Bitmap generator in erlang

bmp.erl

-module(bmp).

-export([generate/0, header/2, i/2, c/1]).

header(FileSz, Offset) ->
    <<
      "BM",
      FileSz:32/little,
      0:32,
      Offset:32/little
    >>.

info(Width,Height,ImageSize) ->
    <<
      40:32/little,         % Header size
      Width:32/little,
      Height:32/little,
      1:16/little,          % Planes
      24:16/little,         % Bits per pixel
      0:32/little,          % Compression
      ImageSize:32/little,
      0:32/little,          % X Pixels per meter
      0:32/little,          % Y Pixels per meter
      0:32/little,          % Colors in color table
      0:32/little           % Important color count
    >>.

padpx(N) when (N * 3) rem 4 =:= 0 -> 0;
padpx(N) -> 4 - ((N * 3) rem 4).

padrow(R, W) -> [R| lists:duplicate(padpx(W),0)].

generate() ->
    W = 3,
    H = 3,
    Matrix = [
              [<<213, 45, 133>>,<<64, 76, 32>>, <<0, 0, 0>>],
              [<<76,  0,  0>>,  <<234,32, 15>>, <<0, 0, 0>>],
              [<<76,  0,  0>>,  <<234,32, 15>>, <<0, 0, 0>>]
             ],
    write_image(W, H, l(c(Matrix),1,1,<<128,128,128>>)).

write_image(W, H, Matrix) ->
    MatrixReversed = lists:reverse(Matrix),
    MatrixPadded = lists:map(fun(R) -> padrow(R,W) end, MatrixReversed),
    Pixels = erlang:list_to_bitstring(MatrixPadded),
    PixelsSize = erlang:bit_size(Pixels),
    Offset = 14 + 40, % Header Size + InfoSize
    Size = Offset + PixelsSize,
    Header = header(Size, Offset),
    Info = info(W, H, PixelsSize),
    file:write_file("test.bmp",
                    [
                     Header,
                     Info,
                     Pixels,
                     <<10:8/little>>
                    ]).


whitePixel() -> <<255,255,255>>.

changePixel(L, P, E) ->
    lists:sublist(L, P-1) ++ [E] ++ lists:nthtail(P,L).

% getPixel(Matrix, X, Y) ->
%    lists:nth(X,lists:nth(Y,Matrix)).

% I M N
% Create MxN matrix.
i(M, N) -> lists:duplicate(N, lists:duplicate(M, whitePixel())).

% C
% Clean matrix
c(Matrix) when is_list(Matrix) ->
    lists:map(fun(E) -> c(E) end, Matrix);
c(_) -> whitePixel().

% L X Y C
% color pixel (X,Y) in color C.
l(Matrix, X, Y, C) ->
    lists:sublist(Matrix, Y-1) ++
    [changePixel(lists:nth(Y, Matrix), X, C)] ++
    lists:nthtail(Y, Matrix).