AV1 Bitstream & Decoding Process Specification

Last modified: 2023-05-25 12:27 PT

Authors
Peter de Rivaz, Argon Design Ltd
Jack Haughton, Argon Design Ltd

Codec Working Group Chair
Adrian Grange, Google LLC

Document Design
Lou Quillio, Google LLC

Abstract

This document defines the bitstream formats and decoding process for the Alliance for Open Media AV1 video codec.

Contents

• Scope
• Terms and definitions
• Symbols and abbreviated terms
• Conventions
  • General
  • Arithmetic operators
  • Logical operators
  • Relational operators
  • Bitwise operators
  • Assignment
  • Mathematical functions
  • Method of describing bitstream syntax
  • Functions
  • Descriptors
    • General
    • f(n)
    • uvlc()
    • le(n)
    • leb128()
    • su(n)
    • ns(n)
    • L(n)
    • S()
    • NS(n)
• Syntax structures
  • General
  • Low overhead bitstream format
  • OBU syntax
    • General OBU syntax
    • OBU header syntax
    • OBU extension header syntax
    • Trailing bits syntax
    • Byte alignment syntax
  • Reserved OBU syntax
  • Sequence header OBU syntax
    • General sequence header OBU syntax
    • Color config syntax
    • Timing info syntax
    • Decoder model info syntax
    • Operating parameters info syntax
  • Temporal delimiter obu syntax
  • Padding OBU syntax
  • Metadata OBU syntax
    • General metadata OBU syntax
    • Metadata ITUT T35 syntax
    • Metadata high dynamic range content light level syntax
    • Metadata high dynamic range mastering display color volume syntax
    • Metadata scalability syntax
    • Scalability structure syntax
    • Metadata timecode syntax
  • Frame header OBU syntax
    • General frame header OBU syntax
    • Uncompressed header syntax
    • Get relative distance function
    • Reference frame marking function
    • Frame size syntax
    • Render size syntax
    • Frame size with refs syntax
    • Superres params syntax
    • Compute image size function
    • Interpolation filter syntax
    • Loop filter params syntax
    • Quantization params syntax
    • Delta quantizer syntax
    • Segmentation params syntax
    • Tile info syntax
    • Tile size calculation function
    • Quantizer index delta parameters syntax
    • Loop filter delta parameters syntax
    • CDEF params syntax
    • Loop restoration params syntax
    • TX mode syntax
    • Skip mode params syntax
    • Frame reference mode syntax
    • Global motion params syntax
    • Global param syntax
    • Decode signed subexp with ref syntax
    • Decode unsigned subexp with ref syntax
    • Decode subexp syntax
    • Inverse recenter function
    • Film grain params syntax
    • Temporal point info syntax
  • Frame OBU syntax
  • Tile group OBU syntax
    • General tile group OBU syntax
    • Decode tile syntax
    • Clear block decoded flags function
    • Decode partition syntax
    • Decode block syntax
    • Mode info syntax
    • Intra frame mode info syntax
    • Intra segment ID syntax
    • Read segment ID syntax
    • Skip mode syntax
    • Skip syntax
    • Quantizer index delta syntax
    • Loop filter delta syntax
    • Segmentation feature active function
    • TX size syntax
    • Block TX size syntax
    • Var TX size syntax
    • Inter frame mode info syntax
    • Inter segment ID syntax
    • Is inter syntax
    • Get segment ID function
    • Intra block mode info syntax
    • Inter block mode info syntax
    • Filter intra mode info syntax
    • Ref frames syntax
    • Assign MV syntax
    • Read motion mode syntax
    • Read inter intra syntax
    • Read compound type syntax
    • Get mode function
    • MV syntax
    • MV component syntax
    • Compute prediction syntax
    • Residual syntax
    • Transform block syntax
    • Transform tree syntax
    • Get TX size function
    • Get plane residual size function
    • Coefficients syntax
    • Compute transform type function
    • Get scan function
    • Intra angle info luma syntax
    • Intra angle info chroma syntax
    • Is directional mode function
    • Read CFL alphas syntax
    • Palette mode info syntax
    • Transform type syntax
    • Get transform set function
    • Palette tokens syntax
    • Palette color context function
    • Is inside function
    • Is inside filter region function
    • Clamp MV row function
    • Clamp MV col function
    • Clear CDEF function
    • Read CDEF syntax
    • Read loop restoration syntax
    • Read loop restoration unit syntax
  • Tile list OBU syntax
    • General tile list OBU syntax
    • Tile list entry syntax
• Syntax structures semantics
  • General
  • OBU semantics
    • General OBU semantics
    • OBU header semantics
    • OBU extension header semantics
    • Trailing bits semantics
    • Byte alignment semantics
  • Reserved OBU semantics
