Motion detection with camera in Raspberry Pi - Step-by-Step Execution

Concept Flow - Motion detection with camera

Start Camera

↓

Capture Frame

↓

Compare with Previous Frame

↓

Detect Differences?

No→Repeat Capture

Yes↓

Motion Detected: Trigger Action

↓

Update Previous Frame

↓

Repeat Capture

The camera captures frames continuously, compares each new frame with the previous one to find differences. If differences exceed a threshold, motion is detected and an action is triggered.

Execution Sample

Raspberry Pi

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

This code captures two frames from the camera and calculates the difference between them to detect motion.

Execution Table

Step	Action	Frame1	Frame2	Difference Computed	Motion Detected	Next Step
1	Start camera and capture first frame	Captured	N/A	N/A	No	Capture next frame
2	Capture second frame	Stored from step 1	Captured	Computed pixel difference	No	Check difference threshold
3	Compare difference to threshold	Same as step 1	Same as step 2	Difference value calculated	Yes (if difference > threshold)	Trigger motion action or continue
4	If motion detected, trigger action	Same	Same	Same	Yes	Update previous frame to current
5	Update previous frame	Updated to current frame2	N/A	N/A	No	Repeat capture for next frames
6	Repeat capture and detection	Updated frame	New capture	New difference	Depends	Loop continues

💡 Loop runs continuously until program is stopped manually

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 5	Final
frame1	None	Captured frame	Captured frame	Captured frame	Updated to frame2	Updated continuously
frame2	None	None	Captured frame	Captured frame	None	Captured continuously
diff	None	None	Pixel difference array	Pixel difference array	None	Computed each loop
motion_detected	False	False	Depends on diff	True or False	False	Updated each loop

Key Moments - 3 Insights

Why do we compare two frames instead of just one?

We compare two frames (see execution_table step 2 and 3) to find changes between them, which indicate motion. A single frame alone cannot show movement.

What happens if the difference is very small?

If the difference is below the threshold (step 3), motion_detected stays False and the program continues capturing frames without triggering actions.

Why do we update frame1 after detecting motion?

Updating frame1 to the current frame (step 5) ensures the next comparison is always between the latest frames, keeping motion detection accurate and continuous.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what determines if motion is detected?

A The color of frame1

B The size of the camera frame

C The difference value compared to a threshold

D The time between frames

💡 Hint

Check the 'Motion Detected' and 'Difference Computed' columns in step 3 of execution_table

According to variable_tracker, what happens to frame1 after step 5?

A It is updated to the current frame2

B It becomes None

C It remains the same as the first captured frame

D It stores the difference image

💡 Hint

Look at the 'frame1' row under 'After Step 5' in variable_tracker

If the difference between frames is always zero, what will happen in the loop?

A The program will stop

B Motion will never be detected and loop continues

C Motion will always be detected

D The camera will reset

💡 Hint

Refer to execution_table step 3 and the 'motion_detected' variable in variable_tracker

Concept Snapshot

Motion detection with camera:
- Capture two frames continuously
- Compute pixel difference between frames
- Compare difference to threshold
- If difference > threshold, motion detected
- Trigger action and update previous frame
- Repeat loop for continuous detection

Full Transcript

This visual execution shows how motion detection works with a camera on Raspberry Pi. The program starts by capturing a frame from the camera. Then it captures a second frame and calculates the difference between the two frames. If the difference is larger than a set threshold, it means motion is detected. When motion is detected, an action can be triggered, such as saving an image or sending an alert. After that, the previous frame is updated to the current frame to continue detecting motion in the next frames. This loop runs continuously until the program is stopped. Variables like frame1, frame2, and diff change as frames are captured and compared. Motion detection depends on comparing two frames, not just one, and updating frames keeps detection accurate.