ISO/IEC 18477-8:2020
(Main)Information technology — Scalable compression and coding of continuous-tone still images — Part 8: Lossless and near-lossless coding
Information technology — Scalable compression and coding of continuous-tone still images — Part 8: Lossless and near-lossless coding
This document specifies a coding format, referred to as JPEG XT, which is designed primarily for continuous-tone photographic content. This document defines extensions that allow lossless coding of such content while staying compatible with the core coding system specified in ISO/IEC 18477-1.
Technologies de l'information — Compression échelonnable et codage d'images plates en ton continu — Partie 8: Codage sans perte et quasi sans perte
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INTERNATIONAL ISO/IEC
STANDARD 18477-8
Second edition
2020-05
Information technology — Scalable
compression and coding of
continuous-tone still images —
Part 8:
Lossless and near-lossless coding
Technologies de l'information — Compression échelonnable et codage
d'images plates en ton continu —
Partie 8: Codage sans perte et quasi sans perte
Reference number
ISO/IEC 18477-8:2020(E)
©
ISO/IEC 2020
---------------------- Page: 1 ----------------------
ISO/IEC 18477-8:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2020
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
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO/IEC 2020 – All rights reserved
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ISO/IEC 18477-8:2020(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Symbols . 5
3.3 Abbreviated terms . 6
4 Conventions . 6
4.1 Conformance language . 6
4.2 Operators . 6
4.2.1 Arithmetic operators . 6
4.2.2 Logical operators . 7
4.2.3 Relational operators . 7
4.2.4 Precedence order of operators . 7
4.2.5 Mathematical functions . 7
5 General . 8
5.1 General definitions . 8
5.2 Overview of this document . 8
5.3 Profiles .10
5.4 Encoder requirements .10
5.5 Decoder requirements.10
Annex A (normative) Encoding and decoding process .12
Annex B (normative) Boxes .17
Annex C (normative) Multi-component decorrelation transformation .25
Annex D (normative) Entropy coding of residual data in the DCT-bypass and large range mode .28
Annex E (normative) Discrete cosine transformation .39
Annex F (normative) Component upsampling .52
Annex G (normative) Quantization and noise shaping for the DCT-bypass process .54
Annex H (normative) Profiles .57
Bibliography .58
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ISO/IEC 18477-8:2020(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).
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 http:// 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.
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.
This second edition cancels and replaces the first edition (ISO/IEC 18477-8:2016), which has been
technically revised.
The main changes compared to the previous edition are as follows:
— Annex F.2 has been revised to adopt centred upsampling by default;
— minor editorial changes throughout.
A list of all parts in the ISO/IEC 18477 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.
iv © ISO/IEC 2020 – All rights reserved
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ISO/IEC 18477-8:2020(E)
Introduction
This document specifies a coded codestream format for storage of continuous-tone high and low
dynamic range photographic content. This is a scalable lossy to lossless image coding system supporting
multiple component images consisting of integer samples between 8- and 16-bit resolution, or floating
point samples of 16-bit resolution. It is by itself an extension of ISO/IEC 18477-6 and ISO/IEC 18477-7,
which specify intermediate range and high-dynamic range image decoding algorithms. Both of these
are based on the box-based file format specified in ISO/IEC 18477-3, which is again an extension of
ISO/IEC 18477-1; the codestream is composed in such a way that legacy applications conforming to
Rec. ITU-T T.81 | ISO/IEC 10918-1 are able to reconstruct a lossy, low dynamic range, 8 bits per sample
version of the image.
Today, the most widely used digital photography format, a minimal implementation of JPEG (specified
in Rec. ITU-T T.81 | ISO/IEC 10918-1), uses a bit depth of 8; each of the three channels that together
compose an image pixel is represented by 8 bits, providing 256 representable values per channel.
For more demanding applications, it is not uncommon to use a bit depth of 16, providing 65 536
14
representable values to describe each channel within a pixel, resulting in over 2.8×10 representable
colour values. In some less common scenarios, even greater bit depths are used, requiring a floating-
point sample representation.
