MFC: I read this code which is to draw an ellipse (not solid interior), but I cannot understand why function "pDC->Ellipse(...)" is needed twice here?

 CDC *pDC=GetDC();
 CPen pen;
     CBrush brush;
 getpen(pen,pDC,col,bol);
if(do_what>=DRAW_LINE&&do_what<=DRAW_RRECT){
    p->p[0]=start;
    p->p[1]=end;
    if(sol==1){
             getbrush(brush,pDC,col);
        }
    if(do_what==DRAW_LINE){
             pDC->MoveTo(start);
             pDC->LineTo(end);
        }
    else if(do_what==DRAW_ELLIPSE||do_what==DRAW_CIRCLE){
         pDC->SetROP2(R2_NOT);
         assist=start;
         if(do_what==DRAW_CIRCLE){
                  assist.y=end.y-end.x+start.x;
             }
         pDC->Ellipse(start.x,assist.y,end.x,end.y);
         pDC->SetROP2(R2_COPYPEN);
         if(sol==0){
                   pDC->SelectStockObject(NULL_BRUSH);
             }
         if(do_what==DRAW_CIRCLE){
                   assist.y=point.y-point.x+start.x;
             }
         pDC->Ellipse(start.x,assist.y,point.x,point.y);
         end=point;
          }
     }

If I remove the first one, the ellipse will be black solid inside. If I remove the second one, the ellipse will never be drawn but disappears when left mouse button is up.

the dialog: when moving mouse: mouse moving

when mouse button pops: mouse button pops

Besides, what color of CBrush is if I use "CBrush brush; pDC->Ellipse(start.x,assist.y,end.x,end.y);"