//奇妙的异或运算, 就这些简单的代码, 反复点击按钮...

procedure TForm1.Button1Click(Sender: TObject);
begin
  Canvas.Pen.Color := clYellow; {指定笔颜色为黄色}
  Canvas.Pen.Width := 20;       {笔宽 20}
  Canvas.Pen.Mode := pmXor;     {指定画笔模式为: 画笔色与背景色的异或运算}

  {画线}
  Canvas.MoveTo(0,0);
  Canvas.LineTo(ClientWidth,ClientHeight);
end;


//效果图:

//做一个橡皮线的示例:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

var
  x1,y1,x2,y2: Integer; {直线两个端点的坐标}
  f: Boolean;           {判断鼠标是否点下}


procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  x1 := X; y1 := Y;
  x2 := X; y2 := Y;
  Canvas.Pen.Color := clRed;
  Canvas.Pen.Width := 2;
  Canvas.Pen.Mode := pmXor;
  f := True;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if f then
  begin
    {擦除}
    Canvas.MoveTo(x1, y1);
    Canvas.LineTo(x2, y2);

    x2 := X; y2 := y;

    {重绘}
    Canvas.MoveTo(x1, y1);
    Canvas.LineTo(x2, y2);
  end;
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if f then
  begin
    {画出确定的线}
    Canvas.Pen.Mode := pmCopy;
    Canvas.MoveTo(x1, y1);
    Canvas.LineTo(x2, y2);
    f := not f;
  end;
end;

end.


//效果图: