procedure TForm1.TrackBar1Change(Sender: TObject);
begin
  ImageEnView1.Update();
end;

// 0=Black, 100=Full Brightness
procedure DarkenBitmap(Bitmap: TBitmap; BrightnessFactor: Integer);
type
  TRGBTripleArray = ARRAY[Word] of TRGBTriple;
  pRGBTripleArray = ^TRGBTripleArray;
var
  x, y: Integer;
  Row: PRGBTripleArray;
  R, G, B: Byte;
begin
  if ( Bitmap.PixelFormat <> pf24bit ) or ( BrightnessFactor < 0 ) or ( BrightnessFactor > 100 ) then
    raise Exception.create( 'Cannot darken' );

  for y := 0 to Bitmap.Height - 1 do
  begin
    Row := Bitmap.ScanLine[y];
    for x := 0 to Bitmap.Width - 1 do
    begin
      // Get RGB components
      R := Row[x].rgbtRed;
      G := Row[x].rgbtGreen;
      B := Row[x].rgbtBlue;

      // Apply brightness scaling
      Row[x].rgbtRed   := Round( R * BrightnessFactor / 100.0 );
      Row[x].rgbtGreen := Round( G * BrightnessFactor / 100.0 );
      Row[x].rgbtBlue  := Round( B * BrightnessFactor / 100.0 );
    end;
  end;
end;

procedure TForm1.ImageEnView1DrawBackBuffer(Sender: TObject);
begin
  DarkenBitmap( ImageEnView1.BackBuffer, TrackBar1.Position );
end;

procedure TForm1.ImageEnView1DrawLayer(Sender: TObject; Dest: TIEBitmap; LayerIndex: Integer; DestRect: TRect);
var
  brightness: Integer;
begin
  brightness := TrackBar1.Position; // 0=Black, 100=Full Brightness
  IEAdjustBrightnessContrastSaturation( Dest, Round(brightness * 2.56) { move into range 0-256}, 0, 256, nil, nil );
end;