Reading Images

In OpenCV, the cv::imread() function is used to load images from a file. This function returns a cv::Mat object, which is the primary data structure in OpenCV for storing image data.

// Load a color image
cv::Mat imgData = cv::imread("sample_image.png", cv::IMREAD_COLOR);

The cv::imread() function accepts two arguemnts:

1. The file path of the image to load.
2. The loading mode, which can be:
   • cv::IMREAD_COLOR (default, loads as a color image)
   • cv::IMREAD_GRAYSCALE (loads as a grayscale image)
   • cv::IMREAD_UNCHANGED (loads with alpha channel if present)

These flags can also be represented by integers: 1, 0, or -1, respectively.

If the file path is incorrect, cv::imread() returns an empty cv::Mat object without throwing an error. To handle this, use the empty() method to check for successful loading.

if (imgData.empty()) {
    std::cerr << "Failed to load image!" << std::endl;
    return EXIT_FAILURE;
}

Dispplaying Images

To display an image, create a window using cv::namedWindow() and then show the image with cv::imshow().

// Create a window for displaying the image
cv::namedWindow("Display", cv::WINDOW_AUTOSIZE);
// Display the image in the window
cv::imshow("Display", imgData);
std::cout << "Image loaded successfully!" << std::endl;

cv::namedWindow() takes two parameters:

1. The window name.
2. A window property flag, such as:
   • cv::WINDOW_NORMAL (resizable window)
   • cv::WINDOW_AUTOSIZE (window size adjusts to the image)
   • cv::WINDOW_FREERATIO (window can be resized without aspect ratio constraints)

cv::imshow() requires:

1. The window name (must match the one used in cv::namedWindow()).
2. The image object to display.

To keep the window open, use cv::waitKey() to wait for a key press.

cv::waitKey(0);

The parameter to cv::waitKey() specifies the maximum wait time in milliseconds. A value of 0 means it will wait indefinitely until a key is pressed.

Closing Windows

Windows can be closed using cv::destroyAllWindows() to close all windows, or cv::destroyWindow() to close a specific window.

// Close all windows
cv::destroyAllWindows();
// Close a specific window
cv::destroyWindow("Display");

Writing Images

To save an image to a file, use the cv::imwrite() function.

cv::imwrite("saved_image.jpg", imgData);

cv::imwrite() takes two argumants:

1. The output filename.
2. The image object to save.

The image will be saved in the current working directory with the specified format.

Complete Example

Here is a full program that demonstrates loading, displaying, and saving an image.

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

int main() {
    // Load a color image
    cv::Mat imgData = cv::imread("input.jpg", cv::IMREAD_COLOR);

    // Check if the image was loaded successfully
    if (imgData.empty()) {
        std::cerr << "Failed to load image!" << std::endl;
        return EXIT_FAILURE;
    }

    // Create a window and display the image
    cv::namedWindow("Display", cv::WINDOW_AUTOSIZE);
    cv::imshow("Display", imgData);
    std::cout << "Image loaded successfully!" << std::endl;

    // Save a copy of the image
    cv::imwrite("output.jpg", imgData);

    // Wait for a key press, then close windows
    cv::waitKey(0);
    cv::destroyAllWindows();

    return EXIT_SUCCESS;
}