Information technology — JPEG XL image coding system — Part 2: File format

This document specifies the transport and container formats for JPEG XL codestreams as specified in ISO/IEC 18181-1. This document specifies how to add metadata and extensions to JPEG XL codestreams. A file as described by this document is called a JPEG XL file.

Technologies de l'information - Systѐme de codage d'images JPEG XL — Partie 2: Format de fichiers

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Published
Publication Date
12-Oct-2021
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9092 - International Standard to be revised
Completion Date
16-Jun-2022
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INTERNATIONAL ISO/IEC
STANDARD 18181-2
First edition
2021-10
Information technology — JPEG XL
image coding system —
Part 2:
File format
Technologies de l'information - Systѐme de codage d'images JPEG
XL —
Partie 2: Format de fichiers
Reference number
ISO/IEC 18181-2:2021(E)
© ISO/IEC 2021
---------------------- Page: 1 ----------------------
ISO/IEC 18181-2:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on

the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below

or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
© ISO/IEC 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 18181-2:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ..................................................................................................................................................................................... 1

3 Terms and definitions .................................................................................................................................................................................... 1

4 General ........................................................................................................................................................................................................................... 2

5 File organization ..................................................................................................................................................................................................2

6 Data types and numerical values ....................................................................................................................................................... 4

7 Graphical descriptions .................................................................................................................................................................................. 4

8 Binary format of a box ........................................................................................................................................... .........................................5

9 Box types ...................................................................................................................................................................................................................... 6

9.1 JPEG XL signature box ...................................................................................................................................................................... 6

9.2 File type box ...................................................................... ........................................................................................................................ 6

9.3 Level box ...................................................................................................................................................................................................... 6

9.4 JUMBF box ................................................................................................................................................................................................... 6

9.5 Exif box .......................................................................................................................................................................................................... 6

9.6 XML box ........................................................................................................................................................................................................ 7

9.7 Brotli-compressed box .................................................................................................................................................................... 7

9.8 Frame index box .................................................................................................................................................................................... 7

9.9 JPEG XL codestream box ................................................................................................................................................................ 9

9.10 JPEG XL partial codestream box ............................................................................................................................................. 9

9.11 JPEG bitstream reconstruction data box......................................................................................................................... 9

9.11.1 Syntax ...................................................................... ..................................................................................................................... 9

9.11.2 Reconstruction procedure......................................................................................................................................12

Bibliography .............................................................................................................................................................................................................................16

iii
© ISO/IEC 2021 – All rights reserved
---------------------- Page: 3 ----------------------
ISO/IEC 18181-2:2021(E)
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).

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

Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents) or the IEC

list of patent declarations received (see https://patents.iec.ch).

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/IEC 18181 series can be found on the ISO website.

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.
© ISO/IEC 2021 – All rights reserved
---------------------- Page: 4 ----------------------
INTERNATIONAL STANDARD ISO/IEC 18181-2:2021(E)
Information technology — JPEG XL image coding system —
Part 2:
File format
1 Scope

This document specifies the transport and container formats for JPEG XL codestreams as specified in

ISO/IEC 18181-1. This document specifies how to add metadata and extensions to JPEG XL codestreams.

A file as described by this document is called a JPEG XL file.
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 18181-1:— , Information technology — JPEG XL image coding system — Part 1: Core coding

system

ISO/IEC 19566-5, Information technologies — JPEG systems — Part 5: JPEG universal metadata box format

(JUMBF)

INTERNET ENGINEERING TASK FORCE (IETF) RFC 7932, Brotli Compressed Data Format [online]. July

2016 [viewed 2021-04-19]. Available at https:// tools .ietf .org/ html/ rfc7932
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO/IEC 18181-1, ISO/IEC 19566-5

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
box
structured collection of data describing the image or the image decoding process
3.2
box content
data wrapped within the box structure
3.3
box type
kind of information stored within the box
3.4
file format

set of data structures for the storage of metadata and extensions of a codestream

1) Under preparation. Stage at the time of publication: ISO/IEC FDIS 18181-1:2021.

© ISO/IEC 2021 – All rights reserved
---------------------- Page: 5 ----------------------
ISO/IEC 18181-2:2021(E)
3.5
JPEG XL file
data file encoded in the file format defined by this document
3.6
superbox
box that carries other boxes as payload data
4 General
This document defines the file format of a JPEG XL file.

