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type TBuffer = Array of Byte;

procedure ROT13(var aBuffer: TBuffer);
var i: Integer;
begin
  for i := 0 to {...} do
    case {...} of
      97..122: aBuffer[i] := (ord(aBuffer[i]) - 97 + 13) MOD 26 + 97;
       65..90: {...}
    end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var abuffer: TBuffer;
  fs: TFileStream;
begin
  if OpenDialog1.Execute then
  begin
    fs := TFileStream.Create({...}, fmOpenRead  );
    setlength({}, fs.size);
    fs.Read(aBuffer[0], length(abuffer));
    fs.Free;
    ROT13({...});
    if SaveDialog1.Execute then
    begin
      fs := TFileStream.Create(SaveDialog1.FileName, fmCreate);
      {...}
      fs.Free;
    end;
  end;
end;					

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function GetG(aColor: TColor):Byte;
begin
  result := (aColor shr 8) AND ($0000FF);
end;
//...
for x := 0 to Image1.Picture.Bitmap.Width - 1 do
  for y := 0 to Image1.Picture.Bitmap.Height - 1 do
    if ((GetG(Image1.Picture.Bitmap.Canvas.Pixels[x,y])) MOD 2 = 0) then
      Image2.Canvas.Pixels[x,y] := clblack
    else
      {...}