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There is no standard event to handle mousewheel down or up on the onMouse method. You can, however, combine more than one event to create a drag mouse effect, which would complete your task.

Something like

void MouseCB::onMouse( int event, int x, int y, int flags, void* userData)
{
    if (event == CV_EVENT_LBUTTONDOWN)
           {
               int leftcornerX = x;
               int leftcornerY = y;

               while( event != CV_EVENT_LBUTTONUP)
               {
                 //method to retrieve your originalframe with no rectangles drawn -> either the original frame is a
                 //global variable, or you have to pass it using the void* userData input argument.

                 Mat copy = originalframe;
                 rectangle(copy, Point(leftcornerX, leftcornerY), Point(x, y), Scalar(255, 0, 0), 1, 8, 0);
                 imshow("Frame", copy);
                 waitKey(10);


               }
           }

}

This draws a rectangle between the point you first clicked with the left mouse button and the current mouse position, until you lift the mouse button.

There is no standard event to handle mousewheel down or up on the onMouse method. You can, however, combine more than one event to create a drag mouse effect, which would complete your task.

Something like

void MouseCB::onMouse( int event, int x, int y, int flags, void* userData)
{
    if (event == CV_EVENT_LBUTTONDOWN)
        {
         int leftcornerX = x;
         int leftcornerY = y;

         while( event != CV_EVENT_LBUTTONUP)
              {
             //method to retrieve your originalframe with no rectangles drawn -> either the original frame is a
             //global variable, or you have to pass it using the void* userData input argument.

                 Mat copy = originalframe;
             rectangle(copy, Point(leftcornerX, leftcornerY), Point(x, y), Scalar(255, 0, 0), 1, 8, 0);
             imshow("Frame", copy);
             waitKey(10);


            }
       }

}

This draws a rectangle between the point you first clicked with the left mouse button and the current mouse position, until you lift the mouse button.