How to Read, Write, and Display Images in OpenCV with Python

Chapter 1.4: Reading, Writing, and Displaying Images with OpenCV

Once you have installed OpenCV, the first step in image processing is learning how to read, write, and display images. OpenCV provides simple functions for these tasks: cv2.imread(), cv2.imwrite(), and cv2.imshow().

📥 Reading an Image

Use cv2.imread() to read an image. By default, OpenCV loads images in BGR format.

import cv2

# Read an image from file
img = cv2.imread("sample.jpg")

# Check if image is loaded
if img is None:
    print("Error: Image not found!")
else:
    print("Image loaded successfully!")

🖼️ Displaying an Image

To display an image, use cv2.imshow(). You must also include cv2.waitKey() to keep the window open until a key is pressed.

cv2.imshow("My Image", img)      # Window title + image
cv2.waitKey(0)                   # Wait until a key is pressed
cv2.destroyAllWindows()          # Close all windows

💾 Saving an Image

Use cv2.imwrite() to save an image to your system.

# Save the image as PNG
cv2.imwrite("output.png", img)

🔧 Reading Modes

The cv2.imread() function supports different modes:

• 🔹 cv2.IMREAD_COLOR (default) – Loads image in color (BGR).
• 🔹 cv2.IMREAD_GRAYSCALE – Loads image in grayscale (single channel).
• 🔹 cv2.IMREAD_UNCHANGED – Loads image as is (including alpha channel if present).

🚀 Example: Load and Save Grayscale

gray = cv2.imread("sample.jpg", cv2.IMREAD_GRAYSCALE)
cv2.imshow("Grayscale Image", gray)
cv2.waitKey(0)
cv2.destroyAllWindows()

cv2.imwrite("output_gray.jpg", gray)

✅ In this lesson, you learned how to read, display, and save images with OpenCV. This forms the foundation of image processing tasks.
👉 Next, we will explore basic image operations such as resize, crop, rotate, and flip.