ISO/IEC 23094-2:2021
(Main)Information technology – General video coding
Information technology – General video coding
This document specifies low complexity enhancement video coding.
Technologies de l'information – Codage vidéo général
General Information
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Standards Content (Sample)
INTERNATIONAL ISO/IEC
STANDARD 23094-2
First edition
2021-11
Information technology – General
video coding —
Part 2:
Low complexity enhancement video
coding
Technologies de l'information – Codage vidéo général —
Partie 2: Codage vidéo d'amélioration de faible complexité
Reference number
© ISO/IEC 2021
© ISO/IEC 2021
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Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 6
5 Conventions . 7
5.1 General . 7
5.2 Arithmetic operators . 7
5.3 Logical operators . 8
5.4 Relational operators . 8
5.5 Bit-wise operators . 8
5.6 Assignment operators . 9
5.7 Range notation . 9
5.8 Mathematical functions . 9
5.9 Order of operation precedence . 10
5.10 Variables, syntax elements and tables. 11
5.11 Text description of logical operations .12
5.12 Processes . 13
6 Bitstream and picture formats, partitioning, scanning processes and neighbouring
relationships .13
6.1 Bitstream formats . 13
6.2 Source, decoded and output picture formats . 14
6.3 Partitioning of pictures . 17
6.3.1 Organization of the hierarchical structure . 17
6.3.2 Partitioning of residuals plane . 17
7 Syntax and semantics .18
7.1 Method of specifying syntax in tabular form . . 18
7.2 Specification of syntax functions and descriptors . 19
7.3 Syntax in tabular form . 20
7.3.1 Syntax order .20
7.3.2 NAL unit and NAL unit header syntax . 20
7.3.3 Process block syntax . 21
7.3.4 Process payload – sequence configuration . 22
7.3.5 Process payload – global configuration . 22
7.3.6 Process payload – picture configuration . 24
7.3.7 Process payload – encoded data . 25
7.3.8 Process payload – encoded tiled data . 26
7.3.9 Process payload – surface .28
7.3.10 Process payload – additional info .28
7.3.11 Process payload – filler .29
7.3.12 Byte alignment syntax .29
7.4 Semantics . 29
7.4.1 General .29
7.4.2 NAL unit semantics . 30
7.4.3 Data block unit configuration semantics . 32
8 Decoding process .41
8.1 General decoding process . 41
8.2 Payload data block unit process . 41
8.3 Picture enhancement decoding process . 42
8.3.1 General enhancement decoding process . 42
8.3.2 Decoding process for picture enhancement encoded data (payload_
encoded_data) . 42
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© ISO/IEC 2021 – All rights reserved
8.3.3 Decoding process for picture enhancement encoded tiled data (payload_
encoded_tiled_data) . 45
8.3.4 Decoding process for enhancement sub-layer 1 (L-1) encoded data .50
8.3.5 Decoding process for enhancement sub-layer 2 (L-2) encoded data . 52
8.4 Decoding process for the temporal prediction . 53
8.4.1 General decoding process for temporal prediction .53
8.4.2 Tiled temporal refresh .54
8.5 Decoding process for the dequantization .54
8.5.1 Decoding process for the dequantization overview .54
8.5.2 Scaling process for transform coefficients .54
8.5.3 Derivation of dequantization offset and stepwidth modifier .55
8.5.4 Derivation of quantization matrix .56
8.6 Decoding process for the transform . 59
8.6.1 General upscaling process description . . 59
8.6.2 Transform inputs and outputs, transform types, and residual samples
derivation . .63
8.6.3 2x2 directional decomposition transform .64
8.6.4 4x4 directional decomposition transform .65
8.7 Decoding process for the upscaling .66
8.7.1 Nearest sample upsampler kernel description .66
8.7.2 Bilinear upsampler kernel description .68
8.7.3 Cubic upsampler kernel description . 71
8.7.4 Modified Cubic upsampler kernel description .73
8.7.5 Predicted residual process description .74
8.7.6 Adaptive Cubic upsampler kernel description . 75
8.8 Decoding process for the residual reconstruction . 75
8.8.1 Reconstructed residual of each block derivation . 75
8.8.2 Residual reconstruction for L-1 block . 76
8.8.3 Residual reconstruction for L-2 block . 76
8.9 Decoding process for the L-1 filter .77
8.9.1 L-1 residual filter overview .77
8.9.2 Decoding process for filtering L-1 block .77
8.10 Decoding process for base decoder data extraction . 79
8.11 Decoding process for dither filter . 79
9 Parsing process .79
9.1 Parsing process inputs and outputs, process overview . 79
9.1.1 Parsing process for entropy encoded transform coefficients .79
9.1.2 Parsing process for entropy encoded temporal signal coefficient group .82