Most common photo and image formats use an 8-bit or 16-bit unsigned integer value to represent some
function of the intensity of each colour channel. While it might be theoretically possible to agree on
one method for assigning specific numerical values to real world colours, doing so is not practical.
Since any specific device has its own limited range for colour reproduction, the device’s range may be a
small portion of the agreed-upon universal colour range. As a result, such an approach is an extremely
inefficient use of the available numerical values, especially when using only 8 bits (or 256 unique
values) per channel. To represent pixel values as efficiently as possible, devices use a numeric encoding
optimized for their own range of possible colours or gamut.
This document is primarily designed to encode intermediate or high dynamic image sample values
without loss, or with a precisely controllable bounded loss using the tools defined in ISO/IEC 18477-1
and some minimal extensions of those tools. The goal is to provide a backwards-compatible coding
specification that allows legacy applications and existing toolchains to continue to operate on
codestreams conforming to this document.
JPEG XT has been designed to be backwards compatible to legacy applications while at the same time
having a small coding complexity; JPEG XT uses, whenever possible, functional blocks of Rec. ITU-T T.81
| ISO/IEC 10918-1 to extend the functionality of the legacy JPEG coding system. It is optimized for
storage and transmission of intermediate and high dynamic range and wide colour gamut 8- to 16-
bit integer or 16-bit floating point images while also enabling low-complexity encoder and decoder
implementations.
This document is an extension of ISO/IEC 18477-1, a compression system for continuous tone digital
still images which is backwards compatible with Rec. ITU-T T.81 | ISO/IEC 10918-1. That is, legacy
applications conforming to Rec. ITU-T T.81 | ISO/IEC 10918-1 will be able to reconstruct streams
generated by an encoder conforming to this document, though will possibly not be able to reconstruct
such streams in full dynamic range, full quality or without loss.
This document is itself based on ISO/IEC 18477-3 that defines a box-based file format similar to
other JPEG standards. It also contains elements of ISO/IEC 18477-6 and ISO/IEC 18477-7. The aim
of this document is to provide a migration path for legacy applications to support lossless coding of
intermediate and high dynamic range images, that is images that are either represented by sample
values requiring 8- to 16-bit precision, or even using 16-bit floating point sample resolution. While Rec.
ITU-T T.81 | ISO/IEC 10918-1 already defines a lossless mode for integer samples, images encoded in
this mode cannot be decoded by applications only supporting the lossy 8-bit-mode; the coding engine
for lossless coding in Rec. ITU-T T.81 | ISO/IEC 10918-1 is completely different from the lossy coding
mode. Unlike the legacy standard, this document defines a lossless scalable coding engine supporting
all bit depths between 8 and 16 bits per sample, including 16-bit floating point samples, while also
staying compatible with legacy applications. Such applications will continue to work, but will only able
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ISO/IEC 18477-8:2020(E)
to reconstruct a lossy 8-bit standard low dynamic range (LDR) version of the full image contained in
the codestream. The ISO/IEC 18477 series specifies a coded file format, referred to as JPEG XT, which is
designed primarily for storage and interchange of continuous-tone photographic content.
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INTERNATIONAL STANDARD ISO/IEC 18477-8:2020(E)
Information technology — Scalable compression and
coding of continuous-tone still images —
Part 8:
Lossless and near-lossless coding
1 Scope
This document specifies a coding format, referred to as JPEG XT, which is designed primarily for
continuous-tone photographic content. This document defines extensions that allow lossless coding of
such content while staying compatible with the core coding system specified in ISO/IEC 18477-1.
