17 Face Recognition Based on OpenCV

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This chapter introduces how to use OpenCV to compare feature databases and achieve face recognition. Although this method is not as efficient as MediaPipe's solution, it allows for the detection of other objects by replacing the feature database file.

Preparation

Since the product automatically runs the main program at startup, which occupies the camera resource, this tutorial cannot be used in such situations. You need to terminate the main program or disable its automatic startup before restarting the robot.
It's worth noting that because the robot's main program uses multi-threading and is configured to run automatically at startup through crontab, the usual method sudo killall python typically doesn't work. Therefore, we'll introduce the method of disabling the automatic startup of the main program here.
If you have already disabled the automatic startup of the robot's main demo, you do not need to proceed with the section on Terminate the Main Demo.

Terminate the Main Demo

1. Click the "+" icon next to the tab for this page to open a new tab called "Launcher."
2. Click on "Terminal" under "Other" to open a terminal window.
3. Type bash into the terminal window and press Enter.
4. Now you can use the Bash Shell to control the robot.
5. Enter the command: crontab -e.
6. If prompted to choose an editor, enter 1 and press Enter to select nano.
7. After opening the crontab configuration file, you'll see the following two lines:
@reboot ~/ugv_pt_rpi/ugv-env/bin/python ~/ugv_pt_rpi/app.py >> ~/ugv.log 2>&1
@reboot /bin/bash ~/ugv_pt_rpi/start_jupyter.sh >> ~/jupyter_log.log 2>&1
8. Add a # character at the beginning of the line with ……app.py >> …… to comment out this line.
#@reboot ~/ugv_pt_rpi/ugv-env/bin/python ~/ugv_pt_rpi/app.py >> ~/ugv.log 2>&1
@reboot /bin/bash ~/ugv_pt_rpi/start_jupyter.sh >> ~/jupyter_log.log 2>&1
9. Press Ctrl + X in the terminal window to exit. It will ask you Save modified buffer? Enter Y and press Enter to save the changes.
10. Reboot the device. Note that this process will temporarily close the current Jupyter Lab session. If you didn't comment out ……start_jupyter.sh >>…… in the previous step, you can still use Jupyter Lab normally after the robot reboots (JupyterLab and the robot's main program app.py run independently). You may need to refresh the page.
11. One thing to note is that since the sub-controller continues to communicate with the upper machine through the serial port, the host may not start up properly during the restart process due to the continuous change of serial port levels. Taking the case where the upper machine is a Raspberry Pi, after the Raspberry Pi is shut down and the green LED is constantly on without the green LED blinking, you can turn off the power switch of the robot, then turn it on again, and the robot will restart normally.
12. Enter the reboot command: sudo reboot.
13. After waiting for the device to restart (during the restart process, the green LED of the Raspberry Pi will blink, and when the frequency of the green LED blinking decreases or goes out, it means that the startup is successful), refresh the page and continue with the remaining part of this tutorial.

Example

The following code block can be run directly:

1. Select the code block below.
2. Press Shift + Enter to run the code block.
3. Watch the real-time video window.
4. Press STOP to close the real-time video and release the camera resources.

If you cannot see the real-time camera feed when running:

  • Click on Kernel -> Shut down all kernels above.
  • Close the current section tab and open it again.
  • Click STOP to release the camera resources, then run the code block again.
  • Reboot the device.

Features of This Chapter

The face feature database file is located in the same path as this .ipynb file. You can change the faceCascade variable to modify what needs to be detected. You'll need to replace the current haarcascade_frontalface_default.xml file with other feature files.
When the code block runs successfully, you can position the robot's camera on a face, and the area containing the face will be automatically highlighted on the screen. 

import cv2  # Import the OpenCV library for image processing
from picamera2 import Picamera2  # Library for accessing the Raspberry Pi Camera
import numpy as np  # Library for mathematical calculations
from IPython.display import display, Image  # Library for displaying images in Jupyter Notebook
import ipywidgets as widgets  # Library for creating interactive widgets like buttons
import threading  # Library for creating new threads to execute tasks asynchronously

# Load the Haar cascade classifier for face detection
faceCascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')

# Create a "Stop" button for users to stop the video stream by clicking on it
# ================
stopButton = widgets.ToggleButton(
    value=False,
    description='Stop',
    disabled=False,
    button_style='danger', # 'success', 'info', 'warning', 'danger' or ''
    tooltip='Description',
    icon='square' # (FontAwesome names without the `fa-` prefix)
)


# Define a display function to process video frames and perform face detection
# ================
def view(button):
    # If you are using a CSI camera you need to comment out the picam2 code and the camera code.
    # Since the latest versions of OpenCV no longer support CSI cameras (4.9.0.80), you need to use picamera2 to get the camera footage
    # picam2 = Picamera2()  # Create an instance of Picamera2
    # picam2.configure(picam2.create_video_configuration(main={"format": 'XRGB8888', "size": (640, 480)}))  # Configure camera parameters
    # picam2.start()  # Start the camera

    camera = cv2.VideoCapture(-1) #Create camera example 
    #Set resolution
    camera.set(cv2.CAP_PROP_FRAME_WIDTH, 640)
    camera.set(cv2.CAP_PROP_FRAME_HEIGHT, 480)
    
    display_handle=display(None, display_id=True)  # Create a display handle to update the displayed image
    i = 0
    
    avg = None
    
    while True:
        # frame = picam2.capture_array()
        _, frame = camera.read()
        # frame = cv2.flip(frame, 1) # if your camera reverses your image

        img = cv2.cvtColor(frame, cv2.COLOR_RGB2BGR)  # Convert the image from RGB to BGR because OpenCV defaults to BGR
        gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)  # Convert the image to grayscale because face detection is typically performed on grayscale images

        # Perform face detection using the cascade classifier
        faces = faceCascade.detectMultiScale(
                gray,     
                scaleFactor=1.2,
                minNeighbors=5,     
                minSize=(20, 20)
            )

        if len(faces):
            for (x,y,w,h) in faces: # Loop through all detected faces
                cv2.rectangle(frame,(x,y),(x+w,y+h),(64,128,255),1) # Draw a rectangle around the detected face
        
        _, frame = cv2.imencode('.jpeg', frame) # Encode the frame as JPEG format
        display_handle.update(Image(data=frame.tobytes()))
        if stopButton.value==True:
            #picam2.close()  # If yes, close the camera
            cv2.release() # if yes, close the camera
            display_handle.update(None)

            
# Display the "Stop" button and start a thread to execute the display function
# ================
display(stopButton)
thread = threading.Thread(target=view, args=(stopButton,))
thread.start()
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