ISO/IEC 23090-14:2023
(Main)Information technology — Coded representation of immersive media — Part 14: Scene description
Information technology — Coded representation of immersive media — Part 14: Scene description
This document specifies extensions to existing scene description formats in order to support MPEG media, in particular immersive media. MPEG media includes but is not limited to media encoded with MPEG codecs, media stored in MPEG containers, MPEG media and application formats as well as media provided through MPEG delivery mechanisms. Extensions include scene description format syntax and semantics and the processing model when using these extensions by a Presentation Engine. It also defines a Media Access Function (MAF) API for communication between the Presentation Engine and the Media Access Function for these extensions. While the extensions defined in this document can be applicable to other scene description formats, they are provided for ISO/IEC 12113.
Technologies de l'information — Représentation codée de média immersifs — Partie 14: Description de scènes
General Information
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Standards Content (Sample)
INTERNATIONAL ISO/IEC
STANDARD 23090-14
First edition
2023-06
Information technology — Coded
representation of immersive media —
Part 14:
Scene description
Technologies de l'information — Représentation codée de média
immersifs —
Partie 14: Description de scènes
Reference number
© ISO/IEC 2023
© ISO/IEC 2023
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Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, abbreviated terms, and conventions . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 3
3.3 Conventions . 3
3.3.1 General . 3
3.3.2 Arithmetic operators . 3
3.3.3 Logical operators . 4
3.3.4 Relational operators . 4
3.3.5 Bit-wise operators . 4
3.3.6 Assignment operators . 4
3.3.7 Other operators . 5
3.3.8 Order of operation precedence . 5
3.3.9 Text description of logical operations . 5
4 Overview and architecture .7
4.1 Overview . 7
4.2 Architecture . 7
4.3 Timing model . 11
5 Scene description extensions .11
5.1 General . 11
5.1.1 Overview of extensions . 11
5.1.2 Formatting and typing .12
5.2 Generic extensions . 13
5.2.1 MPEG_media extension . 13
5.2.2 MPEG_accessor_timed extension . 16
5.2.3 MPEG_buffer_circular extension . 19
5.2.4 MPEG_scene_dynamic extensions . 21
5.3 Visual Extensions . 23
5.3.1 MPEG_texture_video extensions . 23
5.3.2 MPEG_mesh_linking extensions . 24
5.4 Audio extensions. 26
5.4.1 MPEG_audio_spatial extensions . 26
5.5 Metadata extensions .29
5.5.1 MPEG_viewport_recommended extensions .29
5.5.2 MPEG_animation_timing extensions.30
6 Media access function and buffer API .31
6.1 General . 31
6.2 Media access function API . 32
6.3 Buffer API . 35
7 Carriage formats .37
7.1 General . 37
7.2 Carriage format for glTF JSON and JSON patch .38
7.2.1 General .38
7.2.2 glTF patch config box .39
7.3 Carriage format for glTF object and glTF source object as non-timed item .39
7.3.1 General .39
7.3.2 glTF Items .40
7.3.3 glTF source items .40
7.4 Carriage format for mesh correspondence values . 41
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© ISO/IEC 2023 – All rights reserved
7.4.1 General . 41
7.4.2 Vertices correspondence sample entry . 41
7.4.3 Vertices correspondence sample format . 42
7.5 Carriage format for pose and weight . 42
7.5.1 General . 42
7.5.2 Pose transformation sample entry . 43
7.5.3 Pose transformation sample format . 43
7.6 Carriage format for animation timing .44
7.6.1 General .44
7.6.2 Animation sample entry .44
7.6.3 Animation sample format .44
7.7 Sample redundancies .46
7.8 Brands .46
Annex A (informative) JSON schema reference .47
Annex B (normative) Attribute registry . .49
Annex C (normative) Support for real-time media.50
Annex D (normative) Audio attenuation functions .51
Annex E (informative) Linking a dependent mesh and its associated shadow mesh .53
Annex F (informative) glTF extension usage examples .55
Bibliography .57
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© ISO/IEC 2023 – 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
committees established by the respective organization to deal with particular fields of technical
activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international
organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the
work.