  • Sequence header OBU semantics
    • General sequence header OBU semantics
    • Color config semantics
    • Timing info semantics
    • Decoder model info semantics
    • Operating parameters info semantics
  • Temporal delimiter OBU semantics
  • Padding OBU semantics
  • Metadata OBU semantics
    • General metadata OBU semantics
    • Metadata ITUT T35 semantics
    • Metadata high dynamic range content light level semantics
    • Metadata high dynamic range mastering display color volume semantics
    • Metadata scalability semantics
    • Scalability structure semantics
      • General
      • L1T2 (Informative)
      • L1T3 (Informative)
      • L2T1 / L2T1h (Informative)
      • L2T2 / L2T2h (Informative)
      • L2T3 / L2T3h (Informative)
      • S2T1 / S2T1h (Informative)
      • S2T2 / S2T2h (Informative)
      • S2T3 / S2T3h (Informative)
      • L3T1 (Informative)
      • L3T2 (Informative)
      • L3T3 (Informative)
      • S3T1 (Informative)
      • S3T2 (Informative)
      • S3T3 (Informative)
    • Metadata timecode semantics
  • Frame header OBU semantics
    • General frame header OBU semantics
    • Uncompressed header semantics
    • Reference frame marking semantics
    • Frame size semantics
    • Render size semantics
    • Frame size with refs semantics
    • Superres params semantics
    • Compute image size semantics
    • Interpolation filter semantics
    • Loop filter semantics
    • Quantization params semantics
    • Delta quantizer semantics
    • Segmentation params semantics
    • Tile info semantics
    • Quantizer index delta parameters semantics
    • Loop filter delta parameters semantics
    • Global motion params semantics
    • Global param semantics
    • Decode subexp semantics
    • Film grain params semantics
    • TX mode semantics
    • Skip mode params semantics
    • Frame reference mode semantics
    • Temporal point info semantics
  • Frame OBU semantics
  • Tile group OBU semantics
    • General tile group OBU semantics
    • Decode tile semantics
    • Clear block decoded flags semantics
    • Decode partition semantics
    • Decode block semantics
    • Intra frame mode info semantics
    • Intra segment ID semantics
    • Read segment ID semantics
    • Inter segment ID semantics
    • Skip mode semantics
    • Skip semantics
    • Quantizer index delta semantics
    • Loop filter delta semantics
    • CDEF params semantics
    • Loop restoration params semantics
    • TX size semantics
    • Block TX size semantics
    • Var TX size semantics
    • Transform type semantics
    • Is inter semantics
    • Intra block mode info semantics
    • Inter block mode info semantics
    • Filter intra mode info semantics
    • Ref frames semantics
    • Assign mv semantics
    • Read motion mode semantics
    • Read inter intra semantics
    • Read compound type semantics
    • MV semantics
    • MV component semantics
    • Compute prediction semantics
    • Residual semantics
    • Transform block semantics
    • Coefficients semantics
    • Intra angle info semantics
    • Read CFL alphas semantics
    • Palette mode info semantics
    • Palette tokens semantics
    • Palette color context semantics
    • Read CDEF semantics
    • Read loop restoration unit semantics
  • Tile list OBU semantics
    • General tile list OBU semantics
    • Tile list entry semantics
• Decoding process
  • Overview
  • General decoding process
  • Large scale tile decoding process
    • General
    • Decode camera tile process
  • Decode frame wrapup process
  • Ordering of OBUs
  • Random access decoding
    • General
    • Definitions
    • Conformance requirements
    • Encoder consequences
    • Decoder consequences
  • Frame end update CDF process
  • Set frame refs process
  • Motion field estimation process
    • General
    • Projection process
    • Get MV projection process
    • Get block position process
  • Motion vector prediction processes
    • General
    • Find MV stack process
      • Setup global MV process
      • Scan row process
      • Scan col process
      • Scan point process
      • Temporal scan process
      • Temporal sample process
      • Add reference motion vector process
      • Search stack process
      • Compound search stack process
      • Lower precision process
      • Sorting process
      • Extra search process
      • Add extra MV candidate process
      • Context and clamping process
    • Has overlappable candidates process
    • Find warp samples process
      • General
      • Add sample process
  • Prediction processes
    • General
    • Intra prediction process
      • General
      • Basic intra prediction process
      • Recursive intra prediction process
      • Directional intra prediction process
      • DC intra prediction process
