Cannot Assign TBitmapCanvas to TBitmapCanvas

[sk.Silvia]

hi alle

ich hab probleme mit diesen part fo code

Delphi-Quellcode:

  
  procedure TEngine.Show;
  var tmp:TBitmap;
      begin
      tmp:=TBitmap.Create;
      tmp.Canvas.Assign(Screen.Canvas);

      ShowScr.Canvas.Draw(-10,-10,tmp);
      ShowScr.Canvas.LineTo(50,50);
      end;

it creates a runtime error "Cannot Assign TBitmapCanvas to TBitmapCanvas" what is nonesence..isnt it??
i just only want to save the canvas into TBitmap (and dont want to make it over Pixels[], cuz its too slow)