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 18477-1:2020, Information technology — Scalable compression and coding of continuous-tone still
images — Part 1: Scalable compression and coding of continuous-tone still images
ISO/IEC 18477-3:2015, Information technology — Scalable compression and coding of continuous-tone still
images — Part 3: Box file format
ISO/IEC 18477-6:2016, Information technology — Scalable compression and coding of continuous-tone
still images — Part 6: IDR Integer Coding
ISO/IEC 18477-7:2017, Information technology — Scalable compression and coding of continuous-tone still
images — Part 7: HDR Floating-Point Coding
ITU-T T.81 | ISO/IEC 10918-1, Information technology — Digital compression and coding of continuous
tone still images — Requirements and guidelines
ITU-T BT.601, Studio encoding parameters of digital television for standard 4:3 and wide screen 16:9
aspect ratios
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1.1
AC coefficient
any DCT coefficient for which the frequency is not zero in at least one dimension
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ISO/IEC 18477-8:2020(E)
3.1.2
ASCII encoding
encoding of text characters and text strings
Note 1 to entry: In accordance with ISO/IEC 10646.
3.1.3
base decoding path
process of decoding legacy codestream and refinement data to the base image, jointly with all further
steps until residual data is added to the values obtained from the residual codestream
3.1.4
base image
collection of sample values obtained by entropy decoding the DCT coefficients of the legacy codestream
and the refinement codestream, and inversely DCT transforming them jointly
3.1.5
box
structured collection of data describing the image or the image decoding process embedded into one or
multiple APP marker segments
11
Note 1 to entry: See ISO/IEC 18477-3:2015, Annex B for the definition of boxes.
3.1.6
coding process
encoding process, a decoding process, or both
3.1.7
DC coefficient
DCT coefficient for which the frequency is zero in both dimensions
3.1.8
DCT coefficient
amplitude of a specific cosine basis function
Note 1 to entry: May refer to an original DCT coefficient, to a quantized DCT coefficient, or to a dequantized DCT
coefficient.
3.1.9
decoder
embodiment of a decoding process
3.1.10
decoding process
process which takes as its input compressed image data and outputs a continuous-tone image
3.1.11
dequantization
inverse procedure to quantization by which the decoder recovers a representation of the DCT
coefficients
3.1.12
encoder
embodiment of an encoding process
3.1.13
encoding process
process which takes as its input a continuous-tone image and outputs compressed image data
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ISO/IEC 18477-8:2020(E)
3.1.14
extension image
residual image
sample values as reconstructed by inverse quantization and inverse DCT transformation applied to the
entropy-decoded coefficients described by the residual scan and residual refinement scans
[SOURCE: ISO/IEC 18477-6:2016, 3.1.54]
3.1.15
fixed point discrete cosine transformation
implementation of the discrete cosine transformation based on fixed point arithmetic
Note 1 to entry: As specified in Annex E.
3.1.16
forward DCT bypass
transformation that takes an 8×8 sample block and prepares it for entropy coding without applying a
discrete cosine transformation
3.1.17
forward fixed point DCT
transformation of an 8×8 sample block from the spatial domain to the frequency domain using the fixed
point arithmetic
Note 1 to entry: As specified in Annex E.
3.1.18
forward integer DCT
transformation of an 8×8 sample block from the spatial domain to the frequency domain using the
integer approximation of the discrete cosine transformation
Note 1 to entry: As specified in Annex E.
3.1.19
inverse DCT bypass
transformation that takes an 8×8 sample block as generated by entropy decoding and level-shifts it
without applying a discrete cosine transformation
3.1.20
inverse fixed point DCT
transformation of an 8×8 sample block from the frequency domain to the spatial domain using the fixed
point arithmetic
Note 1 to entry: As specified in Annex E.
3.1.21
inverse integer DCT
the transformation of an 8×8 sample block from the frequency domain to the spatial domain using the
integer approximation of the discrete cosine transformation
Note 1 to entry: As specified in Annex E.
