How to Flip Image Using OpenCV in Computer Vision

To flip an image in OpenCV, use the cv2.flip() function with the image and a flip code: 0 for vertical, 1 for horizontal, and -1 for both axes. This function returns the flipped image which you can display or save.

Syntax

The cv2.flip() function syntax is:

• cv2.flip(src, flipCode)

Where:

• src is the source image to flip.
• flipCode determines the flip direction: 0 for vertical flip, 1 for horizontal flip, and -1 for both vertical and horizontal flips.

python

flipped_image = cv2.flip(src, flipCode)

Example

This example loads an image, flips it horizontally, vertically, and both ways, then displays all results in separate windows.

python

import cv2

# Load image from file
image = cv2.imread('input.jpg')

if image is None:
    print('Error: Image not found or unable to load.')
else:
    # Flip horizontally
    flip_horizontal = cv2.flip(image, 1)

    # Flip vertically
    flip_vertical = cv2.flip(image, 0)

    # Flip both axes
    flip_both = cv2.flip(image, -1)

    # Show images
    cv2.imshow('Original', image)
    cv2.imshow('Flip Horizontal', flip_horizontal)
    cv2.imshow('Flip Vertical', flip_vertical)
    cv2.imshow('Flip Both', flip_both)

    cv2.waitKey(0)
    cv2.destroyAllWindows()

Output

Four windows open showing the original image and the three flipped versions.

Common Pitfalls

Common mistakes when flipping images with OpenCV include:

• Using incorrect flipCode values other than 0, 1, or -1, which will not flip the image as expected.
• Not checking if the image loaded correctly before flipping, which can cause errors.
• Forgetting to call cv2.waitKey() and cv2.destroyAllWindows() after cv2.imshow(), so windows may not display or close properly.

python

import cv2

image = cv2.imread('input.jpg')

if image is None:
    print('Error: Image not found or unable to load.')
else:
    # Wrong flipCode example (no flip)
    flip_wrong = cv2.flip(image, 2)  # Invalid flipCode

    # Correct flipCode
    flip_correct = cv2.flip(image, 1)  # Horizontal flip

    cv2.imshow('Wrong FlipCode', flip_wrong)
    cv2.imshow('Correct FlipCode', flip_correct)
    cv2.waitKey(0)
    cv2.destroyAllWindows()

Output

The 'Wrong FlipCode' window shows the original image unchanged; 'Correct FlipCode' shows the horizontally flipped image.

Quick Reference

Flip Code	Flip Direction
0	Flip vertically (upside down)
1	Flip horizontally (mirror image)
-1	Flip both vertically and horizontally

Key Takeaways

Use cv2.flip(image, flipCode) with flipCode 0, 1, or -1 to flip images vertically, horizontally, or both.

Always check if the image loaded successfully before flipping to avoid errors.

Remember to use cv2.waitKey() and cv2.destroyAllWindows() to properly display and close image windows.

Invalid flipCode values will not flip the image and may cause confusion.

Flipping images is useful for data augmentation and image preprocessing in computer vision.