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Membrane H264 decoder and encoder based on FFmpeg and x264
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lib/membrane_h264_ffmpeg/parser/nalu.ex
defmodule Membrane.H264.FFmpeg.Parser.NALu do
@moduledoc false
# See https://yumichan.net/video-processing/video-compression/introduction-to-h264-nal-unit/
@nalu_types %{
0 => :unspecified,
1 => :non_idr,
2 => :part_a,
3 => :part_b,
4 => :part_c,
5 => :idr,
6 => :sei,
7 => :sps,
8 => :pps,
9 => :aud,
10 => :end_of_seq,
11 => :end_of_stream,
12 => :filler_data,
13 => :sps_extension,
14 => :prefix_nal_unit,
15 => :subset_sps,
(16..18) => :reserved,
19 => :auxiliary_non_part,
20 => :extension,
(21..23) => :reserved,
(24..31) => :unspecified
}
|> Enum.flat_map(fn
{k, v} when is_integer(k) -> [{k, v}]
{k, v} -> Enum.map(k, &{&1, v})
end)
|> Map.new()
@typedoc """
Type representing a single option that can be passed to `parse/2` function call
- `complete_nalu?` - determines if the last NALu in the binary will be considered complete despite lacking the delimiter marking the end of it.
"""
@type parse_option_t() :: {:complete_nalu?, boolean()}
@type parse_options_t() :: [parse_option_t()]
@spec parse(binary, parse_options_t()) :: {list, %{h264: any}, binary}
def parse(access_unit, options \\ []) do
{nalus, au_info} =
access_unit
|> extract_nalus(options)
|> Enum.map_reduce(%{key_frame?: false}, &parse_nalu(&1, &2, access_unit))
nalus =
nalus
|> List.update_at(0, &put_in(&1, [:metadata, :h264, :new_access_unit], au_info))
|> List.update_at(-1, &put_in(&1, [:metadata, :h264, :end_access_unit], true))
length_of_parsed_data =
List.last(nalus, %{prefixed_poslen: {0, 0}}).prefixed_poslen
|> then(fn {pos, len} -> pos + len end)
<<_parsed::binary-size(length_of_parsed_data), unparsed::binary>> = access_unit
{nalus, %{h264: au_info}, unparsed}
end
defp extract_nalus(access_unit, options) do
access_unit =
if Keyword.get(options, :complete_nalu?, false),
do: access_unit <> <<0, 0, 1>>,
else: access_unit
access_unit
|> :binary.matches([<<0, 0, 0, 1>>, <<0, 0, 1>>])
|> Enum.chunk_every(2, 1, :discard)
|> Enum.map(fn [{from, prefix_len}, {to, _}] ->
len = to - from
%{prefixed_poslen: {from, len}, unprefixed_poslen: {from + prefix_len, len - prefix_len}}
end)
end
defp parse_nalu(nalu, access_unit_info, access_unit) do
<<0::1, _nal_ref_idc::unsigned-integer-size(2), nal_unit_type::unsigned-integer-size(5),
_rest::bitstring>> = :binary.part(access_unit, nalu.unprefixed_poslen)
type = @nalu_types |> Map.fetch!(nal_unit_type)
new_au_info = if type == :idr, do: %{key_frame?: true}, else: %{}
nalu = Map.put(nalu, :metadata, %{h264: %{type: type}})
{nalu, Map.merge(access_unit_info, new_au_info)}
end
end