winforms - C# Windows Forms. Moveable areas on the PictureBox -

i'm working images like: http://imgur.com/a2vkb

i've managed find vertical line between pages on scanned image. there errors , need make option user change line position , angle. think nice within picturebox.

i need somehow draw line between 2 movable points on picturebox current image. when move point, position of line , it's angle have changed properly.

here sample code can use need. uses 4 events :
- paint
- mousedown
- mousemove
- mouseup

you can copy-paste code form called form1, picture box called picturebox1

    int handleradius = 3;     int mpointmoveinprogress = 0;     point mpoint1, mpoint2;      public form1()     {         initializecomponent();          mpoint1 = new point(50, 50); // set correct default values         mpoint1 = new point(50, 300); // set correct default values     }      private void picturebox1_paint(object sender, painteventargs e)     {         // draw line         e.graphics.drawline(new pen(color.black, 2), mpoint1, mpoint2);          rectangle rectangle;          // draw first handle         rectangle = new rectangle(mpoint1.x - handleradius, mpoint1.y - handleradius, handleradius * 2, handleradius * 2);         e.graphics.fillrectangle(brushes.white, rectangle);         e.graphics.drawrectangle(pens.black, rectangle);          // draw second handle         rectangle = new rectangle(mpoint2.x - handleradius, mpoint2.y - handleradius, handleradius * 2, handleradius * 2);         e.graphics.fillrectangle(brushes.white, rectangle);         e.graphics.drawrectangle(pens.black, rectangle);     }      private void picturebox1_mousedown(object sender, mouseeventargs e)     {         // determine if point under cursor. if so, declare move in progress         if (math.abs(e.x - mpoint1.x) < handleradius && math.abs(e.y - mpoint1.y) < handleradius) mpointmoveinprogress = 1;         else if (math.abs(e.x - mpoint2.x) < handleradius && math.abs(e.y - mpoint2.y) < handleradius) mpointmoveinprogress = 2;         else mpointmoveinprogress = 0;     }      private void picturebox1_mousemove(object sender, mouseeventargs e)     {         if (mpointmoveinprogress == 1) // if moving first point         {             mpoint1.x = e.x ;             mpoint1.y = e.y ;             refresh();         }         else if (mpointmoveinprogress == 2) // if moving second point         {             mpoint2.x = e.x ;             mpoint2.y = e.y ;             refresh();         }         else // if moving in picturebox: change cursor hand if above handle         {             if (math.abs(e.x - mpoint1.x) < handleradius && math.abs(e.y - mpoint1.y) < handleradius) cursor.current = cursors.hand;             else if (math.abs(e.x - mpoint2.x) < handleradius && math.abs(e.y - mpoint2.y) < handleradius) cursor.current = cursors.hand;             else cursor.current = cursors.default;         }     }      private void picturebox1_mouseup(object sender, mouseeventargs e)     {         // declare no move in progress         mpointmoveinprogress = 0;     }