annex.b.superframes.md

Annex B: Length delimited bitstream format

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Overview

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[Section 5][] defines the syntax for OBUs. [Section 5.2][] defines the low-overhead bitstream format. This annex defines a length-delimited format for packing OBUs into a format that enables skipping through temporal units and frames more easily.

Length delimited bitstream syntax

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| --------------------------------------------------------- | ---------------- | | bitstream( ) { | Type | while ( more_data_in_bitstream() ) { | @@temporal_unit_size | leb128() | temporal_unit( temporal_unit_size ) | } |} {:.syntax }

| --------------------------------------------------------- | ---------------- | | temporal_unit( sz ) { | Type | while ( sz > 0 ) { | @@frame_unit_size | leb128() | sz -= Leb128Bytes | frame_unit( frame_unit_size ) | sz -= frame_unit_size | } |} {:.syntax }

| --------------------------------------------------------- | ---------------- | | frame_unit( sz ) { | Type | while ( sz > 0 ) { | @@obu_length | leb128() | sz -= Leb128Bytes | open_bitstream_unit( obu_length ) | sz -= obu_length | } |} {:.syntax }

Length delimited bitstream semantics

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more_data_in_bitstream() is a system-dependent method of determining whether the end of the bitstream has been reached. It returns 1 when there is more data to be read, or 0 when the end of the bitstream has been reached.

temporal_unit_size specifies the length in bytes of the next temporal unit.

frame_unit_size specifies the length in bytes of the next frame unit.

obu_length specifies the length in bytes of the next OBU.

Note: It is allowed for the OBU to set obu_has_size_field equal to 1 to indicate that the obu_size syntax element is present. In this case, the decoding process assumes that obu_size and obu_length are set consistently. If obu_size and obu_length are both present, but inconsistent, then the packed bitstream is deemed invalid. {:.alert .alert-info }

The first OBU in the first frame_unit of each temporal_unit must be a temporal delimiter OBU (and this is the only place temporal delimiter OBUs can appear).

All the frame header and tile group OBUs required for decoding a single frame must be within the same frame_unit (and a frame_unit must not contain frame headers for more than one frame).