Membrane element providing parser for H264 encoded video stream.

The parser:

• prepares and sends the appropriate stream format, based on information provided in the stream and via the element's options
• splits the incoming stream into H264 access units - each buffer being output is a Membrane.Buffer struct with a binary payload of a single access unit or network abstraction layer unit.
• enriches the output buffers with the metadata describing the way the access unit is split into NAL units, type of each NAL unit making up the access unit and the information if the access unit hold a keyframe.
• converts the stream's structure (Annex B, avc1 or avc3) to the one provided via the element's options.

The parser works in one of three possible modes, depending on the structure of the input buffers:

• :bytestream - each input buffer contains some part of H264 stream's payload, but not necessary a logical H264 unit (like NAL unit or an access unit). Can be used for i.e. for parsing the stream read from the file.
• :nalu_aligned - each input buffer contains a single NAL unit's payload
• :au_aligned - each input buffer contains a single access unit's payload

The parser's mode is set automatically, based on the input stream format received by that element:

• Receiving %Membrane.RemoteStream{type: :bytestream} results in the parser mode being set to :bytestream
• Receiving %Membrane.H264{alignment: :nalu} results in the parser mode being set to :nalu_aligned
• Receiving %Membrane.H264{alignment: :au} results in the parser mode being set to :au_aligned

The distinction between parser modes was introduced to eliminate the redundant operations and to provide a reliable way for rewriting of timestamps:

• in the :bytestream mode:
  • if option :framerate is set to nil, the output buffers have their :pts and :dts set to nil
  • if framerate is specified, :pts and :dts will be generated automatically, based on that framerate, starting from 0 This may only be used with H264 profiles :baseline and :constrained_baseline, where PTS==DTS.
• in the :nalu_aligned mode, the output buffers have their :pts and :dts set to :pts and :dts of the input buffer that was holding the first NAL unit making up given access unit (that is being sent inside that output buffer).
• in the :au_aligned mode, the output buffers have their :pts and :dts set to :pts and :dts of the input buffer (holding the whole access unit being output)

The parser also allows for conversion between stream structures. The available structures are:

• Annex B, :annexb - In a stream with this structure each NAL unit is prefixed by three or four-byte start code (0x(00)000001) that allows to identify boundaries between them.
• avc1, :avc1 - In such stream a DCR (Decoder Configuration Record) is included in stream_format and NALUs lack the start codes, but are prefixed with their length. The length of these prefixes is contained in the stream's DCR. PPSs and SPSs (Picture Parameter Sets and Sequence Parameter Sets) are transported in the DCR.
• avc3, :avc3 - The same as avc1, only that parameter sets may be also present in the stream (in-band).

Element options

Passed via struct Membrane.H264.Parser.t/0

• spss

  [binary()]

  Default value: []
  Sequence Parameter Set NAL unit binary payloads - if absent in the stream, should be provided via this option (only available for :annexb output stream structure).

• ppss

  [binary()]

  Default value: []
  Picture Parameter Set NAL unit binary payloads - if absent in the stream, should be provided via this option (only available for :annexb output stream structure).

• output_alignment

  :au | :nalu

  Default value: :au
  Alignment of the buffers produced as an output of the parser. If set to :au, each output buffer will be a single access unit. Otherwise, if set to :nalu, each output buffer will be a single NAL unit. Defaults to :au.

• skip_until_keyframe

  boolean()

  Default value: true
  Determines whether to drop the stream until the first key frame is received.

• repeat_parameter_sets

  boolean()

  Default value: false
  Repeat all parameter sets (sps and pps) on each IDR picture.

  Parameter sets may be retrieved from:

  • The stream
  • Parser options.
  • Decoder Configuration Record, sent in :acv1 and :avc3 stream types

• output_stream_structure

  nil | :annexb | :avc1 | :avc3 | {:avc1 | :avc3, nalu_length_size :: pos_integer()}

  Default value: nil
  format of the outgoing H264 stream, if set to :annexb NALUs will be separated by a start code (0x(00)000001) or if set to :avc3 or :avc1 they will be prefixed by their size. Additionally for :avc1 and :avc3 a tuple can be passed containing the atom and nalu_length_size that determines the size in bytes of each NALU's field describing their length (by default 4). In avc1 output streams the PPSs and SPSs will be transported in the DCR, when in avc3 they will be present only in the stream (in-band). If not provided or set to nil the stream's structure will remain unchanged.

• generate_best_effort_timestamps

  false | %{:framerate => {pos_integer(), pos_integer()}, optional(:add_dts_offset) => boolean()}

  Default value: false
  Generates timestamps based on given framerate.

  This option works only when Membrane.RemoteStream format arrives.

  Keep in mind that the generated timestamps may be inaccurate and lead to video getting out of sync with other media, therefore h264 should be kept in a container that stores the timestamps alongside.

  By default, the parser adds negative DTS offset to the timestamps, so that in case of frame reorder (which always happens when B frames are present) the DTS was always bigger than PTS. If that is not desired, you can set add_dts_offset: false.

Pads

:input

Accepted formats:

%RemoteStream{type: :bytestream}

H264

:output

Accepted formats:

%H264{alignment: alignment, nalu_in_metadata?: true} when alignment in [:nalu, :au]