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).
ISO and IEC draw attention to the possibility that the implementation of this document may involve
the use of (a) patent(s). ISO and IEC take no position concerning the evidence, validity or applicability
of any claimed patent rights in respect thereof. As of the date of publication of this document, ISO and
IEC had received notice of (a) patent(s) which may be required to implement this document. However,
implementers are cautioned that this may not represent the latest information, which may be obtained
from the patent database available at www.iso.org/patents and https://patents.iec.ch. ISO and IEC shall
not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see
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 23090 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 2023 – All rights reserved
Introduction
This document defines the MPEG-I Scene Description. It provides an architecture for the MPEG-I Scene
Description, a set of extensions based on ISO/IEC 12113, a set of APIs, and storage formats for scene
description documents and scene description updates documents.
vi
© ISO/IEC 2023 – All rights reserved
INTERNATIONAL STANDARD ISO/IEC 23090-14:2023(E)
Information technology — Coded representation of
immersive media —
Part 14:
Scene description
1 Scope
This document specifies extensions to existing scene description formats in order to support MPEG
media, in particular immersive media. MPEG media includes but is not limited to media encoded with
MPEG codecs, media stored in MPEG containers, MPEG media and application formats as well as media
provided through MPEG delivery mechanisms. Extensions include scene description format syntax
and semantics and the processing model when using these extensions by a Presentation Engine. It also
defines a Media Access Function (MAF) API for communication between the Presentation Engine and
the Media Access Function for these extensions. While the extensions defined in this document can be
applicable to other scene description formats, they are provided for ISO/IEC 12113.
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 12113, Information technology — Runtime 3D asset delivery format — Khronos glTF™ 2.0
ISO/IEC 14496-12, Information technology — Coding of audio-visual objects — Part 12: ISO base media file
format
ISO/IEC 21778, Information technology — The JSON data interchange syntax
IEEE 754-2019, IEEE Standard for Floating-Point Arithmetic
IETF RFC 6902, JavaScript Object Notation (JSON) Patch
IETF RFC 8259, The JavaScript Object Notation (JSON) Data Interchange Format
3 Terms, definitions, abbreviated terms, and conventions
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC 12113 and the following
apply.
ISO and IEC maintain terminology 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/
3.1.1
asset
3D scene described by a scene description document (3.1.10) together with corresponding scene
description data (3.1.9)
© ISO/IEC 2023 – All rights reserved
3.1.2
node
element in the scene graph (3.1.12)
3.1.3
media access function
function that retrieves and prepares media for rendering on request by the presentation engine (3.1.7)
3.1.4
media pipeline
chain of media processing components to process media
3.1.5
object
node in a scene description document (3.1.10)
3.1.6
patch document
document that contains update instructions
Note 1 to entry: For example, update instruction can be provided as defined in RFC 6902.
3.1.7
presentation engine
engine that processes and renders the asset (3.1.1)
3.1.8
scene activation time
time on the media timeline at which the scene described by a scene description document (3.1.10) takes
effect in the presentation engine (3.1.7)
3.1.9
scene description data
binary data that is described by scene description document (3.1.10)
3.1.10
scene description document
document describing a 3D scene
Note 1 to entry: For example, scene description document is containing description of node hierarchy, materials,
cameras, as well as description information for meshes, animations, and other constructs.
