Information technology — Digital compression and coding of continuous-tone still images — Part 7: Reference software

This document provides reference software for the coding technology specified in Recommendation ITU-T T.81 | ISO/IEC 10918-1. While the reference implementations also provide an encoder, conformance testing of their encoding process is beyond the scope of this document.

Technologies de l'information — Compression numérique et codage des images fixes à modelé continu — Partie 7: Logiciel de référence

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Status
Published
Publication Date
14-Sep-2021
Current Stage
9092 - International Standard to be revised
Completion Date
02-Sep-2022
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INTERNATIONAL ISO/IEC
STANDARD 10918-7
Second edition
2021-09
Information technology — Digital
compression and coding of
continuous-tone still images —
Part 7:
Reference software
Technologies de l'information — Compression numérique et codage
des images fixes à modelé continu —
Partie 7: Logiciel de référence
Reference number
ISO/IEC 10918-7:2021(E)
ISO/IEC 2021
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ISO/IEC 10918-7:2021(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
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ISO/IEC 10918-7:2021(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 (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 ITU-T as Rec. ITU-T T.873 (06/2021) and drafted in accordance with

its editorial rules, in collaboration with Joint Technical Committee ISO/IEC JTC 1, Information

technology, Subcommittee SC 29, Coding of audio, picture, multimedia and hypermedia information.

This second edition cancels and replaces the first edition (ISO/IEC 10918-7:2019), which has been

technically revised.
The main changes compared to the previous edition are as follows:

‒ the reference software implementations are updated to version 1.58 for reference software 1, and

to version 2.0.5 for reference software 2. These versions do not include any new features, they

correct implementation errors and improve the overall stability of the software.

A list of all parts in the ISO/IEC 10918 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.
© ISO/IEC 2021 – All rights reserved iii
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ISO/IEC 10918-7:2021 (E)
CONTENTS
Page

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

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

3 Definitions........................................................................................................................................................ 1

4 Abbreviations ................................................................................................................................................... 1

5 Conventions ..................................................................................................................................................... 2

6 Reference software ........................................................................................................................................... 2

6.1 Purpose .................................................................................................................................................. 2

6.2 Examples of use ..................................................................................................................................... 2

6.3 General .................................................................................................................................................. 2

Annex A – Unpacking and compiling reference software A ......................................................................................... 4

Annex B – Using reference software A ........................................................................................................................ 5

B.1 General .................................................................................................................................................. 5

B.2 Encoder options defining the quality of the image .................................................................................. 5

B.3 Options controlling the colour space ...................................................................................................... 5

B.4 Options controlling the scan generation and entropy coding ................................................................... 5

B.5 Options controlling the quantizer ........................................................................................................... 6

B.6 Options controlling the subsampling of components .............................................................................. 6

B.7 Miscellaneous options ........................................................................................................................... 6

B.8 Decoder options ..................................................................................................................................... 6

Annex C – Unpacking and compiling reference software B ......................................................................................... 7

Annex D – Using reference software B ........................................................................................................................ 9

D.1 General .................................................................................................................................................. 9

D.2 Encoder options defining the quality of the base and full image ............................................................. 9

D.3 Encoder options controlling the colour space ......................................................................................... 9

D.4 Encoder options controlling the scan generation and entropy coding ...................................................... 9

D.5 Encoder options controlling the DCT implementation .......................................................................... 10

D.6 Encoder options controlling the subsampling of components ............................................................... 10

D.7 Miscellaneous encoder options ............................................................................................................ 10

D.8 Decoder options controlling the choice of the inverse discrete cosine transform ................................... 11

D.9 Decoder options selecting the output file format .................................................................................. 11

D.10 Decoder options controlling the rendering of the output image............................................................. 11

D.11 Miscellaneous decoder options ............................................................................................................ 12

D.12 Decompressing to pgx ......................................................................................................................... 12

Bibliography ............................................................................................................................................................. 13

Electronic attachment with two reference implementations of Rec. ITU-T T.81 | ISO/IEC 10918-1.

© ISO/IEC 2021 – All rights reserved Rec. ITU-T T.873 (06/2021) iv
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ISO/IEC 10918-7:2021 (E)
Introduction

Rec. ITU-T T.81 | ISO/IEC 10918-1 specifies the codestream format and the decoding process, and is designed primarily

for compression of continuous-tone photographic content.

