控制组件上的透明位图

我基于TGraphic控件创建了一个组件。 当我直接在画布上画线时,它是透明的。但当我试图在画布上绘制位图时,它不会显示为透明。 如何在画布上绘制透明位图? 谢谢

unit Test;

interface

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

type
  TTest = class(TgraphicControl)
  private
    { Private declarations }
    BMP:TBitmap;
  protected
    { Protected declarations }
    Procedure Paint; Override;
  public
    { Public declarations }
    Constructor create(Aoner: Tcomponent); Override;
    Destructor Destroy; Override;
  published
    { Published declarations }
  end;

procedure Register;

implementation

procedure Register;
begin
  RegisterComponents('Automation', [TTest]);
end;

Constructor TTest.create(Aoner: TComponent);
begin
  Inherited Create(Aoner);
  Width:=100;
  Height:=100;
  Bmp := TBitmap.Create;
  BMp.Width:=50;
  BMP.Height:=50;
  BMP.Transparent:=true;
  BMP.TransparentColor:=BMp.Canvas.Pixels[0,0];
  BMP.Canvas.MoveTo(1,1);
  BMP.Canvas.LineTo(50,50);
End;

Procedure TTest.Paint;
Begin
  Inherited Paint;
  Canvas.MoveTo(0,0);
  Canvas.LineTo(100,100);
  Canvas.Draw(0,0,BMp);
End;

Destructor TTest.Destroy;
begin
  Inherited Destroy;
end;