Stretch Bitmap

procedure StretchGraphic(const src, dest: TGraphic;
  DestWidth, DestHeight: integer; Smooth: Boolean = true);
var
  temp, aCopy: TBitmap;
  faktor: double;
begin
  Assert(Assigned(src) and Assigned(dest));
  if (src.Width = 0) or (src.Height = 0) then
    raise Exception.CreateFmt('Invalid source dimensions: %d x %d',[src.Width, src.Height]);
  if src.Width > DestWidth then
    begin
      faktor := DestWidth / src.Width;
      if (src.Height * faktor) > DestHeight then
        faktor := DestHeight / src.Height;
    end
  else
    begin
      faktor := DestHeight / src.Height;
      if (src.Width * faktor) > DestWidth then
        faktor := DestWidth / src.Width;
    end;
  try
    aCopy := TBitmap.Create;
    try
      aCopy.PixelFormat := pf24Bit;
      aCopy.Assign(src);
      temp := TBitmap.Create;
      try
        temp.Width := round(src.Width * faktor);
        temp.Height := round(src.Height * faktor);
        if Smooth then
          SetStretchBltMode(temp.Canvas.Handle, HALFTONE);
        StretchBlt(temp.Canvas.Handle, 0, 0, temp.Width, temp.Height,
          aCopy.Canvas.Handle, 0, 0, aCopy.Width, aCopy.Height, SRCCOPY);
        dest.Assign(temp);
      finally
        temp.Free;
      end;
    finally
      aCopy.Free;
    end;
  except
    on E: Exception do
      MessageBox(0, PChar(E.Message), nil, MB_OK or MB_ICONERROR);
  end;
end;

Beispiel :

procedure TForm1.Button1Click(Sender: TObject);
begin
  StretchGraphic(Image1.Picture.Bitmap, Image2.Picture.Bitmap,
                 600, 600,  true);
end;