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type
  TRGB = record
    b, g, r: byte;
  end;
  ARGB = array[0..1] of TRGB;
  PARGB = ^ARGB;

var
  b: TBitMap;

procedure TForm1.FormCreate(sender: TObject);
begin
  b := TBitMap.Create;
  b.pixelformat := pf24bit;
  b.width := Clientwidth;
  b.height := Clientheight;
end;

procedure TForm1.Tim1OnTimer(sender: TObject);
var
  p: PARGB;
  x, y: integer;
begin
  for y := 0 to b.height - 1 do
  begin
    p := b.scanline[y];
    for x := 0 to b.width - 1 do
    begin
      p[x].r := random(256);
      p[x].g := random(256);
      p[x].b := random(256);
    end;
  end;
  canvas.draw(0, 0, b);
end;

procedure TForm1.FormDestroy(sender: TObject);
begin
  b.free;
end;