9.2 Prefix Coding decoder .83
9.2.1 Prefix Coding decoder description .83
9.2.2 Prefix Coding decoder table generation .84
9.2.3 Prefix Coding decoder for tile data sizes .88
9.3 RLE decoder .89
9.3.1 RLE process inputs and outputs .89
9.3.2 RLE decoder for coefficient groups .89
9.3.3 RLE decoder description .90
9.3.4 RLE decoder for temporal signal coefficient group . 91
9.3.5 RLE decoder for tile entropy_enabled_flag fields. 93
9.4 Parsing process for 0-th order Exp-Golomb codes.94
Annex A (normative) Profiles and levels .96
Annex B (normative) Byte stream format .98
Annex C (normative) Hypothetical reference decoder . 101
Annex D (normative) Supplemental enhancement information . 108
Annex E (normative) Video usability information . 115
Bibliography .119
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© ISO/IEC 2021 – All rights reserved
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are
members of ISO or IEC participate in the development of International Standards through technical
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The procedures used to develop this document and those intended for its further maintenance
are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria
needed for the different types of document should be noted. This document was drafted in
accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives or
www.iec.ch/members_experts/refdocs).
Attention is drawn to the possibility that some of the elements of this document may be the subject
of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent
rights. Details of any patent rights identified during the development of the document will be in the
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list of patent declarations received (see patents.iec.ch).
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www.iso.org/iso/foreword.html. In the IEC, see www.iec.ch/understanding-standards.
This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 29, Coding of audio, picture, multimedia and hypermedia information.
A list of all parts in the ISO/IEC 23094 series can be found on the ISO and IEC websites.
Any feedback or questions on this document should be directed to the user’s national standards
body. A complete listing of these bodies can be found at www.iso.org/members.html and
www.iec.ch/national-committees.
v
© ISO/IEC 2021 – All rights reserved
INTERNATIONAL STANDARD ISO/IEC 23094-2:2021(E)
Information technology – General video coding —
Part 2:
Low complexity enhancement video coding
1 Scope
This document specifies low complexity enhancement video coding.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 11578:1996, Information technology — Open systems interconnection — Remote procedure call
(RPC)
ITU-T H.273 | ISO/IEC 23091-2:2019, Information technology — Coding-independent code points — Part
2: Video
ITU-T Recommendation T.35:2000, Procedure for the allocation of ITU-T defined codes for non-standard
facilities
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
access unit
AU
set of NAL units (3.35) that are associated with a particular output time, are consecutive in decoding
order (3.20), and contain exactly one coded picture (3.9)
3.2
bitstream
sequence of bits, in the form of a NAL unit stream (3.36) or a byte stream (3.6), that forms the
representation of coded pictures (3.9), and associated data forming one or more coded video sequences
(CVSs)
3.3
block
MxN (M-column by N-row) array of samples, or an MxN array of transform coefficients (3.57)
3.4
byte
sequence of 8 bits, within which, when written or read as a sequence of bit values, the left-most and
right-most bits represent the most and least significant bits, respectively
© ISO/IEC 2021 – All rights reserved
3.5
byte-aligned
position in a bitstream (3.2) in which the position is an integer multiple of 8 bits from the position of the
first bit in the bitstream
Note 1 to entry: A bit, byte (3.4) or syntax element (3.53) is said to be byte-aligned when the position at which it
appears in a bitstream (3.2) is byte-aligned.
3.6
byte stream
encapsulation of a NAL unit stream (3.36) containing start code prefixes (3.51) and NAL units (3.35)
3.7
chroma
sample array or single sample is representing one of the two colour difference signals related to the
primary colours, represented by the symbols Cb and Cr
Note 1 to entry: The term chroma is used rather than the term chrominance in order to avoid the implication of
the use of linear light transfer characteristics that is often associated with the term chrominance.
