How to reduce colors to a specified number in Delphi?

Question

How can I reduce colors to a specified number (<=256) in Delphi? I don't want to just use:

 Bmp.PixelFormat := pf8bit;

because that way I cannot control number of colors. I don't want dithering, because I already know how to dither an image with 256 or less colors.

I found this Median Cut implementation but it is pure Pascal from 1990 and:

1. doesn't compile in Delphi
2. Says it's shareware and costs 25 Deutche Marks
3. Looks (somehow) unnecessary complicated

I want to reduce TBitmap32 only (Graphics32 bitmap class, supports only 32bit colors) to <= 8bit bmp. I don't need to reduce to 15/16bit, I don't need to reduce from 24 or 15/16bit images. Just 32bit => 8bit-

Delphi I use: 7, 2005, XE3.

Answer 1

An fast implemented,cheap way with many options would be the usage of TGIFImage

uses
  gifimg;



 Procedure ReduceTo8Bit(var bmp:TBitmap; ColorReduction: TColorReduction; DitherMode: TDitherMode);
var
 GI:TGifImage;
begin
   GI:=TGifImage.Create;
   try
     GI.DitherMode := DitherMode;
     GI.ColorReduction := ColorReduction;
     GI.Assign(bmp);
     bmp.Assign(GI.Bitmap);
   finally
     GI.Free;
   end;
end;

TEST

procedure TForm3.Button2Click(Sender: TObject);
var
 bmp:TBitmap;
begin
  bmp:=TBitmap.Create;
  try
     bmp.LoadFromFile('C:\bilder\bummi.bmp');
     ReduceTo8Bit(bmp,rmQuantizeWindows,dmSierra);
     bmp.SaveToFile('C:\bilder\bummi_8bit.bmp');
  finally
    bmp.Free;
  end;
end;

a easier way will be using ReduceColors with rmQuantize from gifimg, if bits per pixel have to be set

// BytesPerPixel integer with range of Range 3 - 8

DestBMP := ReduceColors(SourceBMP,rmQuantize,dmNearest,BytesPerPixel,0);