A JPEG XL file shall contain a codestream as specified in ISO/IEC 18181-1, and may contain additional

metadata and extensions.
A JPEG XL file shall come in one of the following forms.
— A box structure, as defined in Clause 5.
— A direct JPEG XL codestream without box structure.

The rest of this document only defines the box structure, the codestream without box structure is valid

but is completely specified in ISO/IEC 18181-1.

A decoder shall require the file format to follow either the structure of a codestream without box

structure, or follow the box structure as defined in Clause 5 and follow all box requirements in

Clauses 6 to 9. A decoder can extract the codestream from the box structure and decode the image from

the codestream using the procedure specified in ISO/IEC 18181-1, and can decode the contents of other

boxes following their respective specifications in this document.

NOTE A direct JPEG XL codestream without box structure is also a valid JPEG XL file. This allows, for

example, a more efficient encoding of images for the web, in cases where information encoded in other boxes

than the codestream is not required.
5 File organization

A JPEG XL file using the box structure is formed as a series of boxes. These boxes contain all data within

the file, including the initial signature required by the file format.

NOTE This box-based file format is based on the same syntax as described in ISO/IEC 15444-1:2019, Annex I

or ISO/IEC 15444-2:2004, Annex M, or ISO/IEC 21122-3. The binary format of a box is also described in Clause 8.

Boxes of different types contain different types of data, such as the file signature, metadata and the

codestream. Clause 9 defines box types that may appear in a JPEG XL file and their requirements. Boxes

with an unrecognized type shall be ignored and skipped by the decoder.

A JPEG XL file shall contain a JPEG XL codestream. The codestream can be split across multiple boxes:

JPEG XL partial codestream boxes. In this case, the codestream is formed by the concatenation of the

content of all those boxes.

Any boxes, content and codestreams present in a superbox, such as another JPEG XL file in a JUMBF

superbox, shall not be taken into account for the syntactic requirements of this document; they

recursively follow their applicable specification.

Figures 1 and 2 each show a conceptual box structure of a JPEG XL file, that is a possible series of

different box types that form the file, respectively with a single full codestream box and with multiple

partial codestream boxes. Boxes that may appear multiple times are indicated with '...' and optional

boxes are indicated in a dashed rectangle. These figures are only an indication and do not imply any

ordering or counting requirements for the boxes. The decoder shall not make any assumptions about

the ordering of any boxes after the first two, except where indicated.
© ISO/IEC 2021 – All rights reserved
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ISO/IEC 18181-2:2021(E)
Figure 1 — Conceptual structure of a JPEG XL file using a full codestream box
© ISO/IEC 2021 – All rights reserved
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ISO/IEC 18181-2:2021(E)
Figure 2 — Conceptual structure of a JPEG XL file using partial codestream boxes
6 Data types and numerical values
Data types used in this document shall be interpreted by the decoder as follows.
— u32: a 32-bit unsigned integer encoded in big endian order (4 bytes).
— u64: a 64-bit unsigned integer encoded in big endian order (8 bytes).

— Varint(): an unsigned integer value of up to 63 bits as a variable length integer in little endian order

as specified in ISO/IEC 18181-1:—, 9.2.1.5.

Numerical values for bytes are given as hexadecimal values, each individually prefixed by 0x.

Hexadecimal byte values are given in the order as they appear in the file. In some cases, these bytes

spell out text in ASCII, this is informatively indicated after the hexadecimal values.

7 Graphical descriptions

Box definitions contain graphical description figures to illustrate the structure of the box. These figures

should be interpreted as follows.

— A sequence of rectangles is used to indicate the fields of the box and their order.

— Optional fields have a dashed border.
© ISO/IEC 2021 – All rights reserved
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ISO/IEC 18181-2:2021(E)

— A grey background indicates a variable length field. Exact data types or sizes are indicated by name

either in the rectangle after the name of the field, or in a description of the fields outside of the

figure.

— Larger outer rectangles may show fields that are grouped in a larger named structure.

Figure 3 shows an example of a box with 3 fields.
— A: a name given to a group of the three fields contained within.
— B: required field with a fixed length data type: the type u32.

— C: optional field with a fixed length data type (e.g. u32, u64 or a fixed amount of bytes).

