OpenCV Getting Started Example

Next, let's look at a simple OpenCV example. We will implement the following features:

  1. Read an image.

  2. Display the image.

  3. Save the image.

  4. Add simple user interaction.

You can use the following image as an example:

The following code is saved indisplay_image.pyfile:

Example (Python)

# Import the OpenCV library
import cv2

# 1. Read the image
# Replace with the actual image path, here it is "bird.jpg" in the current directory
image_path = "./bird.jpg"
image = cv2.imread(image_path)

# Check whether the image was successfully read
if image is None:
    print(Error: Unable to load image, please check whether the path is correct.)
    exit()

# 2. Display the image
# Create a window named "Display Image" and display the image in it
cv2.imshow("Display Image", image)

# 3. Wait for the user to press a key
# Parameter 0 means wait indefinitely until the user presses any key
key = cv2.waitKey(0)

# 4. Perform actions based on the user's key press
if key == ord('s'):  # If the 's' key is pressed
    # Save the image
    output_path = "saved_image.jpg"
    cv2.imwrite(output_path, image)
    print(fImage saved as {output_path})
else:  # If another key is pressed
    print(Image was not saved.)

# 5. Close all windows
cv2.destroyAllWindows()
Run the following command:
python display_image.py

A pop-up window displays as follows:

In the displayed image window, press theskey to save the image, or press any other key to exit.

The C++ code for the above example is as follows:

Example (C++)

#include <opencv2/opencv.hpp>
#include <iostream>

using namespace cv;
using namespace std;

int main() {
    // 1. Read the image
    // Replace with the actual image path, here it is "bird.jpg" in the current directory
    string image_path = "./bird.jpg";
    Mat image = imread(image_path);

    // Check whether the image was successfully read
    if (image.empty()) {
        cout << Error: Unable to load image, please check whether the path is correct. << endl;
        return -1;
    }

    // 2. Display the image
    // Create a window named "Display Image" and display the image in it
    namedWindow("Display Image", WINDOW_AUTOSIZE);
    imshow("Display Image", image);

    // 3. Wait for the user to press a key
    // Parameter 0 means wait indefinitely until the user presses any key
    int key = waitKey(0);

    // 4. Perform actions based on the user's key press
    if (key == 's') {  // If the 's' key is pressed
        // Save the image
        string output_path = "saved_image.jpg";
        imwrite(output_path, image);
        cout << Image saved as << output_path << endl;
    } else {  // If another key is pressed
        cout << Image was not saved. << endl;
    }

    // 5. Close all windows
    destroyAllWindows();

    return 0;
}

Extended Exercises

Next, we can try making some code modifications.

1. Change the window title:Change "Display Image" to another name, for example "My Image".

cv2.imshow("My Image", image)

2. Save in a different format:Change "saved_image.jpg" to "saved_image.png" and observe the saved result.

# 4. 根据用户按键执行操作
if key == ord('s'):  # 如果按下 's' 键
    # 保存图像
    output_path = "saved_image.png"
    cv2.imwrite(output_path, image)
    print(f"图像已保存为 {output_path}")

3. Add more interactions:For example, press the 'q' key to exit the program directly.

if key == ord('s'):  # 如果按下 's' 键
    # 保存图像
    output_path = "saved_image.jpg"
    cv2.imwrite(output_path, image)
    print(f"图像已保存为 {output_path}")
elif key == ord('q'):  # 如果按下 'q' 键
    print("程序已退出。")
else:  # 如果按下其他键
    print("图像未保存，程序已退出。")

  • ord('q')Return the character'q''s ASCII code value.

  • If the user presses'q'key, print a prompt message"The program has exited.", then the program will continue to executecv2.destroyAllWindows()close the window and exit.

Other Extensions