Solutions: A

Logger.configure(level: :info)

# All necessary dependencies are installed by installing the package below
Mix.install([
  {:workshop_elixir_conf_us_2024, path: Path.join(__DIR__, "../..")}
])

Exercise A1

The missing child is |> child(:converter, %Membrane.FFmpeg.SWScale.Converter{output_width: 640}). It should be put between :h264_decoder and :vp9_encoder.

defmodule TransmuxingPipeline do
  use Membrane.Pipeline

  @impl true
  def handle_init(_ctx, _options) do
    priv = "#{__DIR__}/../../priv/" |> Path.expand()
    mp4_path = Path.join(priv, "fixtures/bunny_without_sound.mp4")
    mkv_path = Path.join(priv, "outputs/bunny_without_sound.mkv")

    spec = [
      child(:source, %Membrane.File.Source{location: mp4_path})
      |> child(:mp4_demuxer, Membrane.MP4.Demuxer.ISOM)
      |> via_out(:output, options: [kind: :video])
      |> child(:h264_parser_1, %Membrane.H264.Parser{output_stream_structure: :annexb})
      |> child(:h264_decoder, Membrane.H264.FFmpeg.Decoder)
      |> child(:converter, %Membrane.FFmpeg.SWScale.Converter{output_width: 640})
      |> child(:h264_encoder, %Membrane.H264.FFmpeg.Encoder{preset: :fast})
      |> child(:h264_parser_2, %Membrane.H264.Parser{output_stream_structure: :avc1})
      |> child(:matroska_muxer, Membrane.Matroska.Muxer)
      |> child(:file_sink, %Membrane.File.Sink{location: mkv_path})
    ]

    {[spec: spec], %{}}
  end

  @impl true
  def handle_element_end_of_stream(:file_sink, _input, _ctx, state) do
    {[terminate: :normal], state}
  end

  @impl true
  def handle_element_end_of_stream(_element, _input, _ctx, state), do: {[], state}
end

Exercise A2

This spec should be added to spec returned in handle_init/2:

get_child(:mp4_demuxer)
|> via_out(:output, options: [kind: :audio])
|> get_child(:matroska_muxer)

defmodule TransmuxingPipeline do
  use Membrane.Pipeline

  @impl true
  def handle_init(_ctx, _options) do
    priv = "#{__DIR__}/../../priv/" |> Path.expand()
    mp4_path = Path.join(priv, "fixtures/bunny_with_sound.mp4")
    mkv_path = Path.join(priv, "outputs/bunny_with_sound.mkv")

    spec = [
      child(:source, %Membrane.File.Source{location: mp4_path})
      |> child(:mp4_demuxer, Membrane.MP4.Demuxer.ISOM)
      |> via_out(:output, options: [kind: :video])
      |> child(:h264_parser_1, %Membrane.H264.Parser{output_stream_structure: :annexb})
      |> child(:h264_decoder, Membrane.H264.FFmpeg.Decoder)
      |> child(:converter, %Membrane.FFmpeg.SWScale.Converter{output_width: 640})
      |> child(:h264_encoder, %Membrane.H264.FFmpeg.Encoder{preset: :fast})
      |> child(:h264_parser_2, %Membrane.H264.Parser{output_stream_structure: :avc1})
      |> child(:matroska_muxer, Membrane.Matroska.Muxer)
      |> child(:file_sink, %Membrane.File.Sink{location: mkv_path}),
      get_child(:mp4_demuxer)
      |> via_out(:output, options: [kind: :audio])
      |> get_child(:matroska_muxer)
    ]

    {[spec: spec], %{}}
  end

  @impl true
  def handle_element_end_of_stream(:file_sink, _input, _ctx, state) do
    {[terminate: :normal], state}
  end

  @impl true
  def handle_element_end_of_stream(_element, _input, _ctx, state), do: {[], state}
end

Exercise A3

Possible handle_buffer/4 implementation:

@impl true
def handle_buffer(:input, buffer, _ctx, state) do
  payload = for <>, into: <<>>, do: <<255 - byte>>
  buffer = %{buffer | payload: payload}
  {[buffer: {:output, buffer}], state}
end

The following children should be put between the :h264_decoder and the :h264_encoder:

|> child(:rgb_converter, %Membrane.FFmpeg.SWScale.Converter{format: :RGB})
|> child(:color_reverter, ColorReverter)
|> child(:yuv_converter, %Membrane.FFmpeg.SWScale.Converter{format: :I420})

defmodule ColorReverter do
  use Membrane.Filter
  alias Membrane.RawVideo

  def_input_pad(:input, accepted_format: %RawVideo{pixel_format: :RGB})
  def_output_pad(:output, accepted_format: %RawVideo{pixel_format: :RGB})

  @impl true
  def handle_buffer(:input, buffer, _ctx, state) do
    payload = for <>, into: <<>>, do: <<255 - byte>>
    buffer = %{buffer | payload: payload}
    {[buffer: {:output, buffer}], state}
  end
end

defmodule ColorRevertingPipeline do
  use Membrane.Pipeline

  @impl true
  def handle_init(_ctx, _options) do
    priv = "#{__DIR__}/../../priv/" |> Path.expand()
    mp4_path = Path.join(priv, "fixtures/bunny_with_sound.mp4")
    mkv_path = Path.join(priv, "outputs/bunny_with_reverted_colors.mkv")

    spec = [
      child(:source, %Membrane.File.Source{location: mp4_path})
      |> child(:mp4_demuxer, Membrane.MP4.Demuxer.ISOM)
      |> via_out(:output, options: [kind: :video])
      |> child(:h264_parser_1, %Membrane.H264.Parser{output_stream_structure: :annexb})
      |> child(:h264_decoder, Membrane.H264.FFmpeg.Decoder)
      |> child(:rgb_converter, %Membrane.FFmpeg.SWScale.Converter{format: :RGB})
      |> child(:color_reverter, ColorReverter)
      |> child(:yuv_converter, %Membrane.FFmpeg.SWScale.Converter{format: :I420})
      |> child(:h264_encoder, %Membrane.H264.FFmpeg.Encoder{preset: :fast})
      |> child(:h264_parser_2, %Membrane.H264.Parser{output_stream_structure: :avc1})
      |> child(:matroska_muxer, Membrane.Matroska.Muxer)
      |> child(:file_sink, %Membrane.File.Sink{location: mkv_path}),
      get_child(:mp4_demuxer)
      |> via_out(:output, options: [kind: :audio])
      |> get_child(:matroska_muxer)
    ]

    {[spec: spec], %{}}
  end

  @impl true
  def handle_element_end_of_stream(:file_sink, _input, _ctx, state) do
    {[terminate: :normal], state}
  end

  @impl true
  def handle_element_end_of_stream(_element, _input, _ctx, state), do: {[], state}
end