Learning OpenCV

Displaying Images

Now we are ready for what we really want to do, and that is to load an image and to put it into the window where we can view it and appreciate its profundity. We do this via one simple function, cvShowImage():

void cvShowImage(
  const char*  name,
  const CvArr* image
);

The first argument here is the name of the window within which we intend to draw. The second argument is the image to be drawn.

Let's now put together a simple program that will display an image on the screen. We can read a filename from the command line, create a window, and put our image in the window in 25 lines, including comments and tidily cleaning up our memory allocations!

int main(int argc, char** argv)
{

  // Create a named window with the name of the file.
  cvNamedWindow( argv[1], 1 );

  // Load the image from the given file name.
  IplImage* img = cvLoadImage( argv[1] );

  // Show the image in the named window
  cvShowImage( argv[1], img );

  // Idle until the user hits the "Esc" key.
  while( 1 ) {
    if( cvWaitKey( 100 ) == 27 ) break;
  }

  // Clean up and don't be piggies
  cvDestroyWindow( argv[1] );
  cvReleaseImage( &img );
  exit(0);
}

For convenience we have used the filename as the window name. This is nice because OpenCV automatically puts the window name at the top of the window, so we can tell which file we are viewing (see Figure 4-1). Easy as cake.

Figure 4-1. A simple image displayed with cvShowImage()

Before ...