Drag mouse to get rectangular region

void on_mouse( int event, int x, int y, int flags, void* param )
{
if( !image )
return;
if( image->origin )
y = image->height - y;
if( select_object )
{
selection.x = MIN(x,origin.x);
selection.y = MIN(y,origin.y);
selection.width = selection.x + CV_IABS(x - origin.x);
selection.height = selection.y + CV_IABS(y - origin.y);
selection.x = MAX( selection.x, 0 );
selection.y = MAX( selection.y, 0 );
selection.width = MIN( selection.width, image->width );
selection.height = MIN( selection.height, image->height );
selection.width -= selection.x;
selection.height -= selection.y;
}

switch( event )
{
case CV_EVENT_LBUTTONDOWN:
origin = cvPoint(x,y);
selection = cvRect(x,y,0,0);
select_object = 1;
break;

case CV_EVENT_LBUTTONUP:
select_object = 0;
if( selection.width > 0 && selection.height > 0 )
track_object = -1;
break;
}
}///////////////// show sub-image

myImg = cvCloneImage(image);
cvSetImageROI( myImg, selection );
cvNamedWindow("help", 1);
cvShowImage("help", myImg);
///////////// or
CvMat *sub=cvCreateMat(selection.width,selection.height,myImg->depth);
cvGetSubRect(image, sub, selection);
IplImage *res=cvGetImage(sub, myImg);
cvShowImage("Histogram", res);