      • Smooth intra prediction process
      • Filter corner process
      • Intra filter type process
      • Intra edge filter strength selection process
      • Intra edge upsample selection process
      • Intra edge upsample process
      • Intra edge filter process
    • Inter prediction process
      • General
      • Rounding variables derivation process
      • Motion vector scaling process
      • Block inter prediction process
      • Block warp process
      • Setup shear process
      • Resolve divisor process
      • Warp estimation process
      • Overlapped motion compensation process
      • Overlap blending process
      • Wedge mask process
      • Difference weight mask process
      • Intra mode variant mask process
      • Mask blend process
      • Distance weights process
    • Palette prediction process
    • Predict chroma from luma process
  • Reconstruction and dequantization
    • General
    • Dequantization functions
    • Reconstruct process
  • Inverse transform process
    • General
    • 1D transforms
      • Butterfly functions
      • Inverse DCT array permutation process
      • Inverse DCT process
      • Inverse ADST input array permutation process
      • Inverse ADST output array permutation process
      • Inverse ADST4 process
      • Inverse ADST8 process
      • Inverse ADST16 process
      • Inverse ADST process
      • Inverse Walsh-Hadamard transform process
      • Inverse identity transform 4 process
      • Inverse identity transform 8 process
      • Inverse identity transform 16 process
      • Inverse identity transform 32 process
      • Inverse identity transform process
    • 2D inverse transform process
  • Loop filter process
    • General
    • Edge loop filter process
    • Filter size process
    • Adaptive filter strength process
    • Adaptive filter strength selection process
    • Sample filtering process
      • General
      • Filter mask process
      • Narrow filter process
      • Wide filter process
  • CDEF process
    • CDEF block process
    • CDEF direction process
    • CDEF filter process
  • Upscaling process
  • Loop restoration process
    • Loop restore block process
    • Self guided filter process
    • Box filter process
    • Wiener filter process
    • Wiener coefficient process
    • Get source sample process
  • Output process
    • General
    • Intermediate output preparation process
    • Film grain synthesis process
      • General
      • Random number process
      • Generate grain process
      • Scaling lookup initialization process
      • Add noise synthesis process
  • Motion field motion vector storage process
  • Reference frame update process
  • Reference frame loading process
• Parsing process
  • Parsing process for f(n)
  • Parsing process for symbol decoder
    • General
    • Initialization process for symbol decoder
    • Boolean decoding process
    • Exit process for symbol decoder
    • Parsing process for read_literal
    • Symbol decoding process
  • Parsing process for CDF encoded syntax elements
    • General
    • Cdf selection process
• Additional tables
  • General
  • Scan tables
  • Conversion tables
  • Default CDF tables
  • Quantizer matrix tables
    • General
    • Derivation process (Informative)
    • Tables
• Annex A: Profiles and levels
  • General
  • Profiles
  • Levels
  • Decoder Conformance
• Annex B: Length delimited bitstream format
  • Overview
  • Length delimited bitstream syntax
  • Length delimited bitstream semantics
• Annex C: Error resilience behavior (informative)
  • General
  • Definition of processable frames
  • Recommendation for processable frames
  • Encoder consequences of processable frames
  • Decoder consequences of processable frames
• Annex D: Large scale tile use case (informative)
• Annex E: Decoder model
  • General
  • Decoder model definitions
  • Operating modes
    • Resource availability mode
    • Decoding schedule mode
    • When timing information is not present in the bitstream
  • Frame timing definitions
    • Start of DFG bits arrival
    • End of DFG bits arrival
    • Scheduled removal times
    • Removal times in decoding schedule mode
    • Removal times in resource availability mode
    • Frame decode timing
    • Frame presentation timing
  • Decoder model
    • Decoder model functions
    • Decoder model process
  • Bitstream conformance
    • General
    • Decoder buffer delay consistency across random access points (applies to decoding schedule mode)
    • Smoothing buffer overflow
    • Smoothing buffer underflow
    • Minimum decode time (applies to decoding schedule mode)
    • Minimum presentation Interval
    • Decode deadline
    • Level imposed constraints
    • Decode Process constraints
• Bibliography