— D: required field with a variable length data type (such as Varint(), or remaining amount of bytes).

Figure 3 — Example of a graphical description of a box definition
8 Binary format of a box

Each box shall have the structure indicated in Figure 4. This structure consists of a header indicating

size and box type, and box content.
NOTE 1 This format is also specified in ISO/IEC 15444-1 and ISO/IEC 15444-2.
Figure 4 — Binary format of a box
The fields given in Figure 4 are the following.

— LBox: has type u32. Gives the size of the box in bytes, including the box header fields. If the value is

1, then XLBox is used instead to indicate the size of the box. If the value is 0, then this box is the last

box of the file, and its data extends to the end of the file. If the value is not 0 or 1, it shall be at least

— TBox: has 4 bytes (e.g. a FourCC code): box type, specifies the type of information found in the box

content, e.g. whether it is a JPEG XL signature box, a file type box, and so on.

— XLBox: has type u64. Only present if LBox == 1. If present this field, instead of the LBox field, indicates

the size of the box in bytes. Its value shall be at least 16.

— DBox: has the remaining bytes. The box content (data). The content is formed by all the remaining

bytes of the box. The size of the content in bytes is the box size minus the size of the box header

fields. The format and meaning of this content are indicated by the box type, and Clause 9 defines

the format of the contents that may appear in a JPEG XL file.

NOTE 2 The box size is a multiple of bytes. This includes the JPEG XL codestream box. The JPEG XL codestream

is zero-padded at the end to align to a byte.
© ISO/IEC 2021 – All rights reserved
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ISO/IEC 18181-2:2021(E)
9 Box types
9.1 JPEG XL signature box

The JPEG XL signature box shall contain exactly the following 12 bytes, given as hexadecimal numbers:

— 0x0 0x0 0x0 0xC
— 0x4A 0x58 0x4C 0x20 (the box type "JXL " in ASCII)
— 0xD 0xA 0x87 0xA

A JPEG XL file shall contain exactly one signature box. The signature box shall be the first box.

9.2 File type box
The file type box shall contain exactly the following 20 bytes:
— 0x0 0x0 0x0 0x14
— 0x66 0x74 0x79 0x70 (the box type "ftyp" in ASCII)
— 0x6A 0x78 0x6C 0x20 ("jxl " in ASCII)
— 0x0 0x0 0x0 0x0
— 0x6A 0x78 0x6C 0x20 ("jxl " in ASCII)

A JPEG XL file shall contain exactly one file type box. The file type box shall be the second box. The

profile of the codestream box contained in this file is the main profile.
9.3 Level box

The type of this box shall be given by the 4 bytes 0x6A 0x78 0x6C 0x6C ("jxll" in ASCII).

Figure 5 shows the contents of a level box, excluding the box header.
Figure 5 — Content of a level box

A JPEG XL file shall contain at most one level box. Encoders are encouraged to write it immediately after

the file type box.

If there is no level box, the level is assumed to be 5. This level applies to the content of the JPEG XL

codestream box, as described in ISO/IEC 18181-1:—, Annex N .
9.4 JUMBF box

The type of this box shall be given by the 4 bytes 0x6A 0x75 0x6D 0x62 ("jumb" in ASCII). This box shall

follow the specification defined by ISO/IEC 19566-5.

A JUMBF box is a superbox that shall contain exactly one JUMBF Description box followed by one or

more Content Boxes.
9.5 Exif box

The type of this box shall be given by the 4 bytes 0x45 0x78 0x69 0x66 ("Exif" in ASCII).

© ISO/IEC 2021 – All rights reserved
---------------------- Page: 10 ----------------------
ISO/IEC 18181-2:2021(E)
Figure 6 shows the contents of an Exif box, excluding the box header.
Figure 6 — Content of an Exif box
[6] [7]

The Exif payload is as described in JEITA CP-3451E or CP-3461B . The tiff header offset denotes, as

specified in JEITA CP-3451B, the number of bytes, counting from the first byte of Exif_payload to the

first byte of the TIFF header of the Exif metadata. The value is zero if the payload starts immediately

with the TIFF header.

NOTE 1 The content of this box is exactly ExifDataBlock as defined in ISO/IEC 23008-12:2017, A.2.