3.1.11
scene description update
patch document (3.1.6) to a scene description document (3.1.10) or a scene description document (3.1.10)
3.1.12
scene graph
data structure used to represent objects (3.1.5) in a 3D scene and their hierarchical relationships
3.1.13
timed accessor
accessor defined in ISO/IEC 12113 that has an MPEG_accessor_timed extension and is used to describe
access to timed data
3.1.14
timed data
timed media
media, which when decoded results in content, possibly containing internal timing values, to be
presented at a given presentation time and for a certain duration
© ISO/IEC 2023 – All rights reserved
3.2 Abbreviated terms
3D Three-Dimensional
3DoF Three Degrees of Freedom
6DoF Six Degrees of Freedom
API Application Programming Interface
AR Augmented Reality
DASH Dynamic Adaptive Streaming over HTTP
dB Decibel
DSR Diffuse to Source Ratio
glTF Graphics Language Transmission Format
HOA Higher Order Ambisonics
ISOBMFF ISO Base Media File Format
JSON JavaScript Object Notation
MAF Media Access Function
MPEG Moving Picture Experts Group
IDL Interface Definition Language
PCM Pulse-Code Modulation
RT60 60 dB Reverberation Time
SDP Session Description Protocol
3.3 Conventions
3.3.1 General
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.
3.3.2 Arithmetic operators
+ addition
− subtraction (as a two-argument operator) or negation (as a unary prefix operator)
* multiplication, including matrix multiplication
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.
© ISO/IEC 2023 – All rights reserved
3.3.3 Logical operators
! Boolean logical "not".
3.3.4 Relational operators
> Greater than.
>= Greater than or equal to.
< Less than.
<= Less than or equal to.
== Equal to.
!= Not equal to.
3.3.5 Bit-wise operators
~ bit-wise "not".
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 "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.
| 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 shifted into the 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 LSBs
as a result of the left shift have a value equal to 0.
3.3.6 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.
© ISO/IEC 2023 – All rights reserved
+= 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).
3.3.7 Other operators
y.z range operator/notation.
This function is defined only for integer values of y and z. When z is larger than or equal
to y, it defines an ordered set of values from y to z in increments of 1. Otherwise, when z is
smaller than y, the output of this function is an empty set. If this operator is used within the
context of a loop, it specifies that any subsequent operations defined are performed using
each element of this set, unless this set is empty.
3.3.8 Order of operation precedence
When 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 (at top of table) to lowest (at bottom of table)
operations (with operands x, y, and z)
"x++", "x--"
"!x", "−x" (as a unary prefix operator)
"x * y", "x / y", "x ÷ y", "x % y"
"x + y", "x − y" (as a two-argument operator)
"x << y", "x >> y"
"x < y", "x <= y", "x > y", "x >= y"
"x == y", "x != y"
"x & y"
"x | y"
"x && y"
"x || y"
"x ? y : z"
"x.y"
"x = y", "x += y", "x −= y"
3.3.9 Text description of logical operations
In the text, a statement of logical operations as would be described mathematically in the following
form:
© ISO/IEC 2023 – All rights reserved
if( condition 0 )
statement 0
else if( condition 1 )
statement 1
...
else /* informative remark on remaining condition */
statement n
may be described in the following manner:
... as follows / ... the following applies:
— If condition 0, statement 0
— Otherwise, if condition 1, statement 1
— .
— Otherwise (informative remark on remaining condition), statement n
Each "If . Otherwise, if . Otherwise, ." statement in the text is introduced with ". as follows" or ".
the following applies" immediately followed by "If . ". The last condition of the "If . Otherwise, if .
Otherwise, ." is always an "Otherwise, .". Interleaved "If . Otherwise, if . Otherwise, ." statements
can be identified by matching ". as follows" or ". the following applies" with the ending "Otherwise, .".
In the text, a statement of logical operations as would be described mathematically in the following
form:
if( condition 0a && condition 0b )
statement 0
else if( condition 1a || condition 1b )
statement 1
...
else
statement n
may be described in the following manner:
... as follows / ... the following applies:
— If all of the following conditions are true, statement 0:
— condition 0a
— condition 0b
— Otherwise, if one or more of the following conditions are true, statement 1:
— condition 1a
— condition 1b
— .