Scope

This document specifies the Alliance for Open Media AV1 bitstream formats and decoding process.

Terms and definitions

For the purposes of this document, the following terms and definitions apply:

AC coefficient

Any transform coefficient whose frequency indices are non-zero in at least one dimension.

Altref

(Alternative reference frame) A frame that can be used in inter coding.

Base layer

The layer with spatial_id and temporal_id values equal to 0.

Bitstream

The sequence of bits generated by encoding a sequence of frames.

Bit string

An ordered string with limited number of bits. The left most bit is the most significant bit (MSB), the right most bit is the least significant bit (LSB).

Block

A square or rectangular region of samples.

Block scan

A specified serial ordering of quantized coefficients.

Byte

An 8-bit bit string.

Byte alignment

One bit is byte aligned if the position of the bit is an integer multiple of eight from the position of the first bit in the bitstream.

CDEF

Constrained Directional Enhancement Filter designed to adaptively filter blocks based on identifying the direction.

CDF

Cumulative distribution function representing the probability times 32768 that a symbol has value less than or equal to a given level.

Chroma

A sample value matrix or a single sample value of one of the two color difference signals.

Note: Symbols of chroma are U and V.

Coded frame

The representation of one frame before the decoding process.

Component

One of the three sample value matrices (one luma matrix and two chroma matrices) or its single sample value.

Compound prediction

A type of inter prediction where sample values are computed by blending together predictions from two reference frames (the frames blended can be the same or different).

DC coefficient

A transform coefficient whose frequency indices are zero in both dimensions.

Decoded frame

The frame reconstructed out of the bitstream by the decoder.

Decoder

One embodiment of the decoding process.

Decoding process

The process that derives decoded frames from syntax elements, including any processing steps used prior to and for the film grain synthesis process.

Dequantization

The process in which transform coefficients are obtained by scaling the quantized coefficients.

Encoder

One embodiment of the encoding process.

Encoding process

A process not specified in this Specification that generates the bitstream that conforms to the description provided in this document.

Enhancement layer

A layer with either spatial_id greater than 0 or temporal_id greater than 0.

Flag

A binary variable - some variables and syntax elements (e.g. obu_extension_flag) are described using the word flag to highlight that the syntax element can only be equal to 0 or equal to 1.

Frame

The representation of video signals in the spatial domain, composed of one luma sample matrix (Y) and two chroma sample matrices (U and V).

Frame context

A set of probabilities used in the decoding process.

Golden frame

A frame that can be used in inter coding. Typically the golden frame is encoded with higher quality and is used as a reference for multiple inter frames.

Inter coding

Coding one block or frame using inter prediction.

Inter frame

A frame compressed by referencing previously decoded frames and which may use intra prediction or inter prediction.

Inter prediction

The process of deriving the prediction value for the current frame using previously decoded frames.

Intra coding

Coding one block or frame using intra prediction.

Intra frame

A frame compressed using only intra prediction which can be independently decoded.

Intra prediction

The process of deriving the prediction value for the current sample using previously decoded sample values in the same decoded frame.

Inverse transform

The process in which a transform coefficient matrix is transformed into a spatial sample value matrix.

Key frame

An Intra frame which resets the decoding process when it is shown.

Layer

A set of tile group OBUs with identical spatial_id and identical temporal_id values.

Level

A defined set of constraints on the values for the syntax elements and variables.

Loop filter

A filtering process applied to the reconstruction intended to reduce the visibility of block edges.

Luma

A sample value matrix or a single sample value representing the monochrome signal related to the primary colors.

Note: The symbol representing luma is Y.

Mode info

Syntax elements sent for a block containing an indication of how a block is to be predicted during the decoding process.

Mode info block

A luma sample value block of size 4x4 or larger and its two corresponding chroma sample value blocks (if present).

Motion vector

A two-dimensional vector used for inter prediction which refers the current frame to the reference frame, the value of which provides the coordinate offsets from a location in the current frame to a location in the reference frame.

OBU

All structures are packetized in “Open Bitstream Units” or OBUs. Each OBU has a header, which provides identifying information for the contained data (payload).

Parse

The procedure of getting the syntax element from the bitstream.

Prediction

The implementation of the prediction process consisting of either inter or intra prediction.

Prediction process

The process of estimating the decoded sample value or data element using a predictor.

Prediction value

The value, which is the combination of the previously decoded sample values or data elements, used in the decoding process of the next sample value or data element.

Profile

A subset of syntax, semantics and algorithms defined in a part.

Quantization parameter

A variable used for scaling the quantized coefficients in the decoding process.

