Motion detection basics in Computer Vision

Introduction

Motion detection helps computers see when something moves in a video. It is useful for security, games, and smart cameras.

To alert when someone enters a room using a security camera

To count people walking in a store

To start recording only when there is movement

To control a game character by moving in front of a camera

To detect traffic flow on roads

Syntax

Computer Vision

frame_diff = cv2.absdiff(frame1, frame2)
gray = cv2.cvtColor(frame_diff, cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(gray, (5,5), 0)
_, thresh = cv2.threshold(blur, 20, 255, cv2.THRESH_BINARY)
contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

This code compares two video frames to find differences.

Contours show where movement happens.

Examples

Convert the difference between frames to grayscale to simplify processing.

Computer Vision

frame_diff = cv2.absdiff(frame1, frame2)
gray = cv2.cvtColor(frame_diff, cv2.COLOR_BGR2GRAY)

Blur the image to reduce noise, then create a binary image to highlight moving areas.

Computer Vision

blur = cv2.GaussianBlur(gray, (5,5), 0)
_, thresh = cv2.threshold(blur, 20, 255, cv2.THRESH_BINARY)

Find shapes of moving parts and ignore small changes like light flickers.

Computer Vision

contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
for contour in contours:
    if cv2.contourArea(contour) > 500:
        print('Motion detected!')

Sample Model

This program uses your webcam to detect motion. It shows a green box around moving parts and prints 'Motion detected!' when movement is found.

Computer Vision

import cv2

cap = cv2.VideoCapture(0)
ret, frame1 = cap.read()
ret, frame2 = cap.read()

while cap.isOpened():
    frame_diff = cv2.absdiff(frame1, frame2)
    gray = cv2.cvtColor(frame_diff, cv2.COLOR_BGR2GRAY)
    blur = cv2.GaussianBlur(gray, (5,5), 0)
    _, thresh = cv2.threshold(blur, 20, 255, cv2.THRESH_BINARY)
    contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

    motion_detected = False
    for contour in contours:
        if cv2.contourArea(contour) > 1000:
            motion_detected = True
            (x, y, w, h) = cv2.boundingRect(contour)
            cv2.rectangle(frame1, (x, y), (x+w, y+h), (0, 255, 0), 2)

    if motion_detected:
        print('Motion detected!')

    cv2.imshow('Motion Detection', frame1)
    frame1 = frame2
    ret, frame2 = cap.read()

    if cv2.waitKey(30) & 0xFF == 27:  # Press ESC to exit
        break

cap.release()
cv2.destroyAllWindows()

Important Notes

Lighting changes can cause false motion detection.

Adjust the contour area threshold to ignore small movements like shadows.

Use a stable camera to get better results.

Summary

Motion detection compares video frames to find changes.

It uses image difference, grayscale, blur, threshold, and contours.

It helps computers notice when something moves in a scene.

Practice

1. What is the main goal of motion detection in computer vision?

easy

A. To increase the brightness of images

B. To find changes between video frames indicating movement

C. To convert color images to grayscale

D. To compress video files for storage

Solution

1. Step 1: Understand motion detection purpose

   Motion detection compares video frames to identify changes that show movement.

2. Step 2: Eliminate unrelated options

   Brightness increase, grayscale conversion, and compression are not about detecting motion.

3. Final Answer:

   To find changes between video frames indicating movement -> Option B

4. Quick Check:

   Motion detection = find changes [OK]

Hint: Motion detection = spotting changes between frames [OK]

Common Mistakes:

• Confusing motion detection with image enhancement
• Thinking motion detection compresses videos
• Mixing grayscale conversion with motion detection

2. Which of the following is the correct order of steps in basic motion detection?

easy

A. Blur, grayscale, difference, threshold, contours

B. Difference, blur, grayscale, contours, threshold

C. Grayscale, blur, difference, threshold, contours

D. Contours, threshold, difference, blur, grayscale

Solution

1. Step 1: Recall typical motion detection steps

   First convert to grayscale, then blur to reduce noise, find difference between frames, apply threshold to highlight changes, and finally find contours.