This Recommendation | International Standard provides reference software for Rec. ITU-T T.81 | ISO/IEC 10918-1. The

software has been successfully compiled and tested on Linux and Windows operating systems and conforms to the

decoder requirements set forth in Rec. ITU-T T.83 | ISO/IEC 10918-2. It has also been tested for conformance to

Rec. ITU-T T.86 | ISO/IEC 10918-4 and ISO/IEC 18477-4.

Instructions for unpacking and building the software are found in Annexes A and C. Instructions for its use are listed in

Annexes B and D.
v Rec. ITU-T T.873 (06/2021)
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ISO/IEC 10918-7:2021 (E)
INTERNATIONAL STANDARD
ITU-T RECOMMENDATION
Information technology – Digital compression and coding of continuous-tone
still images: Reference software
1 Scope

This Specification provides reference software for the coding technology specified in Recommendation ITU-T T.81 |

ISO/IEC 10918-1. While the reference implementations also provide an encoder, conformance testing of their encoding

process is beyond the scope of this Specification.
2 Normative references

The following Recommendations and International Standards contain provisions which, through reference in this text,

constitute provisions of this Recommendation | International Standard. For dated references, only the edition cited applies.

For undated references, the latest edition of the referenced document (including any amendments) applies. At the time of

publication, the editions indicated in dated references were valid. All Recommendations and Standards are subject to

revision, and parties to agreements based on this Recommendation | International Standard are encouraged to investigate

the possibility of applying the most recent edition of the Recommendations and Standards listed below. Members of IEC

and ISO maintain registers of currently valid International Standards. The Telecommunication Standardization Bureau of

the ITU maintains a list of currently valid ITU-T Recommendations.

– Recommendation ITU-T T.81 (latest) | ISO/IEC 10918-1 (latest), Information technology – Digital

compression and coding of continuous-tone still images: Requirements and guidelines.

3 Definitions

For the purposes of this Specification, the terms and definitions specified in Rec. ITU-T T.81 | ISO/IEC 10918-1 and the

following apply.
3.1 codestream; JPEG file: Sequence of bytes.

3.2 pgx format; portable graphics format: Image format describing integer-based continuous-tone images.

NOTE – For the purposes of this Specification, the image format is as specified in Rec. ITU-T T.803 | ISO/IEC 15444-4.

3.3 pnm format; portable any map format: Image format describing integer-based continuous-tone images of

either one or three components consisting of a header determining image dimensions and sample precision and

component-interleaved image samples encoded as 8-bit or 16-bit big-endian integers.

NOTE – For a specification of the pnm format, see Bourke (1997).

3.4 R′G′B′: Colour space that describes a colour by three gamma-corrected coordinates relative to three colour

primaries.

3.5 upsampling: Procedure that increases the spatial or temporal sampling rate of a time-discretely sampled signal.

3.6 Y′CBCR: Colour space that describes a colour by 1 luma coordinate and 2 chroma coordinates derived from a

gamma-corrected R′G′B′ colour space by a linear transformation.
4 Abbreviations

For the purposes of this Recommendation | International Standard, the following abbreviations apply:

DCT Discrete Cosine Transform
DNL Define Number of Lines
IDCT Inverse Discrete Cosine Transform
MCU Minimum Coded Unit
POSIX Portable Operating System Interface
© ISO/IEC 2021 – All rights reserved
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ISO/IEC 10918-7:2021 (E)
5 Conventions

Text in Times New Roman provides instructions, comments or details for the reader.

Text in Courier New indicates program input or output as necessary to either run the software or as generated by the

software on the console.
6 Reference software
6.1 Purpose
The purpose of this Specification is to provide:

– a reference decoder software capable of decoding codestreams that conform to Rec. ITU-T T.81 | ISO/IEC

10918-1;

– a sample encoder software capable of producing codestreams that conform to Rec. ITU-T T.81 | ISO/IEC

10918-1.

The use of the reference software is not required to implement an encoder or decoder in conformance to Rec. ITU-T T.81

| ISO/IEC 10918-1. Requirements established in Rec. ITU-T T.81 | ISO/IEC 10918-1 take precedence over the behaviour

of the reference software.
6.2 Examples of use
Some examples of use for the reference decoder software implementations are:

– as an illustration of how to perform the decoding processes specified in Rec. ITU-T T.81 |

ISO/IEC 10918-1;

– as the starting basis for the implementation of a decoder that conforms to Rec. ITU-T T.81 |

ISO/IEC 10918-1;

– for (non-exhaustive) testing of the conformance of a codestream (or file) to the constraints specified in

Rec. ITU-T T.81 | ISO/IEC 10918-1.

