鼠标画矩形

丿Clover灬夢

procedure TfrmCanvas.imgImageMouseDown(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  with imgImage do
  begin
    oldx := x;
    oldy := y;
    Canvas.Brush.Style := bsClear;//画刷填充方式
    Canvas.Pen.Style := psDot; //线条类型
    Canvas.Pen.Mode := pmXor;
    Canvas.Pen.Color := clRed;
    rold := Rect(0, 0, 0, 0); //创建一个矩形对象
  end;

  if ssright in Shift then
  begin
   imgImage.Picture.Bitmap := nil;
  end;
end;

procedure TfrmCanvas.imgImageMouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
begin
  if not (ssleft in Shift) then Exit;
  with imgImage do
  begin
    Canvas.Rectangle(rold);//Rectangle根据提供的两个点或一个矩形区域绘制一个矩形
    Canvas.Rectangle(oldx, oldy, x, y);
    rold := Rect(oldx, oldy, x, y);
  end;
end;

procedure TfrmCanvas.imgImageMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  with imgImage do
  begin
    Canvas.Pen.Style := psSolid;
    Canvas.Pen.Mode := pmCopy;
    Canvas.Pen.Color := clRed;
    Canvas.Rectangle(rold);
  end;
end;

问题是在MouseMove事件中    Canvas.Rectangle(rold);//Rectangle根据提供的两个点或一个矩形区域绘制一个矩形
   Canvas.Rectangle(oldx, oldy, x, y);
    rold := Rect(oldx, oldy, x, y);
主要是实现了什么，求告知。。。谢谢！！！