3.8
chunk
entropy coded portion of data containing the quantized transform coefficient (3.57) belonging to a
coefficient group
3.9
coded picture
coded representation (3.10) of a picture (3.40) containing all TUs (3.58) of the picture
3.10
coded representation
data element as represented in its coded form
3.11
coded video sequence
CVS
coded sequence of access units (3.1)
3.12
coding block
MxN block (3.3) of samples for some values of M and N
3.13
coding unit
CU
32 x 32 block (3.3) of samples resulting from the parsing of the entropy encoded transform coefficients
(3.57) in the decoding process (3.22)
3.14
coefficient group
CG
syntactical structure containing coded data related to a specific set of transform coefficients (3.57)
3.15
component
array or single sample from one of the three arrays (luma (3.34) and two chroma (3.7)) that compose
a picture (3.40) in 4:2:0, 4:2:2, or 4:4:4 colour format, or the array or a single sample of the array that
compose a picture in monochrome format
© ISO/IEC 2021 – All rights reserved
3.16
data block
syntax structure (3.54) containing bytes (3.4) corresponding to a type of data
3.17
decoded base picture
decoded picture (3.18) derived by decoding a coded picture (3.9) with a base decoder (3.20)
3.18
decoded picture
picture (3.40) derived by decoding a coded picture (3.9), and which is either a decoded frame (3.29) or a
decoded field (3.28)
3.19
decoded picture buffer
DPB
buffer holding decoded pictures (3.18) for reference or output reordering
3.20
decoder
embodiment of a decoding process (3.22)
3.21
decoding order
order in which syntax elements (3.53) are processed by the decoding process (3.22)
3.22
decoding process
process specified that reads a bitstream (3.2) and derives decoded pictures (3.18) from it
3.23
emulation prevention byte
byte (3.4) equal to 0x03 that may be present within a NAL unit (3.35), the presence of which ensures that
no sequence of consecutive byte-aligned (3.5) bytes in the NAL unit contains a start code prefix (3.51)
3.24
encoder
embodiment of an encoding process (3.25)
3.25
encoding process
process that produces a bitstream (3.2) conforming to this document
3.26
enhancement layer
layer (3.32) within the bitstream (3.2) pertaining to the residual planes (3.47)
3.27
enhancement sub-layer
layer (3.32) of the enhancement layer (3.26)
3.28
field
assembly of alternate rows of a frame (3.29)
3.29
frame
array of luma (3.34) samples in monochrome format or array of luma samples and two corresponding
arrays of chroma (3.7) samples in 4:2:0, 4:2:2, and 4:4:4 colour format, and which consists of two fields
(3.28): a top field and a bottom field
© ISO/IEC 2021 – All rights reserved
3.30
instantaneous decoding refresh picture
IDR picture
picture (3.40) for which a NAL unit (3.35) contains a global configuration data block and does not
refer to any other picture for operation of the decoding process (3.22) of this picture and for which no
subsequent pictures in decoding order (3.21) refer to any picture that precedes it in decoding order
Note 1 to entry: An IDR picture shall occur at least when an IDR picture for the base decoder (3.20) occurs. The
IDR picture for a base decoder is not specified in this document.
3.31
inverse transform
part of the decoding process (3.22) by which a set of transform coefficients (3.57) is converted into
residuals (3.46)
3.32
layer
one of a set of syntactical structures in a non-branching hierarchical relationship
3.33
level
defined set of constraints on the values that may be taken by the syntax elements (3.53) and variables of
this document
Note 1 to entry: The same set of levels is defined for all profiles (3.41), with most aspects of the definition of each
level being in common across different profiles. Individual implementations may, within specified constraints,
support a different level for each supported profile.
3.34
luma
sample array or single sample representing the monochrome signal related to the primary colours,
represented by the symbol or subscript Y or L
Note 1 to entry: The term luma is used rather than the term luminance in order to avoid the implication of the
use of linear light transfer characteristics that is often associated with the term luminance. The symbol L is
sometimes used instead of the symbol Y to avoid confusion with the symbol y as used for vertical location.
