dcp::Size inter_size = _viewer->inter_size ();
glVertex2f (inter_position.x, inter_position.y + (canvas_size.GetHeight() - out_size.height) / 2);
glVertex2f (inter_position.x + inter_size.width, inter_position.y + (canvas_size.GetHeight() - out_size.height) / 2);
dcp::Size inter_size = _viewer->inter_size ();
glVertex2f (inter_position.x, inter_position.y + (canvas_size.GetHeight() - out_size.height) / 2);
glVertex2f (inter_position.x + inter_size.width, inter_position.y + (canvas_size.GetHeight() - out_size.height) / 2);