Delphi怎样实现将一幅小位图粘贴到另一幅大位图上去?


procedure TForm1.imgCarPhotoClick(Sender: TObject);
var
  bmpLicense,bmpDigital,bmpClipboard,bmpMask:TBitmap;
  rgn:Hrgn;
  CurrentX,CurrentY:integer;
begin
  FilePath:=ExtractFilePath(Application.ExeName)+'tp\';
  if CombineLicense=1 then      
  begin
    if LicenseType=0 then       //大型汽车
    begin
      FileName:='H!_.bmp';
    end;
    if LicenseType=1 then       //小型汽车
    begin
      FileName:='!.bmp';
    end;
    if LicenseType=2 then       //农用运输车
    begin    end;
    if LicenseType=3 then       //教练汽车
    begin
      FileName:='H%.bmp';
    end;
    if LicenseType=4 then       //警用汽车
    begin
      FileName:='[email protected]';
    end;    bmpLicense:=TBitmap.Create;
    bmpDigital:=TBitmap.Create;
    bmpClipboard:=TBitmap.Create;
    bmpMask:=TBitmap.Create;
    self.DoubleBuffered:=True;      //设置双缓冲
    //调入牌照
    imgLicense.Picture.Bitmap.LoadFromFile(FilePath+FileName);
    bmpLicense.Assign(imgLicense.Picture.Bitmap);
    //调入掩膜图
    {
    FileName:='@.bmp';
    imgLicense.Picture.Graphic:=nil;
    imgLicense.Picture.Bitmap.LoadFromFile(FilePath+FileName);
    bmpMask.Assign(imgLicense.Picture.Bitmap);
    }
    //创建剪切区域
    CurrentX:=Mouse.CursorPos.X-imgCarPhoto.Left-Panel1.Left;
    CurrentY:=Mouse.CursorPos.Y-imgCarPhoto.Top-Panel1.Top-20;
    rgn:=CreateRectRgn(CurrentX,CurrentY,CurrentX+bmpLicense.Width,CurrentY+bmpLicense.Height);
    SelectClipRgn(imgCarPhoto.Canvas.Handle,rgn);    //将该区域剪切到bmpClipboard
    //合并图像
    imgCarPhoto.Picture:=nil;    //此行清空图片
    //用掩膜图处理车牌图片
    //BitBlt(bmpLicense.Canvas.Handle,0,0,bmpLicense.Width,bmpLicense.Height,bmpMask.Canvas.Handle,0,0,MERGEPAINT);
    //用掩膜图处理背景
    //BitBlt(imgCarPhoto.Canvas.Handle,CurrentX,CurrentY,bmpMask.Width,bmpMask.Height,bmpMask.Canvas.Handle,0,0,NOTSRCERASE);
    //把处理过的车牌图片贴到背景上
    BitBlt(imgCarPhoto.Canvas.Handle,CurrentX,CurrentY,bmpLicense.Width,bmpLicense.Height,bmpLicense.Canvas.Handle,0,0,SRCCOPY);    //释放资源
    bmpLicense.Free;
    bmpMask.Free;
    bmpClipboard.Free;
    bmpDigital.Free;
  end;
end;代码如上,只能将原图清空后再粘贴,怎么能不清空才能粘贴上去呢?