Quantized coefficient

A transform coefficient before dequantization.

Raster scan

Maps a two dimensional rectangular raster into a one dimensional raster, in which the entry of the one dimensional raster starts from the first row of the two dimensional raster, and the scanning then goes through the second row and the third row, and so on. Each raster row is scanned in left to right order.

Reconstruction

Obtaining the addition of the decoded residual and the corresponding prediction values.

Reference

One of a set of tags, each of which is mapped to a reference frame.

Reference frame

A storage area for a previously decoded frame and associated information.

Reserved

A special syntax element value which may be used to extend this part in the future.

Residual

The differences between the reconstructed samples and the corresponding prediction values.

Sample

The basic elements that compose the frame.

Sample value

The value of a sample. This is an integer from 0 to 255 (inclusive) for 8-bit frames, from 0 to 1023 (inclusive) for 10-bit frames, and from 0 to 4095 (inclusive) for 12-bit frames.

Segmentation map

One 3-bit number per 4x4 block in the frame specifying the segment affiliation of that block. A segmentation map is stored for each reference frame to allow new frames to use a previously coded map.

Sequence

The highest level syntax structure of coding bitstream, including one or several consecutive coded frames.

Superblock

The top level of the block quadtree within a tile. All superblocks within a frame are the same size and are square. The superblocks may be 128x128 luma samples or 64x64 luma samples. A superblock may contain 1 or 2 or 4 mode info blocks, or may be bisected in each direction to create 4 sub-blocks, which may themselves be further subpartitioned, forming the block quadtree.

Switch Frame

An inter frame that can be used as a point to switch between sequences. Switch frames overwrite all the reference frames without forcing the use of intra coding. The intention is to allow a streaming use case where videos can be encoded in small chunks (say of 1 second duration), each starting with a switch frame. If the available bandwidth drops, the server can start sending chunks from a lower bitrate encoding instead. When this happens the inter prediction uses the existing higher quality reference frames to decode the switch frame. This approach allows a bitrate switch without the cost of a full key frame.

Syntax element

An element of data represented in the bitstream.

Temporal delimiter OBU

An indication that the following OBUs will have a different presentation/decoding time stamp from the one of the last frame prior to the temporal delimiter.

Temporal unit

A Temporal unit consists of all the OBUs that are associated with a specific, distinct time instant. It consists of a temporal delimiter OBU, and all the OBUs that follow, up to but not including the next temporal delimiter.

Temporal group

A set of frames whose temporal prediction structure is used periodically in a video sequence.

Tier

A specified category of level constraints imposed on the values of the syntax elements in the bitstream.

Tile

A rectangular region of the frame that can be decoded and encoded independently, although loop-filtering across tile edges is still applied.

Transform block

A rectangular transform coefficient matrix, used as input to the inverse transform process.

Transform coefficient

A scalar value, considered to be in a frequency domain, contained in a transform block.

Uncompressed header

High level description of the frame to be decoded that is encoded without the use of arithmetic encoding.

Symbols and abbreviated terms

DCT

Discrete Cosine Transform

ADST

Asymmetric Discrete Sine Transform

LSB

Least Significant Bit

MSB

Most Significant Bit

WHT

Walsh Hadamard Transform

The specification makes use of a number of constant integers. Constants that relate to the semantics of a particular syntax element are defined in section 6.

Additional constants are defined below:

Conventions

General

The mathematical operators and their precedence rules used to describe this Specification are similar to those used in the C programming language. However, the operation of integer division with truncation is specifically defined.

In addition, a length 2 array used to hold a motion vector (indicated by the variable name ending with the letters Mv or Mvs) can be accessed using either array notation (e.g. Mv[ 0 ] and Mv[ 1 ]), or by just the name (e.g., Mv). The only operations defined when using the name are assignment and equality/inequality testing. Assignment of an array is represented using the notation A = B and is specified to mean the same as doing both the individual assignments A[ 0 ] = B[ 0 ] and A[ 1 ] = B[ 1 ]. Equality testing of 2 motion vectors is represented using the notation A == B and is specified to mean the same as (A[ 0 ] == B[ 0 ] && A[ 1 ] == B[ 1 ]). Inequality testing is defined as A != B and is specified to mean the same as (A[ 0 ] != B[ 0 ] || A[ 1 ] != B[ 1 ]).

When a variable is said to be representable by a signed integer with x bits, it means that the variable is greater than or equal to -(1 << (x-1)), and that the variable is less than or equal to (1 << (x-1))-1.