For any Exif fields that have equivalents within the codestream, a decoder shall consider the codestream

to take precedence. Encoders are encouraged to ensure the Exif and codestream fields are identical.

NOTE 2 Examples of such fields include orientation and pixel dimensions.
9.6 XML box

The type of this box shall be given by the 4 bytes 0x78 0x6D 0x6C 0x20 ("xml " in ASCII).

[8]
The XML box contains a well-formed XML document as defined by XML 1.0 .
Figure 7 shows the content of an XML box.
Figure 7 — Content of an
...

FINAL
INTERNATIONAL ISO/IEC
DRAFT
STANDARD FDIS
18181-2
ISO/IEC JTC 1/SC 29
Information technology — JPEG XL
Secretariat: JISC
image coding system —
Voting begins on:
2021-07-01
Part 2:
Voting terminates on:
File format
2021-08-26
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/IEC FDIS 18181-2:2021(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN-
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO/IEC 2021
---------------------- Page: 1 ----------------------
ISO/IEC FDIS 18181-2:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO/IEC 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC FDIS 18181-2:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 General ............................................................................................................................................................................................................................ 2

5 File organization ................................................................................................................................................................................................... 2

6 Data types and numerical values ........................................................................................................................................................ 4

7 Graphical descriptions ................................................................................................................................................................................... 4

8 Binary format of a box .................................................................................................................................................................................... 5

9 Box types ....................................................................................................................................................................................................................... 6

9.1 JPEG XL signature box ....................................................................................................................................................................... 6

9.2 File type box .............................................................................................................................................................................................. 6

9.3 Level box ....................................................................................................................................................................................................... 6

9.4 JUMBF box ................................................................................................................................................................................................... 6

9.5 Exif box .......................................................................................................................................................................................................... 6

9.6 XML box ......................................................................................................................................................................................................... 7

9.7 Brotli-compressed box ..................................................................................................................................................................... 7

9.8 Frame index box ..................................................................................................................................................................................... 7

9.9 JPEG XL codestream box ................................................................................................................................................................. 9

9.10 JPEG XL partial codestream box ............................................................................................................................................... 9

9.11 JPEG bitstream reconstruction data box ........................................................................................................................... 9

9.11.1 Syntax ........................................................................................................................................................................................ 9

9.11.2 Reconstruction procedure ....................................................................................................................................12

Bibliography .............................................................................................................................................................................................................................16

© ISO/IEC 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/IEC FDIS 18181-2:2021(E)
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).

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

Introduction and/or on the ISO list of patent declarations received (see www .iso .org/ patents) or the IEC

list of patent declarations received (see patents.iec.ch).

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/IEC 18181 series can be found on the ISO website.

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.
iv © ISO/IEC 2021 – All rights reserved
---------------------- Page: 4 ----------------------
FINAL DRAFT INTERNATIONAL STANDARD ISO/IEC FDIS 18181-2:2021(E)
Information technology — JPEG XL image coding system —
Part 2:
File format
1 Scope

This document specifies the transport and container formats for JPEG XL codestreams as specified in

ISO/IEC 18181-1. This document specifies how to add metadata and extensions to JPEG XL codestreams.

A file as described by this document is called a JPEG XL file.
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 18181-1:— , Information technology — JPEG XL image coding system — Part 1: Core coding

system

ISO/IEC 19566-5, Information technologies — JPEG systems — Part 5: JPEG universal metadata box format

(JUMBF)

INTERNET ENGINEERING TASK FORCE (IETF) RFC 7932, Brotli Compressed Data Format [online]. July

2016 [viewed 2021-04-19]. Available at https:// tools .ietf .org/ html/ rfc7932
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO/IEC 18181-1, ISO/IEC 19566-5

and the following 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
box
structured collection of data describing the image or the image decoding process
3.2
box content
data wrapped within the box structure
3.3
box type
kind of information stored within the box
3.4
file format

set of data structures for the storage of metadata and extensions of a codestream

1) Under preparation. Stage at the time of publication: ISO/IEC DIS 18181-1:2021.

© ISO/IEC 2021 – All rights reserved 1
---------------------- Page: 5 ----------------------
ISO/IEC FDIS 18181-2:2021(E)
3.5
JPEG XL file
data file encoded in the file format defined by this document
3.6
superbox
box that carries other boxes as payload data
4 General
This document defines the file format of a JPEG XL file.