NOTE 1 – The lack of detection of any conformance violation by any reference software implementation cannot be considered as

a definitive proof that the codestream under test conforms to Rec. ITU-T T.81 | ISO/IEC 10918-1.

Some examples of use for reference encoder software are as:

– an illustration of how to implement an encoding process that produces codestreams that conform to Rec.

ITU-T T.81 | ISO/IEC 10918-1;

– a starting point for an implementation of an encoder that conforms to Rec. ITU-T T.81 | ISO/IEC 10918-1;

– a means of generating codestreams conforming to Rec. ITU-T T.81 | ISO/IEC 10918-1 for testing purposes;

– a means of demonstrating and evaluating examples of the quality that can be achieved by an encoding

process that conforms to Rec. ITU-T T.81 | ISO/IEC 10918-1.

NOTE 2 – No guarantee of the quality that will be achieved by an encoder is provided by its conformance to Rec. ITU-T T.81 |

ISO/IEC 10918-1, as the conformance is only defined in terms of specific constraints imposed on the syntax of the generated

codestream and maximum tolerable errors of the discrete cosine transform (DCT) coefficients after reconstruction. In particular,

while sample encoder software implementations could suffice to provide some illustrative examples of which quality can be

achieved within Rec. ITU-T T.81 | ISO/IEC 10918-1, they provide neither an assurance of minimum guaranteed image encoding

quality nor maximum achievable image encoding quality.

NOTE 3 – The computation resource characteristics in terms of program or data memory usage, execution speed, etc. of sample

software encoder or decoder implementations cannot be construed as representative of the typical, minimal or maximal

computational resource characteristics to be exhibited by implementations of some parts of Rec. ITU-T T.81 | ISO/IEC 10918-1.

6.3 General

The reference software implementations for Rec. ITU-T T.81 | ISO/IEC 10918-1 are available from ISO at

https://standards.iso.org/iso-iec/10918/-7/ed-2/en and also from ITU at https://www.itu.int/rec/T-REC-T.873/en. Each of

the two zip archives contains one reference software implementation.

– The file "referenceA.zip" contains a reference implementation for all processes of Rec. ITU-T T.81 |

ISO/IEC 10918-1. Unpacking and compilation of this software is explained in Annex A. Guidance on how

to use this software is given in Annex B.
2 Rec. ITU-T T.873 (06/2021)
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ISO/IEC 10918-7:2021 (E)

– The file "referenceB.zip" contains a reference implementation for the baseline and extended Huffman and

arithmetic coding DCT processes of Rec. ITU-T T.81 | ISO/IEC 10918-1. This software does not

implement the lossless and hierarchical processes of Rec. ITU-T T.81 | ISO/IEC 10918-1. Unpacking and

compilation of this software is explained in Annex C. Guidance on how to use this software is given in

Annex D.

NOTE – In ISO ReferenceA.zip and Reference B.zip are called DIS_10918att1.zip and DIS_10918att2.zip, respectively.

© ISO/IEC 2021 – All rights reserved Rec. ITU-T T.873 (06/2021) 3
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ISO/IEC 10918-7:2021 (E)
Annex A
Unpacking and compiling reference software A
(This annex does not form an integral part of this document.)

Source code of the reference software implementation is provided in a zip archive available at

https://standards.iso.org/iso-iec/10918/-7/ed-2/en, and https://www.itu.int/rec/T-REC-T.873/en. Unpacking a zip file is

operating system specific. Under portable operating system interface (POSIX) compliant operating systems, open a

command line window and enter
unzip referenceA.zip
This will unpack all components of the software into the current directory.
To compile the software, follow these steps:

– for POSIX compliant operating systems, go to the directory into which the electronic attachment was

unpacked, then enter on the command line
./configure
make

This assumes that a POSIX compliant shell is available, and the GNU compiler (make, gcc compiler and linker) are

installed on the system. The reference software will then be built in the current directory, and a binary named "jpeg"

will be created.

– For Microsoft Windows, the Visual Studio VS2010 or VS2013 compiler suite provides another option for

building the software. A VS2010 solution file allowing loading and compiling the project can be found in

the directory "vs10.0/jpeg". A solution file for VS2013 can be found in the directory "vs12.0/jpeg".

