Convert pf24Bit truecolor bitmap to pf8Bit grayscale

[bobbie100]

I want to convert a truecolor bitmap with HandleType bmDIB and PixelFormat pf24Bit into a 256 grayscale with PixelFormat pf8Bit.

I have a function for converting a pf24Bit bitmap to grayscale

  procedure ConvertBmpToGrayscale(var Bmp: TBitmap);
  const
    MaxPixelCount = 32768;
  type
    pRGBArray = ^TRGBArray;
    TRGBArray = ARRAY[0..MaxPixelCount-1] of TRGBTriple;
  var
    I, J, Lum: Integer;
    Row: pRGBArray;  // Scanline
  begin
    for J := 0 to Bmp.Height - 1 do
    begin
      Row := Bmp.ScanLine[j];
      for I := 0 to Bmp.Width - 1 do
      begin
        Lum := MyRound(0.299 * Row[I].rgbtRed + 0.587 * Row[I].rgbtGreen +
                       0.114 * Row[I].rgbtBlue);
        Row[I].rgbtRed := Lum;
        Row[I].rgbtGreen := Lum;
        Row[I].rgbtBlue := Lum;
      end;
    end;
  end;

I want to convert this to a procedure of the form:
procedure ConvertColorToGrayScale(const Bmp24: TBitmap, var Bmp8: TBitmap);
where Bmp8 is the output bitmap in pf8Bit format.

The main question I have is how do I set up the palette for the output bitmap to be a 256 grayscale?

Bob