Motion Detection Alarm

Security camera-style motion detection system.

View Source Code

Overview

Detect motion in video feed and trigger alerts. Perfect for security camera applications and home monitoring.

Key Techniques:

  • Background subtraction
  • Frame differencing
  • Contour analysis
  • Alert triggering

How It Works 

Frame → Background Model → Foreground Mask → Find Motion → Alert
   ↓          ↓                 ↓               ↓          ↓
[Current]  [Learned      [Moving         [Bounding   [Alarm!]
            background]   pixels]         boxes]

Key OpenCV Functions 

# Create background subtractor
bg_subtractor = cv2.createBackgroundSubtractorMOG2(
    history=500,
    varThreshold=16,
    detectShadows=True
)

# Apply to frame
fg_mask = bg_subtractor.apply(frame)

# Remove shadows (gray pixels)
_, fg_mask = cv2.threshold(fg_mask, 250, 255, cv2.THRESH_BINARY)

# Clean mask
kernel = cv2.getStructuringElement(cv2.MORPH_ELLIPSE, (5, 5))
fg_mask = cv2.morphologyEx(fg_mask, cv2.MORPH_OPEN, kernel)
fg_mask = cv2.morphologyEx(fg_mask, cv2.MORPH_CLOSE, kernel)

# Find moving objects
contours, _ = cv2.findContours(fg_mask, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

for contour in contours:
    if cv2.contourArea(contour) > min_area:
        x, y, w, h = cv2.boundingRect(contour)
        cv2.rectangle(frame, (x, y), (x+w, y+h), (0, 255, 0), 2)
        # Trigger alert!

Background Subtractors

Method Speed Quality Best For
MOG2 Fast Good General use
KNN Medium Better Varying lighting
Frame diff Very fast Basic Simple scenes

Simple Frame Differencing 

# Alternative: Simple difference between frames
prev_gray = cv2.cvtColor(prev_frame, cv2.COLOR_BGR2GRAY)
curr_gray = cv2.cvtColor(curr_frame, cv2.COLOR_BGR2GRAY)

# Compute difference
diff = cv2.absdiff(prev_gray, curr_gray)
_, thresh = cv2.threshold(diff, 25, 255, cv2.THRESH_BINARY)

Controls

Key Action
+/- Adjust sensitivity
a Toggle alarm
r Reset background
s Save screenshot
q Quit

Running the Application 

python curriculum/applications/06_motion_alarm.py

Official Documentation