The compiler will generate a command line tool without any graphical interface that compresses images represented in

the pnm format into JPEG and expands JPEG images into pnm or pgx files. pnm-files represent one component integer

grey-scale or three-component integer colour data, whereas pgx files describe non-upsampled N-component raw sample

data bare any colour interpretation.

For the purpose of testing for conformance to Rec. ITU-T T.81 | ISO/IEC 10918-1, the pgx output is preferable as it

includes neither a transformation from Y′C C to R′G′B′ and nor an upsampling process. These processes are not formally

B R

part of Rec. ITU-T T.81 | ISO/IEC 10918-1, but have been included in later standards, such as Rec. ITU-T T.871 | ISO/IEC

10918-5, also known as JFIF, or ISO/IEC 18477-1.
____________________
In the ISO distribution ReferenceA.zip is called DIS_10918att1.zip.
4 Rec. ITU-T T.873 (06/2021)
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ISO/IEC 10918-7:2021 (E)
Annex B
Using reference software A

(This annex does not form an integral part of this Recommendation | International Standard.)

B.1 General

This annex describes the usage of the software prepared and compiled with the instructions given in Annex A.

The software can both compress images to JPEG files and expand such files back to pnm or pgx. The encoder and decoder

are both contained in the same executable, named "jpeg". If a quality parameter (see clause B.2) is included on the

command line, or the command line parameters request lossless coding, the executable encodes images. If only an input

and an output file are provided, the software attempts to expand the input file to the output file.

The encoder requires one input image encoded in the pnm format that is to be compressed. For lossy compression, the

-q parameter defines a quality parameter between 0 and 100. The -p parameter will enable lossless compression.

The command line parameters for the encoders are as follows:
jpeg [options] source target

where options is a set of command line options, all starting with a hyphen, source is the source image to be compressed,

and the target is the output file to which the encoder writes the output codestream. Command line switches control the

operations of the software. They are listed in clauses B.2 to B.8.

Decoding from a JPEG file to a ppm or pfm file requires only the input and output file name:

jpeg [options] source target

The only command line options recognized for decoding are –c and –U, which are described in subclause B.3.

B.2 Encoder options defining the quality of the image
The following options control the quality of the encoded images.
-q quality selects the encoding mode and defines the quality of the base image
B.3 Options controlling the colour space

The following options control how out of gamut colours and gamut extension is handled:

inserts a marker into the codestream that signals that the transformation from Y′C C to R′G′B′

B R
is disabled by the decoder

NOTE - Colour space interpretation and specifically the usage of this marker is beyond the scope of Rec. ITU-T T.81 |

ISO/IEC 10918-1 and is further specified in Rec. ITU-T T.871 | ISO/IEC 10918-5 and ISO/IEC 18447-1.

B.4 Options controlling the scan generation and entropy coding

The following parameters control the entropy coding and generation of scan patterns for JPEG scans. Especially the -h

parameter (optimize Huffman tables) is recommended, and for most configurations even necessary as Rec. ITU-T T.81 |

ISO/IEC 10918-1 does not provide for all modes examples for tables the encoder could choose.

optimize the Huffman tables
use progressive instead of sequential encoding
use the lossless (predictive) mode of Rec. ITU-T T.81 | ISO/IEC 10918-1
-y N

use hierarchical coding with N hierarchical layers. If N is 0, a two-scan hierarchical process is

initiated of which the first scan is a DCT process and the second hierarchical scan is a lossless

coding process
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ISO/IEC 10918-7:2021 (E)
selects arithmetic coding instead of Huffman coding. Can be combined with -v,
-p and –y
B.5 Options controlling the quantizer
The following parameters control the quantizer:
-qt n
defines the quantization table
The following tables are defined:
– n = 0 the example tables from Rec. ITU-T T.81 | ISO/IEC 10918-1 (default)
– n = 1 a completely flat table that should be PSNR-optimal
– n = 2 a MS-SSIM (see Wang et al. (2003)) optimized table
– n = 3 the table suggested by ImageMagick
– n = 4 an HSV-PSNR optimized table
– n = 5 the table from Klein et al. (1992)
– n = 6 the table from Watson et al. (1997)
– n = 7 the table from Ahumada and Peterson (1993)
– n = 8 the table from Peterson et al. (1993)
-dz
enables a deadzone quantizer which replaces the default uniform quantizer
B.6 Options controlling the subsampling of components

The following parameters control the subsampling of components. Unlike other implementations, this software does not

specify the subsampling by means of the minimum coded unit (MCU) sizes, but directly by giving the subsampling

factors. By default, the software uses 4:4:4 sampling. 4:2:0 chroma subsampling is, for example, defined by the parameters