3.35
network abstraction layer unit
NAL unit
syntax structure (3.54) containing an indication of the type of data to follow and bytes (3.4) containing
that data in the form of an RBSP (3.42) interspersed as necessary with emulation prevention bytes (3.23)
3.36
network abstraction layer unit stream
NAL unit stream
sequence of NAL units (3.35)
3.37
output order
order in which the decoded pictures (3.18) are output from the decoded picture buffer (3.19) (for the
decoded pictures that are to be output from the decoded picture buffer)
3.38
partitioning
division of a set into subsets such that each element of the set is in exactly one of the subsets
3.39
plane
collection of data related to plane Y (luma (3.34)) or C (chroma (3.7))
© ISO/IEC 2021 – All rights reserved
3.40
picture
field (3.28) or frame (3.29)
3.41
profile
specified subset of the syntax of this document
3.42
raw byte sequence payload
RBSP
syntax structure (3.54) containing an integer number of bytes (3.4) that is encapsulated in a NAL unit
(3.35) and which is either empty or has the form of a string of data bits (3.52) containing syntax elements
(3.53) followed by an RBSP stop bit (3.43) and followed by zero or more subsequent bits equal to 0
3.43
raw byte sequence payload stop bit
RBSP stop bit
bit equal to 1 present within an RBSP (3.42) after a string of data bits (3.52)
Note 1 to entry: The location of the end of the string of data bits within an RBSP can be identified by searching
from the end of the RBSP for the RBSP stop bit, which is the last non-zero bit in the RBSP.
3.44
reserved
value of a particular syntax element (3.53) for future use by ISO/IEC and not to be used in bitstreams
(3.2) conforming to this document, but could be used in bitstreams conforming to future revised
editions of this document
3.45
reserved_zeros
value of a particular syntax element (3.53) for future use by ISO/IEC and not to be used in bitstreams
(3.2) conforming to this document, but could be used in bitstreams conforming to future revised
editions of this document
Note 1 to entry: In this document, the value of any reserved_zeros bit is zero.
3.46
residual
difference between a prediction of a sample or data element and a reference of that same sample or data
element
3.47
residual plane
collection of residuals (3.46)
3.48
run length encoding
RLE
method for encoding a sequence of values in which consecutive occurrences of the same value are
represented as a single value together with its number of occurrences
3.49
sample aspect ratio
the ratio between the intended horizontal distance between the columns and the intended vertical
distance between the rows of the luma (3.34) sample array in a picture (3.40), which is specified for
assisting the display process (not specified in this Specification) and expressed as h:v, where h is the
horizontal width and v is the vertical height, in arbitrary units of spatial distance
© ISO/IEC 2021 – All rights reserved
3.50
source
video material or some of its attributes before encoding
3.51
start code prefix
unique sequence of three bytes (3.4) equal to 0 x 000001 embedded in the byte stream (3.6) as a prefix
to each NAL unit (3.35)
Note 1 to entry: The location of a start code prefix can be used by a decoder (3.20) to identify the beginning of a
new NAL unit and the end of a previous NAL unit. Emulation of start code prefixes is prevented within NAL units by
the inclusion of emulation prevention bytes (3.23).
3.52
string of data bits
SODB
sequence of some number of bits representing syntax elements (3.53) present within a raw byte sequence
payload (3.42) prior to the raw byte sequence payload stop bit (3.43), and within which the left-most bit
is considered to be the first and most significant bit, and the right-most bit is considered to be the last
and least significant bit
3.53
syntax element
element of data represented in the bitstream (3.2)
3.54
syntax structure
zero or more syntax elements (3.53) present together in the bitstream (3.2) in a specified order
3.55
tile
rectangular region of TUs (3.58) within a particular picture (3.40)
3.56
transform
part of the decoding process (3.22) by which a block (3.3) of transform coefficients (3.57) is converted to
a block of spatial-domain values
3.57
transform coefficient
scalar quantity, considered to be in a transformed domain, that is associated with a particular index in
an inverse transform (3.31) part of the decoding process (3.22)
3.58
transform unit
TU
MxN block (3.3) of samples resulting from a transform (3.56) in the decoding process (3.22)
3.59
unspecified
value of a particular syntax element (3.53) with no specified meaning in this document and that will not
have a specified meaning in any future revised editions of this document
3.60
video coding layer NAL unit
VCL NAL unit
NAL units (3.35) that have reserved values of NalUnitType that are classified as VCL NAL units in this
document
4 Abbreviated terms
© ISO/IEC 2021 – All rights reserved
CLVS coded layer-wise video sequence
CPB coded picture buffer
CPBB coded picture buffer of the base
CPBL coded picture buffer LCEVC
CVS coded video sequence
DPBB decoded picture buffer of the base
DUT decoder under test
HBD hypothetical base decoder
HDM hypothetical demuxer
HRD hypothetical reference decoder
HSS hypothetical stream scheduler
I intra
LCEVC low complexity enhancement video coding
LSB least significant bit
MSB most significant bit
SEI supplemental enhancement information
VUI video usability information
5 Conventions
5.1 General
NOTE The mathematical operators used in this document are similar to those used in the C programming
language. However, the results of integer division and arithmetic shift operations are defined more precisely,
and additional operations are defined, such as exponentiation and real-valued division. Numbering and counting
conventions generally begin from 0, e.g., “the first” is equivalent to the 0-th, “the second” is equivalent to the 1-th,
etc.