— Otherwise, statement n
In the text, a statement of logical operations as would be described mathematically in the following
form:
if( condition 0 )
statement 0
if( condition 1 )
statement 1
© ISO/IEC 2023 – All rights reserved
may be described in the following manner:
When condition 0, statement 0
When condition 1, statement 1
In addition, a “continue” statement, which is used within loops, is defined as follows:
The “continue” statement, when encountered inside a loop, jumps to the beginning of the loop for the
next iteration. This results in skipping the execution of subsequent statements inside the body of the
loop for the current iteration. For example:
for( j =0; j < N; j++ ) {
statement 0
if( condition 1 )
continue
statement 1
statement 2
}
is equivalent to the following:
for( j =0; j < N; j++ ) {
statement 0
if( !condition 1 ) {
statement 1
statement 2
}
}
4 Overview and architecture
4.1 Overview
This document enables inclusion of timed media in a scene description. This is achieved through first
defining features of a scene description that describe how to get the timed media, and second how a
rendering process expects the data once it is decoded. In this version of the document, the features are
defined as extensions to the glTF format defined in ISO/IEC 12113, see Clause 5.
In addition to the extensions, which provide an integration of timed media with the scene description,
the document describes a reference scene description architecture that includes components such as
Media Access Function, Presentation Engine, Buffer Control & Management, and Pipelines. To enable
cross-platform/cross-vendor interoperability, the document defines Media Access Function (MAF) API
and Buffer API, see Clause 6. The MAF API provides an interface between the Media Access Function
and the Presentation Engine. The Buffer API is used to allocate and control buffers for the exchange of
data between Media Access Function and Presentation Engine.
Not only the timed media described by the scene description may change over the time but also the
scene description itself. The document defines how such change of a scene description document is
signalled to the Presentation Engine.
Finally, a scene description may be stored, delivered, or extended in a way that is consistent with
MPEG formats. The document defines a number of new features that allow a carriage utilizing
ISO/IEC 14496-12 and its derived specifications, see Clause 7.
4.2 Architecture
The scene description is consumed by a Presentation Engine to render a 3D scene to the viewer. The
extensions defined in this document, allow for the creation of immersive experiences using timed
media. The scene description extensions are designed with the goal of decoupling the Presentation
Engine from the Media Access Function. Presentation Engine and Media Access Function communicate
through the Media Access Function API, which allows the Presentation Engine to request timed media
© ISO/IEC 2023 – All rights reserved
required for the rendering of the scene. The Media Access Function will retrieve the requested timed
media and make it available in a timely manner and in a format that can be immediately processed by
the Presentation Engine. For instance, a requested timed media asset may be compressed and residing
in the network, so the Media Access Function will retrieve and decode the asset and pass the resulting
decoded media data to the Presentation Engine for rendering. The decoded media data is passed in
form of buffers from the Media Access Function to the Presentation Engine. The requests for timed
media are passed through the Media Access Function API from the Presentation Engine to the Media
Access Function.
Figure 1 depicts the reference architecture.
Figure 1 — Scene description reference architecture
The interfaces (MAF API, Buffer API) and extensions to ISO/IEC 12113 are within the scope of this
document.
The following principles apply:
— The format of the buffers shall be provided by the scene description document and shall be passed
to the MAF through the Media Access Function API
— Pipeline shall perform necessary transformations to match the buffer format and layout declared in
the scene description for that buffer
— The fetching of scene description document and scene description updates may be triggered by the
MAF.
Figure 1 depicts the reference architecture for scene description. The corresponding procedures are
described as follows:
a) The Presentation Engine receives and parses the scene description document and following scene
description updates
b) The Presentation Engine identifies timed media that needs to be presented and identifies the
required presentation time
© ISO/IEC 2023 – All rights reserved
c) The Presentation Engine then uses the MAF API to request the media and provides the following
information:
1) where the MAF can find the requested media
2) what parts of the media and at what level of detail
3) when the requested media has to be made available
4) in which format it wants the data and how it is passed to the Presentation Engine
d) The MAF instantiates the media fetching and decoding pipeline for the requested media at the
appropriate time.