The key words “must”, “must not”, “required”, “shall”, “shall not”, “should”, “should not”, “recommended”, “may”, and “optional” in this document are to be interpreted as described in RFC 2119.

Arithmetic operators

Logical operators

Relational operators

Bitwise operators

Assignment

Mathematical functions

The following mathematical functions (Abs, Clip3, Clip1, Min, Max, Round2 and Round2Signed) are defined as follows:

The definition of Round2 uses standard mathematical power and division operations, not integer operations. An equivalent definition using integer operations is:

Round2( x, n ) {
  if ( n == 0 )
    return x
  return ( x + ( 1 << (n - 1) ) ) >> n
}

The FloorLog2(x) function is defined to be the floor of the base 2 logarithm of the input x.

The input x will always be an integer, and will always be greater than or equal to 1.

This function extracts the location of the most significant bit in x.

An equivalent definition (using the pseudo-code notation introduced in the following section) is:

FloorLog2( x ) {
  s = 0
  while ( x != 0 ) {
    x = x >> 1
    s++
  }
  return s - 1
}

The CeilLog2(x) function is defined to be the ceiling of the base 2 logarithm of the input x (when x is 0, it is defined to return 0).

The input x will always be an integer, and will always be greater than or equal to 0.

This function extracts the number of bits needed to code a value in the range 0 to x-1.

An equivalent definition (using the pseudo-code notation introduced in the following section) is:

CeilLog2( x ) {
  if ( x < 2 )
    return 0
  i = 1
  p = 2
  while ( p < x ) {
    i++
    p = p << 1
  }
  return i
}

Method of describing bitstream syntax

The description style of the syntax is similar to the C programming language. Syntax elements in the bitstream are represented in bold type. Each syntax element is described by its name (using only lower case letters with underscore characters) and a descriptor for its method of coded representation. The decoding process behaves according to the value of the syntax element and to the values of previously decoded syntax elements. When a value of a syntax element is used in the syntax tables or the text, it appears in regular (i.e. not bold) type. If the value of a syntax element is being computed (e.g. being written with a default value instead of being coded in the bitstream), it also appears in regular type (e.g. tile_size_minus_1).

In some cases the syntax tables may use the values of other variables derived from syntax elements values. Such variables appear in the syntax tables, or text, named by a mixture of lower case and upper case letter and without any underscore characters. Variables starting with an upper case letter are derived for the decoding of the current syntax structure and all depending syntax structures. These variables may be used in the decoding process for later syntax structures. Variables starting with a lower case letter are only used within the process from which they are derived. (Single character variables are allowed.)

Constant values appear in all upper case letters with underscore characters (e.g. MI_SIZE).

Constant lookup tables appear as words (with the first letter of each word in upper case, and remaining letters in lower case) separated with underscore characters (e.g. Block_Width[…]).

Hexadecimal notation, indicated by prefixing the hexadecimal number by 0x, may be used when the number of bits is an integer multiple of 4. For example, 0x1a represents a bit string 0001 1010.

Binary notation is indicated by prefixing the binary number by 0b. For example, 0b00011010 represents a bit string 0001 1010. Binary numbers may include underscore characters to enhance readability. If present, the underscore characters appear every 4 binary digits starting from the LSB. For example, 0b11010 may also be written as 0b1_1010.

A value equal to 0 represents a FALSE condition in a test statement. The value TRUE is represented by any value not equal to 0.

The following table lists examples of the syntax specification format. When syntax_element appears (with bold face font), it specifies that this syntax element is parsed from the bitstream.

Functions

Bitstream functions used for syntax description are specified in this section.

Other functions are included in the syntax tables. The convention is that a section is called syntax if it causes syntax elements to be read from the bitstream, either directly or indirectly through subprocesses. The remaining sections are called functions.

The specification of these functions makes use of a bitstream position indicator. This bitstream position indicator locates the position of the bit that is going to be read next.

get_position( ): Return the value of the bitstream position indicator.

init_symbol( sz ): Initialize the arithmetic decode process for the Symbol decoder with a size of sz bytes as specified in section 8.2.2.

exit_symbol( ): Exit the arithmetic decode process as described in section 8.2.4 (this includes reading trailing bits).