5.2 Arithmetic operators
+ addition
− subtraction (as a two-argument operator) or negation (as a unary prefix operator)
* multiplication, including matrix multiplication
exponentiation; specifies x to the power of y
y
x
In other contexts, such notation is used for superscripting not intended for
interpretation as exponentiation.
© ISO/IEC 2021 – All rights reserved
integer division with truncation of the result toward zero
/
For example, 7 / 4 and −7 / −4 are truncated to 1 and −7 / 4 and 7 / −4 are
truncated to −1.
÷ division in mathematical equations where no truncation or rounding is intended
division in mathematical equations where no truncation or rounding is intended
y
summation of f(i) with i taking all integer values from x up to and including y
f(i)
∑
i=x
Modulus
x % y
Remainder of x divided by y, defined only for integers x and y with x >= 0 and y > 0.
5.3 Logical operators
x && y Boolean logical “and” of x and y
x || y Boolean logical “or” of x and y
! Boolean logical “not”
x ? y : z if x is TRUE or not equal to 0, evaluates to the value of y; otherwise, evalu-
ates to the value of z
5.4 Relational operators
> greater than
>= greater than or equal to
< less than
<= less than or equal to
== equal to
!= not equal to
When a relational operator is applied to a syntax element or variable that has been assigned the value
“na” (not applicable), the value “na” is treated as a distinct value for the syntax element or variable. The
value “na” is considered not to be equal to any other value.
5.5 Bit-wise operators
& bit-wise “and”
When operating on integer arguments, operates on a two’s complement
representation of the integer value. When operating on a binary argument
that contains fewer bits than another argument, the shorter argument is
extended by adding more significant bits equal to 0.
© ISO/IEC 2021 – All rights reserved
| bit-wise “or”
When operating on integer arguments, operates on a two’s complement
representation of the integer value. When operating on a binary argument
that contains fewer bits than another argument, the shorter argument is
extended by adding more significant bits equal to 0.
^ bit-wise “exclusive or”
When operating on integer arguments, operates on a two’s complement
representation of the integer value. When operating on a binary argument
that contains fewer bits than another argument, the shorter argument is
extended by adding more significant bits equal to 0.
x >> y arithmetic right shift of a two’s complement integer representation of x by
y binary digits
This function is defined only for non-negative integer values of y. Bits shift-
ed into the most significant bits (MSBs) as a result of the right shift have a
value equal to the MSB of x prior to the shift operation.
x << y arithmetic left shift of a two’s complement integer representation of x by
y binary digits
This function is defined only for non-negative integer values of y. Bits shifted
into the least significant bits (LSBs) as a result of the left shift have a value
equal to 0.