1) It ensures that the requested media is available at the appropriate time in the appropriate
buffers for access by the Presentation Engine
2) It ensures that the media is decoded and reformatted to match the expected format by the
Presentation Engine as described by the scene description document
The exchange of data (media and metadata) shall be done through buffers (circular and static buffers).
The buffer management shall be controlled through the Buffer API. Each buffer should contain sufficient
header information to describe its content and timing.
The information provided to the Media Access Function by the Presentation Engine allows it to
— Select the appropriate source for the media (multiple could be specified) and the MAF may select
based on preferences and capabilities. Capabilities may for example be decoding capabilities or
supported formats. Preferences may for example be user settings.
— For each selected source,
i) access the media by using a media access protocol;
ii) setup the media pipeline to provide the information in the correct buffer format.
The MAF may obtain additional information from the Presentation Engine in order to optimize the
delivery, for example the required quality for each of the buffers, the exact timing information, etc.
The Media Access Function shall setup and manage the pipeline for each requested media or metadata.
A pipeline takes as input one or more media or metadata tracks and outputs one or more buffers. The
pipeline shall perform all the necessary processing, such as streaming, demultiplexing, decoding,
decryption, and format conversion to match the expected buffer format. The final buffer or set of
buffers are then used to exchange data with the Presentation Engine.
An example of pipelines setup is depicted in Figure 2 for the case of a V-PCC compressed point cloud
object that is referenced in the scene description. Pipeline #1 creates four video decoders and one
patch data decoder. The pipeline is also responsible for processing this data and performing 3D
reconstruction based on the received information. The reconstructed data is then fed to the final buffer
that is accessed by the Presentation Engine. Pipeline #2 on the other hand is not performing the 3D
reconstruction process and provides decoded raw data onto the buffers, which are accessed by the
Presentation Engine.
© ISO/IEC 2023 – All rights reserved
Figure 2 — An example of pipelines in scene description
© ISO/IEC 2023 – All rights reserved
4.3 Timing model
A scene and all contained nodes share a global common presentation timeline. An initial glTF document
is used as an entry point for consuming a 3D scene. The scene activation time of that document may be
set externally or may be determined by the user and is considered the presentation time T0 of the 3D
scene. Each media is started at time T0 + T , where T is equal to startTime, when present, or equal
init init
to the earliest time at which the media is available if autoplay is equal to true and autoplayGroup is not
present, or the earliest time at which all media with the same autoplayGroup are available.
The first sample of each media consumed at T0 + T is the one with presentation time equal to
init
startTimeOffset. The media is consumed up to the sample with presentation time equal to the
endTimeOffset, when present, or up to the last sample present in the media. When loop is set to true, at
each loop the timeline is increased by adding the endTimeOffset – startTimeOffset, when endTimeOffset
is present, or duration – startTimeOffset, when endTimeOffset is not present.
When a scene is updated through patch document to a scene description document or a scene description
document, the media timeline remains unchanged and continues to be evaluated in respect to the T0,
i.e. the activation time of the initial glTF document used as an entry point for consuming the 3D scene.
Animations described by the scene description document may be controlled (e.g., activated, paused,
stopped) through MPEG_animation_timing extension. The activation timing of control events is
identified by the timing of a sample in a metadata track. Once an animation event is activated the
timeline of the animation is determined by animation data in the scene description data and the
information provided by the animation sample in the metadata track (e.g., speed, start_frame, end_
frame).
All static media of a scene are assumed to be presented at time T0. Timed media shall start at the
indicated T0 + T . An object that has timed media components, shall not be rendered until the
init
indicated T0 + T of these components. T0 + T of all the timed media components of the same object
init init
shall be equal.
Any extensions that include new primitive attributes shall register the attributes in Annex B.
5 Scene description extensions
5.1 General
5.1.1 Overview of extensions
An extension mechanism that allows to extend glTF 2.0 with new capabilities is defined in ISO/IEC 12113.