2. Step 2: Match the correct sequence

   Grayscale, blur, difference, threshold, contours matches this exact order.

3. Final Answer:

   Grayscale, blur, difference, threshold, contours -> Option C

4. Quick Check:

   Step order = grayscale -> blur -> difference -> threshold -> contours [OK]

Hint: Remember: grayscale first, then blur, difference, threshold, contours [OK]

Common Mistakes:

• Mixing up the order of blur and difference
• Applying contours before threshold
• Skipping grayscale conversion

3. Given the following Python code snippet for motion detection, what will be the output type of thresh?

import cv2
frame1 = cv2.imread('frame1.jpg')
frame2 = cv2.imread('frame2.jpg')
gray1 = cv2.cvtColor(frame1, cv2.COLOR_BGR2GRAY)
gray2 = cv2.cvtColor(frame2, cv2.COLOR_BGR2GRAY)
diff = cv2.absdiff(gray1, gray2)
thresh = cv2.threshold(diff, 25, 255, cv2.THRESH_BINARY)[1]

medium

A. A grayscale image with pixel values 0 or 255

B. A color image with three channels

C. A single integer threshold value

D. A list of contour points

Solution

1. Step 1: Understand cv2.threshold output

   The function returns a tuple; the second element is the thresholded image, which is a grayscale image with pixels either 0 or max value (255).

2. Step 2: Analyze the code

   Since diff is grayscale, thresholding produces a binary grayscale image, not color or contours.

3. Final Answer:

   A grayscale image with pixel values 0 or 255 -> Option A

4. Quick Check:

   Threshold output = binary grayscale image [OK]

Hint: cv2.threshold returns binary grayscale image, not color or contours [OK]

Common Mistakes:

• Thinking threshold returns contours
• Assuming output is color image
• Confusing threshold value with output image

4. You wrote this code for motion detection but it raises an error:

diff = cv2.absdiff(frame1, frame2)
thresh = cv2.threshold(diff, 30, 255, cv2.THRESH_BINARY)[1]
contours, _ = cv2.findContours(thresh, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

What is the likely cause of the error?

medium

A. cv2.RETR_EXTERNAL is an invalid flag

B. Threshold function returns only one value, not a tuple

C. findContours requires color images, not binary

D. Frames are color images; absdiff needs grayscale images

Solution

1. Step 1: Check input types for absdiff

   cv2.absdiff expects images of the same type and single channel for motion detection, usually grayscale.

2. Step 2: Identify missing grayscale conversion

   Frames are likely color images; missing conversion to grayscale causes error in absdiff.

3. Final Answer:

   Frames are color images; absdiff needs grayscale images -> Option D

4. Quick Check:

   absdiff input = grayscale images [OK]

Hint: Convert frames to grayscale before absdiff to avoid errors [OK]

Common Mistakes:

• Assuming threshold returns single value only
• Thinking findContours needs color images
• Using invalid flags for findContours

5. You want to detect motion only when a moving object covers at least 500 pixels in the frame. Which step should you add after finding contours to achieve this?

hard

A. Filter contours by area and ignore those smaller than 500 pixels

B. Apply a stronger blur to the grayscale image

C. Increase the threshold value to 500

D. Convert the frame to color before contour detection

Solution

1. Step 1: Understand contour filtering

   Contours represent detected shapes; filtering by area removes small noise.

2. Step 2: Apply area filter after contours

   Ignoring contours smaller than 500 pixels ensures only significant motion is detected.

3. Final Answer:

   Filter contours by area and ignore those smaller than 500 pixels -> Option A

4. Quick Check:

   Filter contours by area ≥ 500 pixels [OK]

Hint: Filter contours by size to detect significant motion only [OK]

Common Mistakes:

• Confusing threshold value with pixel area
• Applying blur instead of filtering contours
• Converting to color unnecessarily