How to Crop Image Using OpenCV in Computer Vision

To crop an image in OpenCV, use array slicing on the image matrix with image[y1:y2, x1:x2] where x1, y1 is the top-left corner and x2, y2 is the bottom-right corner of the crop area. This extracts the region of interest as a new image.

Syntax

In OpenCV with Python, images are stored as arrays. Cropping uses array slicing with the syntax cropped_image = image[y1:y2, x1:x2].

• image: the original image array
• y1:y2: vertical pixel range (rows)
• x1:x2: horizontal pixel range (columns)
• The result is a new image containing only the selected rectangle.

python

cropped_image = image[y1:y2, x1:x2]

Example

This example loads an image, crops a 100x100 pixel square from coordinates (50, 50) to (150, 150), and displays both original and cropped images.

python

import cv2

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

# Define crop coordinates
x1, y1 = 50, 50
x2, y2 = 150, 150

# Crop the image using slicing
cropped_image = image[y1:y2, x1:x2]

# Show original and cropped images
cv2.imshow('Original Image', image)
cv2.imshow('Cropped Image', cropped_image)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output

Two windows open: one showing the original image, the other showing the cropped 100x100 pixel area.

Common Pitfalls

• Mixing up x and y coordinates: y is row (height), x is column (width).
• Using coordinates outside image bounds causes errors or empty crops.
• Remember slicing excludes the end index pixel, so y1:y2 crops up to y2-1.
• Not checking if the image loaded correctly before cropping can cause crashes.

python

import cv2

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

# Wrong: x and y swapped
# cropped = image[50:150, 50:150]  # This is correct usage

# Wrong: coordinates outside bounds
# cropped = image[1000:1100, 1000:1100]  # Likely empty or error

# Right way: check image shape
if image is not None:
    height, width = image.shape[:2]
    x1, y1 = 50, 50
    x2, y2 = min(150, width), min(150, height)
    cropped = image[y1:y2, x1:x2]

Quick Reference

Operation	Syntax	Description
Crop image	cropped = image[y1:y2, x1:x2]	Extracts a rectangular region from the image
Check image shape	height, width = image.shape[:2]	Get image dimensions (height and width)
Show image	cv2.imshow('title', image)	Display image in a window
Wait for key	cv2.waitKey(0)	Waits for a key press to close window
Close windows	cv2.destroyAllWindows()	Closes all OpenCV windows

Key Takeaways

Crop images in OpenCV using array slicing with image[y1:y2, x1:x2].

Always verify image loaded correctly before cropping to avoid errors.

Remember y is vertical (rows), x is horizontal (columns) in slicing.

Ensure crop coordinates are within image dimensions.

Use OpenCV functions to display images and verify cropping visually.