-s 1x1,2x2,2x2, 4:2:2 subsampling by -s 1x1,2x1,2x1.
-s WxH,…
define subsampling factors for all components
NOTE – The arguments to –s are not MCU sizes.
B.7 Miscellaneous options
The following options control miscellaneous encoder options.
-z mcus

defines the restart interval size, if the argument is 0, restart markers are disabled

signals the image height by a define number of lines (DNL) marker instead of writing the image

height into the frame header
B.8 Decoder options
The following options control the decoding process.
disables the transformation from Y′C C to R′G′B′
B R

NOTE – This transformation is not part of Rec. ITU-T T.81 | ISO/IEC 10918-1 but has been specified in Rec. ITU-T T.871 |

ISO/IEC 10918-5.

-U writes the output in pgx format rather than pnm format and disables upsampling of components

and the transformation from Y′C C to R′G′B′
B R

Neither upsampling nor this transformation are formally part of the Rec. ITU-T T.81 |

ISO/IEC 10918-1 process and hence should be disabled for the purpose of conformance testing

with Rec. ITU-T T.83 | ISO/IEC 10918-2.
© ISO/IEC 2021 – All rights reserved
6 Rec. ITU-T T.873 (06/2021)
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ISO/IEC 10918-7:2021 (E)
Annex C
Unpacking and compiling reference software B

(This annex does not form an integral part of this Recommendation | International Standard.)

The source code of the software is provided in a zip archive available at https://standards.iso.org/iso-iec/10918/-7/ed-2/en

and https://www.itu.int/rec/T-REC-T.873/en. Unpacking a zip file is operating system specific. Under POSIX compliant

operating systems, open a command line window and enter
unzip referenceB.zip
This will unpack all components of the software into the current directory.
To compile the software, follow these steps:

– In order to compile a command line tool that supports the 8 bit baseline, sequential and progressive DCT

coding modes on POSIX compliant operating systems, go to the directory into which the electronic

attachment was unpacked, then enter on the command line
cmake.
make
make –f Makefile-pgx
The following tools are required for successful building:
– a POSIX compliant shell
– CMake 2.8.12 (Kitware (Internet-1))
– The GNU compiler (make, gcc compiler and linker)

– The NASM assembler (NASM Development Team (Internet)) or Yasm assembler (Yasm (Internet))

The reference software will then be built in the current directory consisting of two binaries, the encoder "cjpeg" and

the decoder "djpeg", and a compliance-testing enabled decoder "djpegtopgx". The binaries created this way will

be only capable of encoding and decoding to the 8 bit sequential or progressive DCT modes.

– In order to compile binaries that support the 12 bit sequential and progressive Huffman-coded DCT modes,

enter the following sequence of commands on the command line:
cmake –D WITH_12BIT=1.
make
make –f Makefile-pgx

NOTE – The supplied software cannot be built in such a way that both 8 bit and 12 bit modes are supported simultaneously by the

same executable.
– For Microsoft Windows, the following tools are required to build the software:
– The Visual Studio VS2008, VS2010 or VS2013 compiler suite
– CMake 2.8.12 (Kitware (Internet-1))
The following steps need to be performed to compile the software:

– Generate a Visual Studio project file for building the tool, which can be done either through the CMake

graphical user interface or from the command line. For the latter, go to the directory into which sources

have been unpacked, then enter
cmake -G "Visual Studio 10 2010".

where the argument behind -G needs to be adapted to the target version of Visual Studio. For a list of supported platforms,

see Kitware (Internet-2).

The command in the previous paragraph will generate suitable project files in the current directory that can be opened

directly from the Visual Studio IDE. Details of the building procedure can be found in the file BUILDING.md in the

source directory of the software.
____________________
In the ISO distribution ReferenceB.zip is called DIS_10918att2.zip.
© ISO/IEC 2021 – All rights reserved
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ISO/IEC 10918-7:2021 (E)

– Open the Visual Studio project file generated by the step in the previous paragraph from the graphical user

interface and rebuild the software from the menu.

– The compiler will generate two command line tools: "cjpeg" will compress images represented in pnm

(Picture AnyMap) into JPEG; and "djpeg" expands JPEG and JPEG XT part 1 images into pnm-files.

8 Rec. ITU-T T.873 (06/2021)
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ISO/IEC 10918-7:2021 (E)
Annex D
Using reference software B
(This annex does
...

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