Descriptors

General

The following descriptors specify the parsing of syntax elements. Lower case descriptors specify syntax elements that are represented by an integer number of bits in the bitstream; upper case descriptors specify syntax elements that are represented by arithmetic coding.

f(n)

Unsigned n-bit number appearing directly in the bitstream. The bits are read from high to low order. The parsing process specified in section 8.1 is invoked and the syntax element is set equal to the return value.

uvlc()

Variable length unsigned number appearing directly in the bitstream. The parsing process for this descriptor is specified below:

le(n)

Unsigned little-endian n-byte number appearing directly in the bitstream. The parsing process for this descriptor is specified below:

Note: This syntax element will only be present when the bitstream position is byte aligned.

leb128()

Unsigned integer represented by a variable number of little-endian bytes.

Note: This syntax element will only be present when the bitstream position is byte aligned.

In this encoding, the most significant bit of each byte is equal to 1 to signal that more bytes should be read, or equal to 0 to signal the end of the encoding.

A variable Leb128Bytes is set equal to the number of bytes read during this process.

The parsing process for this descriptor is specified below:

It is a requirement of bitstream conformance that the value returned from the leb128 parsing process is less than or equal to (1 << 32) - 1.

leb128_byte contains 8 bits read from the bitstream. The bottom 7 bits are used to compute the variable value. The most significant bit is used to indicate that there are more bytes to be read.

It is a requirement of bitstream conformance that the most significant bit of leb128_byte is equal to 0 if i is equal to 7. (This ensures that this syntax descriptor never uses more than 8 bytes.)

Note: There are multiple ways of encoding the same value depending on how many leading zero bits are encoded. There is no requirement that this syntax descriptor uses the most compressed representation. This can be useful for encoder implementations by allowing a fixed amount of space to be filled in later when the value becomes known.

su(n)

Signed integer converted from an n bits unsigned integer in the bitstream. (The unsigned integer corresponds to the bottom n bits of the signed integer.) The parsing process for this descriptor is specified below:

ns(n)

Unsigned encoded integer with maximum number of values n (i.e. output in range 0..n-1).

This descriptor is similar to f(CeilLog2(n)), but reduces wastage incurred when encoding non-power of two value ranges by encoding 1 fewer bits for the lower part of the value range. For example, when n is equal to 5, the encodings are as follows (full binary encodings are also presented for comparison):

The parsing process for this descriptor is specified as:

The abbreviation ns stands for non-symmetric. This encoding is non-symmetric because the values are not all coded with the same number of bits.

L(n)

Unsigned arithmetic encoded n-bit number encoded as n flags (a “literal”). The flags are read from high to low order. The syntax element is set equal to the return value of read_literal( n ) (see section 8.2.5 for a specification of this process).

S()

An arithmetic encoded symbol coded from a small alphabet of at most 16 entries.

The symbol is decoded based on a context sensitive CDF (see section 8.3 for the specification of this process).

NS(n)

Unsigned arithmetic encoded integer with maximum number of values n (i.e. output in range 0..n-1).

This descriptor is the same as ns(n) except the underlying bits are coded arithmetically.

The parsing process for this descriptor is specified as:

Syntax structures

General

This section presents the syntax structures in a tabular form. The meaning of each of the syntax elements is presented in Section 6.

Low overhead bitstream format

This specification defines a low-overhead bitstream format as a sequence of the OBU syntactical elements defined in this section. When using this format, obu_has_size_field must be equal to 1. For applications requiring a format where it is easier to skip through frames or temporal units, a length-delimited bitstream format is defined in Annex B.

Derived specifications, such as container formats enabling storage of AV1 videos together with audio or subtitles, should indicate which of these formats they rely on. Other methods of packing OBUs into a bitstream format are also allowed.

OBU syntax

General OBU syntax

OBU header syntax

OBU extension header syntax

Trailing bits syntax

Byte alignment syntax

Reserved OBU syntax

Note: Reserved OBUs do not have a defined syntax. The obu_type reserved values are reserved for future use. Decoders should ignore the entire OBU if they do not understand the obu_type. Ignoring the OBU can be done based on obu_size. The last byte of the valid content of the payload data for this OBU type is considered to be the last byte that is not equal to zero. This rule is to prevent the dropping of valid bytes by systems that interpret trailing zero bytes as a continuation of the trailing bits in an OBU. This implies that when any payload data is present for this OBU type, at least one byte of the payload data (including the trailing bit) shall not be equal to 0.

Sequence header OBU syntax

General sequence header OBU syntax

Color config syntax

Timing info syntax

Decoder model info syntax

Operating parameters info syntax

Temporal delimiter obu syntax

Note: The temporal delimiter has an empty payload.

