[Property] Empty bitmap. Why?

[WojTec]

Delphi-Quellcode:

type
  // Base
  TCustomFilterLayer = class(TPositionedLayer)
  private
    FBitmap: TBitmap32;
    // ...
  protected
    procedure Paint(Buffer: TBitmap32); override;  // get layer rect and repaint buffer with filter; work with FBitmap
    procedure DoFilter(Buffer: TBitmap32); virtual; abstract;
  public
    constructor Create(ALayerCollection: TLayerCollection); override;  // create FBitmap
    destructor Destroy; override;  // free FBitmap
  published
    property Bitmap: TBitmap32 read FBitmap;
    // ...
  end;

  // Example filter
  TInvertFilterLayer = class(TCustomFilterLayer)
  protected
    procedure DoFilter(Buffer: TBitmap32); override;  // buffer is received in TCustomFilterLayer.Paint()
  end;

var
  L: TCustomFilterLayer;
// ...
  L := TInvertFilterLayer.Create(ImgView.Layers);
  TInvertFilterLayer(L).Bitmap.SaveToFile(...);

Only one task for TCustomFilterLayer is receive bitmap from buffer in rect=layer rect. So, in filter class just need to override DoFilter() method to get processed image in layer. It working for any filter I created.

But question is about Bitmap property: when I want to save it, eg. to file, output is empty. Why?