Learning OpenCV

First Program—Display a Picture

OpenCV provides utilities for reading from a wide array of image file types as well as from video and cameras. These utilities are part of a toolkit called HighGUI, which is included in the OpenCV package. We will use some of these utilities to create a simple program that opens an image and displays it on the screen. See Example 2-1.

Example 2-1. A simple OpenCV program that loads an image from disk and displays it on the screen

#include "highgui.h"

int main( int argc, char** argv ) {
    IplImage* img = cvLoadImage( argv[1] );
    cvNamedWindow( "Example1", CV_WINDOW_AUTOSIZE );
    cvShowImage( "Example1", img );
    cvWaitKey(0);
    cvReleaseImage( &img );
    cvDestroyWindow( "Example1" );
}

When compiled and run from the command line with a single argument, this program loads an image into memory and displays it on the screen. It then waits until the user presses a key, at which time it closes the window and exits. Let's go through the program line by line and take a moment to understand what each command is doing.

IplImage* img = cvLoadImage( argv[1] );

This line loads the image.^[10] The function cvLoadImage() is a high-level routine that determines the file format to be loaded based on the file name; it also automatically allocates the memory needed for the image data structure. Note that cvLoadImage() can read a wide variety of image formats, including BMP, DIB, JPEG, JPE, PNG, PBM, PGM, PPM, SR, RAS, and TIFF. A pointer to an allocated image data structure is ...