Padding OBU syntax

Note: obu_padding_length is not coded in the bitstream but can be computed based on obu_size minus the number of trailing bytes. In practice, though, since this is padding data meant to be skipped, decoders do not need to determine either that length nor the number of trailing bytes. They can ignore the entire OBU. Ignoring the OBU can be done based on obu_size. The last byte of the valid content of the payload data for this OBU type is considered to be the last byte that is not equal to zero. This rule is to prevent the dropping of valid bytes by systems that interpret trailing zero bytes as a continuation of the trailing bits in an OBU. This implies that when any payload data is present for this OBU type, at least one byte of the payload data (including the trailing bit) shall not be equal to 0.

Metadata OBU syntax

General metadata OBU syntax

Note: The exact syntax of metadata_obu is not defined in this specification when metadata_type is equal to a value reserved for future use or a user private value. Decoders should ignore the entire OBU if they do not understand the metadata_type. The last byte of the valid content of the data is considered to be the last byte that is not equal to zero. This rule is to prevent the dropping of valid bytes by systems that interpret trailing zero bytes as a padding continuation of the trailing bits in an OBU. This implies that when any payload data is present for this OBU type, at least one byte of the payload data (including the trailing bit) shall not be equal to 0.

Metadata ITUT T35 syntax

Note: The exact syntax of itu_t_t35_payload_bytes is not defined in this specification. External specifications can define the syntax. Decoders should ignore the entire OBU if they do not understand it. The last byte of the valid content of the data is considered to be the last byte that is not equal to zero. This rule is to prevent the dropping of valid bytes by systems that interpret trailing zero bytes as a padding continuation of the trailing bits in an OBU. This implies that when any payload data is present for this OBU type, at least one byte of the payload data (including the trailing bit) shall not be equal to 0.

Metadata high dynamic range content light level syntax

Metadata high dynamic range mastering display color volume syntax

Metadata scalability syntax

Scalability structure syntax

Metadata timecode syntax

Frame header OBU syntax

General frame header OBU syntax

Uncompressed header syntax

Get relative distance function

This function computes the distance between two order hints by sign extending the result of subtracting the values.

Reference frame marking function

Frame size syntax

Render size syntax

Frame size with refs syntax

Superres params syntax

Compute image size function

Interpolation filter syntax

Loop filter params syntax

Quantization params syntax

Delta quantizer syntax

Segmentation params syntax

The constant lookup tables used in this syntax are defined as:

Segmentation_Feature_Bits[ SEG_LVL_MAX ]   = { 8, 6, 6, 6, 6, 3, 0, 0 }
Segmentation_Feature_Signed[ SEG_LVL_MAX ] = { 1, 1, 1, 1, 1, 0, 0, 0 }
Segmentation_Feature_Max[ SEG_LVL_MAX ] = {
  255, MAX_LOOP_FILTER, MAX_LOOP_FILTER,
  MAX_LOOP_FILTER, MAX_LOOP_FILTER, 7,
  0, 0 }

Tile info syntax

Tile size calculation function

tile_log2 returns the smallest value for k such that blkSize << k is greater than or equal to target.

Quantizer index delta parameters syntax

Loop filter delta parameters syntax

CDEF params syntax

Loop restoration params syntax

where Remap_Lr_Type is a constant lookup table specified as:

Remap_Lr_Type[4] = {
  RESTORE_NONE, RESTORE_SWITCHABLE, RESTORE_WIENER, RESTORE_SGRPROJ
}

TX mode syntax

Skip mode params syntax

Frame reference mode syntax

Global motion params syntax

Global param syntax

Note: When force_integer_mv is equal to 1, some fractional bits are still read for the translation components. However, these fractional bits will be discarded during the Setup Global MV process.

Decode signed subexp with ref syntax

Note: decode_signed_subexp_with_ref will return a value in the range low to high - 1 (inclusive).

Decode unsigned subexp with ref syntax

Note: decode_unsigned_subexp_with_ref will return a value in the range 0 to mx - 1 (inclusive).

Decode subexp syntax

Inverse recenter function

Film grain params syntax

Temporal point info syntax

Frame OBU syntax

Tile group OBU syntax

General tile group OBU syntax

Decode tile syntax

where Sgrproj_Xqd_Mid and Wiener_Taps_Mid are constant lookup tables specified as:

Wiener_Taps_Mid[3] = {  3,  -7,  15 }

Sgrproj_Xqd_Mid[2] = { -32,  31 }

Clear block decoded flags function

Decode partition syntax