Transform img to 1/2

IplImage* doPyrDown(
  IplImage* in)
{

    // Best to make sure input image is divisible by two.
    //
    assert( in->width%2 == 0 && in->height%2 == 0 );

    IplImage* out = cvCreateImage( 
        cvSize( in->width/2, in->height/2 ),
        in->depth,
        in->nChannels
    );
    cvPyrDown( in, out );
    return( out );
};

int _tmain(int argc, _TCHAR* argv[])
{
  IplImage* img = cvLoadImage("lena.jpg");
  IplImage* img2;
  cvNamedWindow("Example1", CV_WINDOW_AUTOSIZE );
  cvNamedWindow("Example2", CV_WINDOW_AUTOSIZE );
  cvShowImage("Example1", img );
  img2 = doPyrDown( img );
  cvShowImage("Example2", img2 );
  cvWaitKey(0);
  cvReleaseImage( &img );
  cvReleaseImage( &img2 );
  cvDestroyWindow("Example1");
  cvDestroyWindow("Example2");
}