereby enhancing its accuracy by removing all the gaps between.
  • On iterating over it, we get the resulting contours.

Blur the frame

GaussianBlur from cv2 is responsible for the blurring of the image. It smoothens the pixels and averages out the intensity across pixel height against its width, thus filtering high-intensity noise.

Let’s implement it as shown below:

if mapping1 is None:
	break

mapping1 = imutils.resize(mapping1, width=700)
convclr1 = cv2.cvtColor(mapping1, cv2.COLOR_BGR2GRAY)
convclr2 = cv2.GaussianBlur(convclr1,(21, 21), 0)

In the above code:

  • The blur is performed to remove high-frequency components from the image.
  • We find that the first and second frames are already converted to grayscale.
  • Using imutils we resize the frame to the desired size of 700 pixels wide.

Find the difference between delta frames

Since we stored the first frame in mapping1, we will be able to compute the difference for the later frames using mapping2 as shown:

if mapping1 is None:
	mapping1 = convclr2
	continue

frameDelta = cv2.absdiff(convclr2, convclr1)
thresh = cv2.threshold(frameDelta, 25, 255, cv2.THRESH_BINARY)[1]

In the above code:

  • The absolute differences between the pixel matrices in mapping1 and mapping2 fetch the absolute value of the pixel intensity frameDelta.
  • The THRESH_BINARY function helps turn the color of the passing object to white while setting the color of the background to black.

Find the contours

To enlarge the images, we call a dilate function from cv2 as shown:

thresh = cv2.dilate(thresh, None, iterations=2)
count = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
count = imutils.grab_contours(count)

In the above code:

  • We use the findContours function to get the contours.
  • Two arguments are passed to the function to retrieve and approximate, thus saving on memory by removing repeated points and compressing them.
  • The dilate function accepts two inputs, one of which is our input image, and the other is known as the structuring element or kernel, which determines the nature of the operation.
  • Dilation of the image helps increase the size of the object.

Format the displayable text and time layout

To be able to write text on the footage, you will have to call a cv2 function putText.

It defines the font, color, size, depth, and the family of the text. To display the current time or real-time data we use datetime module as shown:

for count in contours:
    (x, y, w, h)=cv2.boundingRect(count)
    if cv2.contourArea(count)<700:
        continue
		cv2.rectangle(mapping1, (x, y), (x + w, y + h), (0, 0, 255), 2)
    cv2.putText(mapping1, "REPORT: {}".format('PASSING OBJECT DETECTED'), (5, 30), cv2.FONT_HERSHEY_DUPLEX, 1, (255, 0, 0), 4)
    cv2.putText(mapping1, datetime.datetime.now().strftime("%A %d %B %Y %I:%M:%S%p"), (10, mapping1.shape[0]-10),cv2.FONT_HERSHEY_DUPLEX, 0.9, (0, 255, 0), 3)

Display the image and footage

The cv2 function imshow allows us to display and return the called images or video feeds that was stored in the earlier instantiated values as shown:

cv2.imshow("FOOTAGE", mapping1)
cv2.imshow("MARGIN", thresh)
cv2.imshow("DIFFERENCE OF DELTA FRAMES", frameDelta)

mapping1=mapping2
sto, mapping2=rec.read()

Wrapping it up

We will need to assign a key to record the keypress and terminate our program. On terminating the program, we clean the webcam and release any resources in use.

Lastly, we destroy all the windows constructed by OpenCV.

if cv2.waitKey(50)==50:
	break

cv2.destroyAllWindows()

Results

Below are the expected output and outcome for our program.

Output for THRESH: margin

Output for FRAMEDELTA: framedelta

Output for FOOTAGE: footage

Conclusion

In this tutorial, we have learned several concepts that help with spotting passing objects using OpenCV and other important related tools.

We have gone over installation of the required libraries and the usage of the cv2 module. We have also learned how to compute the frame deltas and format the text layout to be displayed.

You can find the full source here.

For further reading, we suggest you read this article.

Happy coding!


Peer Review Contributions by: Srishilesh P S