How to Write Video Using OpenCV in Computer Vision

To write video using OpenCV, create a VideoWriter object specifying the output filename, codec, frame rate, and frame size. Then, write frames to the video file using the write() method inside a loop.

Syntax

The main steps to write video in OpenCV are:

• VideoWriter(filename, fourcc, fps, frameSize): Creates a video writer object.
• filename: Output video file name with extension (e.g., 'output.avi').
• fourcc: Codec code to compress the video (e.g., 'XVID').
• fps: Frames per second (e.g., 20.0).
• frameSize: Size of each frame as (width, height).
• write(frame): Writes a frame to the video file.

python

import cv2

# Define codec and create VideoWriter object
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi', fourcc, 20.0, (640, 480))

# Write frames using out.write(frame)
# Release with out.release() when done

Example

This example captures video from your webcam and writes it to a file named 'output.avi'. It shows how to open the camera, create a VideoWriter, write frames, and release resources.

python

import cv2

cap = cv2.VideoCapture(0)  # Open default camera
fourcc = cv2.VideoWriter_fourcc(*'XVID')
out = cv2.VideoWriter('output.avi', fourcc, 20.0, (640, 480))

while cap.isOpened():
    ret, frame = cap.read()
    if not ret:
        break
    frame = cv2.resize(frame, (640, 480))
    out.write(frame)  # Write frame to video file
    cv2.imshow('frame', frame)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cap.release()
out.release()
cv2.destroyAllWindows()

Output

A window showing live webcam video appears; pressing 'q' stops recording and saves 'output.avi' file.

Common Pitfalls

• Not matching frameSize in VideoWriter with actual frame size causes errors or black video.
• Using wrong codec or unsupported file extension can fail to save video.
• Forgetting to call release() on VideoWriter causes incomplete files.
• Writing frames before checking if capture is successful leads to crashes.

python

import cv2

cap = cv2.VideoCapture(0)
fourcc = cv2.VideoWriter_fourcc(*'XVID')
# Wrong frame size example (should match actual frame size)
out = cv2.VideoWriter('output.avi', fourcc, 20.0, (320, 240))

ret, frame = cap.read()
if ret:
    # Wrong: writing frame without resizing to (320,240) causes issues
    out.write(frame)  # Frame size mismatch

    # Correct way:
    frame_resized = cv2.resize(frame, (320, 240))
    out.write(frame_resized)

cap.release()
out.release()

Quick Reference

Tips for writing video with OpenCV:

• Always match frameSize in VideoWriter with your frame's actual size.
• Use cv2.VideoWriter_fourcc(*'XVID') or other supported codecs.
• Call release() on both VideoCapture and VideoWriter to save files properly.
• Check if frames are read successfully before writing.

Key Takeaways

Create a VideoWriter with correct filename, codec, fps, and frame size before writing frames.

Ensure frame size matches between captured frames and VideoWriter to avoid errors.

Always release VideoWriter and VideoCapture objects to save video files properly.

Check frame capture success before writing frames to avoid crashes.

Use common codecs like 'XVID' for wide compatibility.