A glTF node may have an optional extensions property that lists the extensions that are used by this
node. All extensions that are used in a glTF document shall be listed in the top-level extensionsUsed
array object, while extensions that are required to correctly load/render the scene shall also be listed
in the extensionsRequired array.
A number of extensions, listed in Table 2, that enable support for timed media, are specified in
subclauses 5.2, 5.3, 5.4, and 5.5. Extensions can be defined under Vendor, EXT, KHR, or KHX namespaces.
The extensions defined in this document are under the vendor-specific extension namespaces with an
MPEG prefix. Examples of how to use the extensions are provided in Annex F.
Table 2 — ISO/IEC 12113 extensions defined in this document
Extension Name Brief Description Type Subclause
MPEG_media Extension for referencing external media Generic
5.2.1
sources.
MPEG_accessor_timed An accessor extension to support timed Generic 5.2.2
media.
© ISO/IEC 2023 – All rights reserved
TTaabbllee 22 ((ccoonnttiinnueuedd))
Extension Name Brief Description Type Subclause
MPEG_buffer_circular A buffer extension to support circular buff- Generic 5.2.3
ers.
MPEG_scene_dynamic An extension to support dynamic scenes. Generic 5.2.4
MPEG_texture_video A texture extension to support video tex- Visual 5.3.1
tures.
MPEG_mesh_linking An extension to link two meshes and provide Visual 5.3.2
mapping information
MPEG_audio_spatial Adds support for spatial audio. Audio 5.4.1
MPEG_viewport_recommended An extension to describe a recommended Metadata 5.5.1
viewport.
MPEG_animation_timing An extension to control animation timelines. Metadata 5.5.2
Figure 3 depicts the glTF 2.0 hierarchy that includes the extensions defined in this document.
Figure 3 — An overview of the glTF document structure with MPEG extensions defined in this
document
5.1.2 Formatting and typing
For binary data fields the following applies. The read_bits( n ) function reads the next n bits from the
buffer data and advances the data pointer by n bit positions. When n is equal to 0, read_bits( n ) is
specified to return a value equal to 0 and to not advance the data pointer.
© ISO/IEC 2023 – All rights reserved
The following types specify the types and parsing process for binary data fields:
— bits(n) fixed-pattern bit string using n bits written (from left to right) with the left bit first. The
parsing process for this descriptor is specified by the return value of the function read_bits( n )
— bits(n)[m] array of m fixed-pattern bit strings with length of n bits.
— uint(n) unsigned integer using n bits. The parsing process for this descriptor is specified by the
return value of the function read_bits( n ) interpreted as a binary representation of an unsigned
integer with the most significant bit written first.
— int(n) signed integer using n bits. The parsing process for this descriptor is specified by the return
value of the function read_bits( n ) interpreted as a two's complement integer representation with
the most significant (left) bit written first. In particular, the parsing process for this type is specified
as follows:
int(n) {
value = read_bits( n )
if( value < ( 1 << ( n – 1 ) ) )
return value
else
return ( value | ~( ( 1 << (n – 1)) – 1 ) )
}
— float(n) binary floating point value using n bits. The parsing process for this descriptor is as specified
in IEEE 754-2019.
For JSON data fields, the following type definitions apply:
— number: primitive type defined in ISO/IEC 21778
— string: primitive type defined in ISO/IEC 21778
— boolean: primitive type defined in ISO/IEC 21778
— array: structured type defined in ISO/IEC 21778
— object: structured type defined in ISO/IEC 21778
5.2 Generic extensions
5.2.1 MPEG_media extension
5.2.1.1 General
The MPEG media extension, identified by MPEG_media, provides an array of media items referenced in
a scene description document.
When present, the MPEG_media extension shall be included as a top-level extension.
5.2.1.2 Semantics
The definition of all objects within MPEG_media extension is provided in Tables 3 to 6.
Table 3 — Definitions of top-level objects of MPEG_media extension
Name Type Default Usage Description
media array N/A M An array of items that describe the external media,
referenced in this scene description document.
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