急，在Delphi如何实现橡皮擦功能(100分)

各位仁兄，我现在写一个三层MIS系统，要处理部分图片，现要实现常用图像处理工具的橡皮擦功能。
请问如何实现（附源代码最佳）

gz

implementation

{$R *.DFM}
var
drawing:Boolean=false;
oPoint,sPoint:TPoint;
procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
spoint:=Point(x,y);
opoint:=sPoint;
drawing:=true;
Canvas.Brush.Style := bsclear;
Canvas.Pen.Style := psDot;
Canvas.Pen.Mode :=pmNotXor;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
Y: Integer);
begin
if not(drawing) then exit;
canvas.Ellipse(spoint.x,spoint.y,opoint.x,opoint.y);
canvas.Ellipse(spoint.x,spoint.y,x,y);
oPoint:=point(x,y);
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if not(drawing) then exit;
canvas.Ellipse(spoint.x,spoint.y,opoint.x,opoint.y);
Canvas.Pen.Style := psSolid;
canvas.Ellipse(spoint.x,spoint.y,x,y);
Drawing:=false;

end;

end.

接受答案了.