BMP -> JPEG 灰階化

//灰階處理
  imgBMP := TBitmap.create;
  imgBMP.LoadFromFile(ret_filename);
  pr_Set_Gray(imgBMP);
  imgJPG := TJPEGImage.Create;
  imgJPG.Grayscale :=True;
  imgJPG.Assign(imgBmp);
  imgJPG.SaveToFile(ret_filename);

procedure pr_Set_Gray(var Picture: TBitmap);
var iX, iY:Integer;
  rgb1: TColor;
  f, r, g, b: byte;
begin
  for iX:=0 to Picture.Width-1 do
  begin
    for iY:=0 to Picture.Height-1 do
    begin
      rgb1:= Picture.Canvas.Pixels[iX, iY];
      r := GetRValue(rgb1);
      g := GetGValue(rgb1);
      b := GetBValue(rgb1);

      if b < 200 then
        f := 0
      else
        f := 255;

      Picture.Canvas.Pixels[iX, iY] := RGB(f, f, f);
    end;
  end;
end;