5.6 Assignment operators
= assignment operator
++ increment, i.e., x++ is equivalent to x = x + 1; when used in an array index,
evaluates to the value of the variable prior to the increment operation
− − decrement, i.e., x− − is equivalent to x = x − 1; when used in an array index,
evaluates to the value of the variable prior to the decrement operation
+= increment by amount specified, i.e., x += 3 is equivalent to x = x + 3, and x
+= (−3) is equivalent to x = x + (−3)
−= decrement by amount specified, i.e., x −= 3 is equivalent to x = x − 3, and x
−= (−3) is equivalent to x = x − (−3)
5.7 Range notation
x = y.z x takes on integer values starting from y to z, inclusive, with x, y, and z being
integer numbers and z being greater than y
x = y to z x takes on integer values starting from y to z, inclusive, with x, y, and z being
integer numbers and z being greater than y
5.8 Mathematical functions
x; x >=0
Abs(x) = (1)
−x; x <0
Ceil(x) smallest integer greater than or equal to x (2)
© ISO/IEC 2021 – All rights reserved
x; z
Clip3(x, y, z) = y; z>y (3)
z; otherwise
Floor(x) largest integer less than or equal to x (4)
Ln(x) natural logarithm of x (the base-e logarithm, where e is the natural logarithm base constant
2.718 281 828.) (5)
Log10(x) base-10 logarithm of x (6)
x; x<=y
Min(x, y) = (7)
y; x>y
xx; >= y
Max(x, y) = (8)
yx; < y
Round(x) = Sign(x) * Floor(Abs(x) + 0.5) (9)
10; x>
Sign(x) = 00; x== (10)
−<10; x
Sqrt(x) = > x (11)
5.9 Order of operation precedence
When the order of precedence in an expression is not indicated explicitly by use of parentheses, the
following rules apply:
— Operations of a higher precedence are evaluated before any operation of a lower precedence.
— Operations of the same precedence are evaluated sequentially from left to right.
Table 1 specifies the precedence of operations from highest to lowest. A higher position in the table
indicates a higher precedence.
NOTE For those operators that are also used in the C programming language, the order of precedence used
in this document is the same as used in the C programming language.
Table 1 — Operation precedence from highest (top of the table) to lowest (bottom of the table)
operations (with operands x, y, and z)
“x++”, “x− −“
“!x”, “−x” (as a unary prefix operator)
y
x
“x * y”, “x / y”, “x ÷ y”, “”, “x % y”
y
“x + y”, “ x ÷ y” (as a two-argument operator), “ f(i) ”
∑
i=x
“x << y”, “x >> y”
“x < y”, “x <= y”, “x > y”, “x >= y”
© ISO/IEC 2021 – All rights reserved
Table 1 (continued)
operations (with operands x, y, and z)
“x == y”, “x != y”
“x & y”
“x | y”
“x && y”
“x || y”
“x ? y : z”
“x.y”
5.10 Variables, syntax elements and tables
Syntax elements in the bitstream are represented in bold type. Each syntax element is described by its
name (all lower-case letters with underscore characters), and one 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.
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. Variables
starting with an upper-case letter may be used in the decoding process for later syntax structures
without mentioning the originating syntax structure of the variable. Variables starting with a lower-
case letter are only used within the clause in which they are derived.
In some cases, “mnemonic” names for syntax element values or variable values are used interchangeably
with their numerical values. Sometimes “mnemonic” names are used without any associated numerical
values. The association of values and names is specified in the text. The names are constructed from
one or more groups of letters separated by an underscore character. Each group starts with an upper-
case letter and may contain more upper-case letters.
NOTE The syntax is described in a manner that closely follows the C-language syntactic constructs.
Functions that specify properties of the current position in the bitstream are referred to as syntax
functions. These functions are specified in 7.2 and assume the existence of a bitstream pointer with an
indication of the position of the next bit to be read by the decoding process from the bitstream. Syntax
functions are described by their names, which are constructed as syntax element names and end with
left and right round parentheses including zero or more variable names (for definition) or values (for
usage), separated by commas (if more than one variable).
Functions that are not syntax functions (including mathematical functions specified in 5.8) are
described by their names, which start with an upper-case letter, contain a mixture of lower- and upper-
case letters without any underscore character, and end with left and right parentheses including zero
or more variable names (for definition) or values (for usage) separated by commas (if more than one
variable).
A one-dimensional array is referred to as a list. A two-dimensional array is referred to as a matrix.
Arrays can either be syntax elements or variables. Subscripts or square parentheses are used for the
indexing of arrays. In reference to a visual depiction of a matrix, the first subscript is used as a row
(vertical) index and the second subscript is used as a column (horizontal) index. The indexing order
is reversed when using square parentheses rather than subscripts for indexing. Thus, an element of
a matrix s at horizontal position x and vertical position y may be denoted either as s[x][y] or as s . A
yx
single column of a matrix may be referred to as a list and denoted by omission of the row
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