procedure TForm1.MakeTransparentByLightness(ImageEnView: TImageEnView; Threshold: Integer);
var
  x, y: Integer;
  IEBitmap: TIEBitmap;
  AlphaBitmap: TIEBitmap;
  PixelColor: TRGB;
  Lightness: Integer;
  AlphaScanLine: PByteArray;
begin
  IEBitmap := ImageEnView.IEBitmap; // Reference to the ImageEnView bitmap

  // Ensure the bitmap has an alpha channel
  if not IEBitmap.HasAlphaChannel then
    IEBitmap.AlphaChannel; 

  AlphaBitmap := IEBitmap.AlphaChannel; // Reference the alpha channel bitmap

  // Process each pixel to make those below or above the threshold transparent
  for y := 0 to IEBitmap.Height - 1 do
  begin
    AlphaScanLine := AlphaBitmap.ScanLine[y];
    for x := 0 to IEBitmap.Width - 1 do
    begin
      // Access the pixel color as TRGB
      PixelColor := IEBitmap.Pixels_ie24RGB[x, y];

      // Calculate the Lightness value
      Lightness := PARGBToIntL(PixelColor);

      if Lightness < Threshold then
        AlphaScanLine[x] := 0 // Make pixel fully transparent
      else
        AlphaScanLine[x] := 255; // Make pixel fully opaque
    end;
  end;

  // Ensure the alpha channel is in sync
  IEBitmap.SyncAlphaChannel;

  // Force the component to redraw:
  ImageEnView.Update; // Does NOT update the image!!
end;

procedure TForm1.ButtonLimitImageColorsClick(Sender: TObject);
begin
  MakeTransparentByLightness(ImageEnView1, 300);
end;

function TForm1.PARGBToIntL(ARGB: TRGB): Integer;
// converts TRGB to Integer Luminance value
var
  H, S, L: Double;
begin
  hyieutils.RGB2HSL(ARGB, H, S, L);
  Result := Round(L * 1000);
end;