A JPEG XL file shall contain a codestream as specified in ISO/IEC 18181-1, and may contain additional

metadata and extensions.
A JPEG XL file shall come in one of the following forms.
— A box structure, as defined in Clause 5.
— A direct JPEG XL codestream without box structure.

The rest of this document only defines the box structure, the codestream without box structure is valid

but is completely specified in ISO/IEC 18181-1.

A decoder shall require the file format to follow either the structure of a codestream without box

structure, or follow the box structure as defined in Clause 5 and follow all box requirements in

Clauses 6 to 9. A decoder can extract the codestream from the box structure and decode the image from

the codestream using the procedure specified in ISO/IEC 18181-1, and can decode the contents of other

boxes following their respective specifications in this document.

NOTE A direct JPEG XL codestream without box structure is also a valid JPEG XL file. This allows, for

example, a more efficient encoding of images for the web, in cases where information encoded in other boxes

than the codestream is not required.
5 File organization

A JPEG XL file using the box structure is formed as a series of boxes. These boxes contain all data within

the file, including the initial signature required by the file format.

NOTE This box-based file format is based on the same syntax as described in ISO/IEC 15444-1:2019, Annex I

or ISO/IEC 15444-2:2004, Annex M, or ISO/IEC 21122-3. The binary format of a box is also described in Clause 8.

Boxes of different types contain different types of data, such as the file signature, metadata and the

codestream. Clause 9 defines box types that may appear in a JPEG XL file and their requirements. Boxes

with an unrecognized type shall be ignored and skipped by the decoder.

A JPEG XL file shall contain a JPEG XL codestream. The codestream can be split across multiple boxes:

JPEG XL partial codestream boxes. In this case, the codestream is formed by the concatenation of the

content of all those boxes.

Any boxes, content and codestreams present in a superbox, such as another JPEG XL file in a JUMBF

superbox, shall not be taken into account for the syntactic requirements of this document; they

recursively follow their applicable specification.

Figures 1 and 2 each show a conceptual box structure of a JPEG XL file, that is a possible series of

different box types that form the file, respectively with a single full codestream box and with multiple

partial codestream boxes. Boxes that may appear multiple times are indicated with '...' and optional

boxes are indicated in a dashed rectangle. These figures are only an indication and do not imply any

ordering or counting requirements for the boxes. The decoder shall not make any assumptions about

the ordering of any boxes after the first two, except where indicated.
2 © ISO/IEC 2021 – All rights reserved
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ISO/IEC FDIS 18181-2:2021(E)
Figure 1 — Conceptual structure of a JPEG XL file using a full codestream box
© ISO/IEC 2021 – All rights reserved 3
---------------------- Page: 7 ----------------------
ISO/IEC FDIS 18181-2:2021(E)
Figure 2 — Conceptual structure of a JPEG XL file using partial codestream boxes
6 Data types and numerical values
Data types used in this document shall be interpreted by the decoder as follows.
— u32: a 32-bit unsigned integer encoded in big endian order (4 bytes).
— u64: a 64-bit unsigned integer encoded in big endian order (8 bytes).

— Varint(): an unsigned integer value of up to 63 bits as a variable length integer in little endian order

as specified in ISO/IEC 18181-1:—, 9.2.1.5.

Numerical values for bytes are given as hexadecimal values, each individually prefixed by 0x.

Hexadecimal byte values are given in the order as they appear in the file. In some cases, these bytes

spell out text in ASCII, this is informatively indicated after the hexadecimal values.

7 Graphical descriptions

Box definitions contain graphical description figures to illustrate the structure of the box. These figures

should be interpreted as follows.

— A sequence of rectangles is used to indicate the fields of the box and their order.

— Optional fields have a dashed border.
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ISO/IEC FDIS 18181-2:2021(E)

— A grey background indicates a variable length field. Exact data types or sizes are indicated by name

either in the rectangle after the name of the field, or in a description of the fields outside of the

figure.

— Larger outer rectangles may show fields that are grouped in a larger named structure.

