Srijita Tiwari — May 21, 2021

This article was published as a part of the Data Science Blogathon

## Aim

Crowd counting is one of the interesting works in the Computer Vision domain. People count in and around retail stores, find its use in many applications like dwell time monitoring, staffing changes, queue management, etc.

In this blog, we will discuss popular people count methods, along with some tasks done in video processing to have better results. There are some algorithms like Haar Cascade, HOG, and OpenCV methods that are used in people detection. After having an understanding of these methods along with their advantages, we can employ these methods in the people counting use case as discussed below.

Our aim is to find the number of people inside the store at a particular hour (dwell time) and the number of people at various sections (groceries, beverages, etc) inside the retail store with the help of CCTV footages. To perform this task, CCTV videos at the entry point, and at different sections inside the store are required.

The video below shows a typical CCTV footage of a retail store, having various sections of the store in the field of view.

## Algorithms

Let’s discuss some of the people detection algorithms along with the approach used in this blog:

Below is the code for it :

import numpy as np

import cv2# Create our body classifier

cap = cv2.VideoCapture(‘/moskva.mov’)# Loop once video is successfully loaded

while cap.isOpened():

gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

# Pass frame to our body classifier

bodies = body_classifier.detectMultiScale(gray, 1.1, 3)

# Extract bounding boxes for any bodies identified

for (x,y,w,h) in bodies:

cv2.rectangle(frame, (x, y), (x+w, y+h), (0, 255, 255), 2)

cv2.imshow(‘Pedestrians’, frame)

if cv2.waitKey(1) == 13: #13 is the Enter Key

break

cap.release()

cv2.destroyAllWindows()

Below is the code for it :

import cv2
import imutils
# Initialising HOG person detector
hog = cv2.HOGDescriptor
hog.setSVMDetector(cv2.HOGDescriptor_getDefaultPeopleDetector)
# Resizing the Image
image = imutils.resize(image,
width=min(400, image.shape[1]))
# Detecting all the regions in the Image that has a pedestrians inside it
(regions, _) = hog.detectMultiScale(image, winStride=(4, 4), padding=(4, 4), scale=1.05)
# Drawing the regions in the Image
for (x, y, w, h) in regions:
cv2.rectangle(image, (x, y), (x + w, y + h), (0, 0, 255), 2)
# Showing the output Image
cv2.imshow(“Image”, image)
cv2.waitKey(0)
cv2.destroyAllWindows()

3. OpenCV background subtraction- Background subtraction is a major preprocessing step in many vision-based applications. For example, consider the cases like a visitor counter where a static camera takes the number of visitors entering or leaving the room, or a traffic camera extracting information about the vehicles, etc. In all these cases, first, you need to extract the person or vehicles alone. Technically, you need to extract the moving foreground from static background. It is a relatively faster method for real-time people detection. OpenCV has implemented three such algorithms :

1. BackgroundSubtractorMOG
2. BackgroundSubtractorMOG2
3. BackgroundSubtractorGMG

Below is the implementation of OpenCV background subtraction using BackgroundSubtractorMOG2:

import numpy as np

import cv2

cap = cv2.VideoCapture(‘vtest.avi’)

fgbg = cv2.createBackgroundSubtractorMOG2()

while(1):

k = cv2.waitKey(30) & 0xff

if k == 27:

break

cap.release()

cv2.destroyAllWindows()

Source : https://docs.opencv.org/3.4/d1/dc5/tutorial_background_subtraction.html

The second image shows the OpenCV background subtraction results on the first image.

Our approach uses this method for better results. Contour methods and morphological transformations have been used to count people with more accuracy.

4. HOG with linear SVM algorithm- The accuracy of the HOG detector (discussed in the Simple HOG detection method) can be further improved by using an SVM classifier to classify positive and negative features from sample images.

The extracted positive and negative features from collected positive and negative image samples are used to train the SVM model with HOG detection. This method counts traffic with maximum accuracy and the algorithm can be customized. Negative images (background images of retail stores) can be generated for any new store to increase the accuracy.

## Approach

Comparison of the above-mentioned algorithms :

Source : self project work

Let’s look at the approach used in this blog, based on the above observation, keeping in mind the different type of videos that we get from the retail store:

## Splitting of videos

Splitting of the store layout video is done for effective traffic count at various categories from a single camera view. One footage might cover 2-3 categories like beverage, grocery sections. To get accurate people – count at different sections of the store, splitting is helpful.

As can be seen in the image above, CCTV videos are available at a bay level so to measure the traffic at a category level, the video coverage area is split into categories area-wise.

## Results

In the use case, our main task is to have an estimate of the count of people inside the store(and also in various sections of the store) to analyze dwell time. Having discussed the algorithms and approaches suitable for the given case, let’s look at the results :

Entrance/Exit Camera Video

The algorithm used:- Opencv background subtractor

Reason:- Fast detections are done because people usually enter at a relatively fast speed (as compared to slow movement inside the store). The people are detected when they cross the camera view.

Result:-

Camera videos above various sections inside the store

The algorithm used:- HOG (linear SVM classifier)

Reason:- Accurate detection is needed since people usually walk with trolleys/children. This algorithm is best for this case scenario

Result:-

Grocery section people count in every frame :

Beverage section people count in every frame :

Let us know in the comments in case of any approach that may further enhance the results.

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