Mat imread(const string& filename, int flags=1): This function loads an image from the specified file and returns it. It also returns an empty matrix if the image cannot be read. It supports the most usual image formats of the files, detected by their content rather than by their extension. The flags parameter indicates the color of the image loaded in the memory, which may differ from the original color of the image stored in the file. In the example code, this function is used as follows:

Mat in_image = imread(argv[2], CV_LOAD_IMAGE_GRAYSCALE);

Here, the filename is obtained from the command-line arguments (the second argument after the command name). The CV_LOAD_IMAGE_GRAYSCALE flag indicates that the image should be loaded in the memory as an 8 bit grayscale image. For a description of the available tags, it is recommended to read the OpenCV online documentation (available at http://docs.opencv.org/).

bool imwrite(const string& filename, InputArray img, const vector<int>& params=vector<int>()): This function writes an image to a given file where some optional format parameters are specified after the second argument. The format of the output file is determined by the file extension. In our example code, this function is used without the format parameters as follows:

imwrite(argv[3], out_image);

void namedWindow(const string& winname, int flags=WINDOW_AUTOSIZE): This function creates a window without displaying it. The first argument is a string used as a name for the window and its identifier. The second argument is a flag or flag combination, which controls some window properties (for example, enable resize). Next, we show how this function is used in the example using a constant string as a name for the created window, as follows:

namedWindow("Flipped …"); // Creates a window

Compiling OpenCV with Qt adds some new functionality to the highgui module (more on that later). Then, the window created with Qt and the namedWindow function uses the default flags: CV_WINDOW_AUTOSIZE, CV_WINDOW_KEEPRATIO, or CV_GUI_EXPANDED.

void imshow(const string& winname, InputArray mat): This function displays an array (image) in a window with the properties set previously with the specified flags when the window was created. In the example, this function is used as follows:

imshow(win, out_image); // Shows output image on window

int waitKey(int delay=0): This function waits for a key press or the milliseconds specified by delay (if delay is greater than zero). If delay is less than or equal to zero, it waits infinitely. It returns the key code if pressed or -1 if a key is not pressed after the delay. This function has to be used after creating and activating a window. In the example code, it is used as follows:

waitKey(); // Wait infinitely for key press