Figure 3 shows an example of a box with 3 fields.
— A: a name given to a group of the three fields contained within.
— B: required field with a fixed length data type: the type u32

— C: optional field with a fixed length data type (e.g. u32, u64 or a fixed amount of bytes)

— D: required field with a variable length data type (such as Varint(), or remaining amount of bytes)

Figure 3 — Example of a graphical description of a box definition
8 Binary format of a box

Each box shall have the structure indicated in Figure 4. This structure consists of a header indicating

size and box type, and box content.
NOTE 1 This format is also specified in ISO/IEC 15444-1 and ISO/IEC 15444-2.
Figure 4 — Binary format of a box
The fields given in Figure 4 are the following.

— LBox: has type u32. Gives the size of the box in bytes, including the box header fields. If the value is

1, then XLBox is used instead to indicate the size of the box. If the value is 0, then this box is the last

box of the file, and its data extends to the end of the file. If the value is not 0 or 1, it shall be at least

— TBox: has 4 bytes (e.g. a FourCC code): box type, specifies the type of information found in the box

content, e.g. whether it is a JPEG XL signature box, a file type box, and so on.

— XLBox: has type u64. Only present if LBox == 1. If present this field, instead of the LBox field, indicates

the size of the box in bytes. Its value shall be at least 16.

— DBox: has the remaining bytes. The box content (data). The content is formed by all the remaining

bytes of the box. The size of the content in bytes is the box size minus the size of the box header

fields. The format and meaning of this content are indicated by the box type, and Clause 9 defines

the format of the contents that may appear in a JPEG XL file.

NOTE 2 The box size is a multiple of bytes. This includes the JPEG XL codestream box. The JPEG XL codestream

is zero-padded at the end to align to a byte.
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ISO/IEC FDIS 18181-2:2021(E)
9 Box types
9.1 JPEG XL signature box

The JPEG XL signature box shall contain exactly the following 12 bytes, given as hexadecimal numbers:

— 0x0 0x0 0x0 0xC
— 0x4A 0x58 0x4C 0x20 (the box type "JXL " in ASCII)
— 0xD 0xA 0x87 0xA

A JPEG XL file shall contain exactly one signature box. The signature box shall be the first box.

9.2 File type box
The file type box shall contain exactly the following 20 bytes:
— 0x0 0x0 0x0 0x14
— 0x66 0x74 0x79 0x70 (the box type "ftyp" in ASCII)
— 0x6A 0x78 0x6C 0x20 ("jxl " in ASCII)
— 0x0 0x0 0x0 0x0
— 0x6A 0x78 0x6C 0x20 ("jxl " in ASCII)

A JPEG XL file shall contain exactly one file type box. The file type box shall be the second box. The

profile of the codestream box contained in this file is the Main profile.
9.3 Level box

The type of this box shall be given by the 4 bytes 0x6A 0x78 0x6C 0x6C ("jxll" in ASCII).

Figure 5 shows the contents of a Level box, excluding the box header.
Figure 5 — Content of a level box

A JPEG XL file shall contain at most one Level box. Encoders are encouraged to write it immediately

after the file type box.

If there is no Level box, the level is assumed to be 5. This level applies to the content of the JPEG XL

codestream box, as described in ISO/IEC 18181-1:—, Annex N .
9.4 JUMBF box

The type of this box shall be given by the 4 bytes 0x6A 0x75 0x6D 0x62 ("jumb" in ASCII). This box shall

follow the specification defined by ISO/IEC 19566-5.

A JUMBF box is a superbox that shall contain exactly one JUMBF Description box followed by one or

more Content Boxes.
9.5 Exif box

The type of this box shall be given by the 4 bytes 0x45 0x78 0x69 0x66 ("Exif" in ASCII).

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ISO/IEC FDIS 18181-2:2021(E)
Figure 6 shows the contents of an Exif box, excluding the box header.
Figure 6 — Content of an Exif box
[6] [7]

The Exif payload is as described in JEITA CP-3451E or CP-3461B . The tiff header offset denotes, as

specified in JEITA CP-3451B, the number of bytes, counting from the first byte of Exif_payload to the

first byte of the TIFF header of the Exif metadata. The value is zero if the payload starts immediately

with the TIFF header.

NOTE 1 The content of this box is exactly ExifDataBlock as defined in ISO/IEC 23008-12:2017, A.2.

For any Exif fields that have equivalents within the codestream, a decoder shall consider the codestream

to take precedence. Encoders are encouraged to ensure the Exif and codestream fields are identical.

NOTE 2 Examples of such fields include orientation and pixe
...

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