# `Membrane.H265.Parser`
[🔗](https://github.com/membraneframework/membrane_h26x_plugin/blob/v0.11.2/lib/membrane_h26x_plugin/h265_parser.ex#L1)

Membrane element providing parser for H265 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 H265 access units - each buffer being output is a `Membrane.Buffer` struct with a
binary payload of a single access 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.

The parser handles one of three possible input alignments, depending on the structure of the input buffers:
* `:bytestream` - each input buffer contains some part of H265 stream's payload, but not necessary a logical
H265 unit (like NAL unit or an access unit). Can be used for i.e. for parsing the stream read from the file.
* `:nalu` - each input buffer contains a single NAL unit's payload
* `:au` - each input buffer contains a single access unit's payload

The input alignment is set automatically, based on the input stream format received by that element:
* Receiving `Membrane.RemoteStream` results in the input alignment being set to `:bytestream`
* Receiving `%Membrane.H265{alignment: :nalu}` results in the input alignment being set to `:nalu`.
* Receiving `%Membrane.H265{alignment: :au}` results in the input alignment being set to `:au`.

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.
* hvc1, `:hvc1` - 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, SPSs and VPSs (Picture Parameter Sets, Sequence Parameter Sets and Video Parameter Sets)
are transported in the DCR.
* hev1, `:hev1` - The same as hvc1, only that parameter sets may be also present in the stream (in-band).

## Element options

Passed via struct `t:Membrane.H265.Parser.t/0`

- `vpss`  

  ```
  [binary()]
  ```
  
  Default value: `[]`  
  Video Parameter Set NAL unit binary payloads - if absent in the stream, may
  be provided via this option (only available for `:annexb` output stream format)
  
  Any decoder conforming to the profiles specified in "Annex A" of ITU/IEC H265 (08/21),
  but does not support INBLD may discard all VPS NAL units.

- `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 format).

- `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 format).

- `skip_until_keyframe`  

  ```
  boolean()
  ```
  
  Default value: `true`  
  Determines whether to drop the stream until the first key frame is received.
  
  Defaults to true.

- `repeat_parameter_sets`  

  ```
  boolean()
  ```
  
  Default value: `false`  
  Repeat all parameter sets (`vps`, `sps` and `pps`) on each IRAP picture.
  
  Parameter sets may be retrieved from:
    * The stream
    * `Parser` options.
    * `Decoder Configuration Record`, sent in `:hcv1` and `:hev1` stream types

- `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.

- `output_stream_structure`  

  ```
  nil | stream_structure() | :hvc1 | :hev1
  ```
  
  Default value: `nil`  
  format of the outgoing H265 stream, if set to `:annexb` NALUs will be separated by
  a start code (0x(00)000001) or if set to `:hvc1` or `:hev1` they will be prefixed by their size.
  Additionally for `:hvc1` and `:hev1` 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 hvc1 output streams the PPSs, SPSs and VPSs will be
  transported in the DCR, when in hev1 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 H265 stream
  should be kept in a container that stores the timestamps alongside.
  
  PTS are derived from each frame's presentation order, recovered by
  sorting the access units by their Picture Order Count (POC). Because
  PTS are based on the relative POC order rather than the absolute POC
  values, the timestamps stay correct even when POC values are not
  continuous within a Coded Video Sequence (e.g. when a sub-bitstream is
  produced by dropping a higher temporal sub-layer).
  
  Recovering the presentation order requires buffering (reordering) a
  bounded number of access units before emitting them, which introduces
  a constant latency of that many frame durations. The buffer depth is
  taken from the SPS `sps_max_num_reorder_pics`, so the latency is as low
  as the stream allows; when the stream can't reorder frames (that value
  being 0) the buffering is disabled and no latency is added.
  
  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 is always bigger than PTS. If that is not desired,
  you can set `add_dts_offset: false`.

## Pads

### `:input`

Accepted formats:
```
RemoteStream
```
```
H265
```

Direction: | `:input`
Availability: | `:always`
Flow control: | `:auto`

### `:output`

Accepted formats:
```
%H265{alignment: alignment, nalu_in_metadata?: true} when alignment in [:nalu, :au]
```

Direction: | `:output`
Availability: | `:always`
Flow control: | `:auto`

# `stream_structure`

```elixir
@type stream_structure() ::
  :annexb | {codec_tag :: :hvc1 | :hev1, nalu_length_size :: pos_integer()}
```

Format of the H265 stream, if set to `:annexb` NALUs will be separated by
a start code (0x(00)000001) or if set to `:hvc1` or `:hev1` they will be
prefixed by their size.

# `t`

```elixir
@type t() :: %Membrane.H265.Parser{
  generate_best_effort_timestamps:
    false
    | %{
        :framerate => {pos_integer(), pos_integer()},
        optional(:add_dts_offset) => boolean()
      },
  output_alignment: :au | :nalu,
  output_stream_structure: nil | stream_structure() | :hvc1 | :hev1,
  ppss: [binary()],
  repeat_parameter_sets: boolean(),
  skip_until_keyframe: boolean(),
  spss: [binary()],
  vpss: [binary()]
}
```

Struct containing options for `Membrane.H265.Parser`

# `options`

```elixir
@spec options() :: keyword()
```

Returns description of options available for this module

---

*Consult [api-reference.md](api-reference.md) for complete listing*