3.1.22
frequency
two-dimensional index into the two-dimensional array of DCT coefficients
[SOURCE: ISO/IEC 10918-1:1994, 3.1.61]
3.1.23
high dynamic range
HDR
image or image data comprised of more than eight bits per sample
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ISO/IEC 18477-8:2020(E)
3.1.24
Huffman encoding
entropy encoding procedure which assigns a variable length code to each input symbol
3.1.25
intermediate dynamic range
image or image data comprised of more than eight bits per sample
3.1.26
legacy codestream
collection of markers and syntax elements
Note 1 to entry: The legacy codestream, as defined by Rec. ITU-T T.81 | ISO/IEC 10918-1 and any syntax elements
defined by the ISO/IEC 18477 series, consists of the collection of all markers except those APP markers that
11
describe JPEG XT boxes by the syntax defined in ISO/IEC 18477-3:2015, Annex A.
3.1.27
lossless
encoding and decoding processes and procedures in which the output of the decoding procedure(s) is
identical to the input to the encoding procedure(s)
3.1.28
lossless coding
mode of operation which refers to any one of the coding processes in which all of the procedures are
lossless
Note 1 to entry: Coding processes defined in ISO/IEC 18477-8.
3.1.29
lossy
encoding and decoding processes which are not lossless
3.1.30
low-dynamic range
LDR
image or image data comprised of data with no more than eight bits per sample
3.1.31
noise shaping
signal processing technique that removes quantization noise from the low frequency components and
injects it into the high frequency domain where it can be removed by filtering
3.1.32
point transformation
application of a location independent global function to reconstructed sample values in the spatial domain
3.1.33
residual decoding path
collection of operations applied to the entropy coded data contained in the residual data box and
residual refinement scan boxes up to the point where this data is merged with the legacy data to form
the final output image
3.1.34
residual image
sample values as reconstructed by inverse quantization and inverse DCT transformation applied to the
entropy-decoded coefficients described by the residual scan and residual refinement scans
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ISO/IEC 18477-8:2020(E)
3.1.35
residual scan
additional pass over the image data invisible to legacy decoders which provides additive and/or
multiplicative correction data of the legacy scans to allow reproduction of high-dynamic range or wide
colour gamut data
3.1.36
refinement scan
additional pass over the image data invisible to legacy decoders which provides additional least
significant bits to extend the precision of the DCT transformed coefficients
Note 1 to entry: Refinement scans can be either applied in the legacy or residual decoding path.
3.1.37
superbox
box that carries other boxes as payload data
3.1.38
sub box
box that is contained as payload data within a superbox
3.1.39
uniform quantization
procedure by which DCT coefficients are linearly scaled in order to achieve compression
3.1.40
upsampling
procedure by which the spatial resolution of a component is increased
3.2 Symbols
X width of the sample grid in positions
Y height of the sample grid in positions
Nf number of components in an image
s subsampling factor of component i in horizontal direction
i,x
s subsampling factor of component i in vertical direction
i,y
H subsampling indicator of component i in the frame header
i
V subsampling indicator of component i in the frame header
i
v sample value at the sample grid position x,y
x,y
R additional number of DCT coefficients bits represented by refinement scans in the base image,
h
8+R is the number of non-fractional bits (i.e. bits in front of the "binary dot") of the output of
h
the inverse DCT process in the base image
R additional number of DCT coefficients bits represented by refinement scans in the residual,
r
P+R is the number of non-fractional bits (i.e. bits in front of the "binary dot") of the output of
h
the inverse DCT process in the residual image where P is the bit depth indicated in the frame
header of the residual codestream
R additional bits in the HDR image. 8+R is the sample precision of the reconstructed HDR image
b b
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ISO/IEC 18477-8:2020(E)
3.3 Abbreviated terms
ASCII American Standard Code for Information Interchange
LSB least significant bit
MSB most significant bit
TMO tone mapping operator
DCT discrete cosine transformation
FCT fixed point multi-component transformation
ICT irreversible multi-component transformation
RCT reversible multi-component transformation
JPEG joint photographic experts group
4 Conventions
4.1 Conformance language
The keyword "reserved" indicates a provision that is not specified at this time, shall not be used, and
may be specified in the future. The keyword "forbidden" indicates "reserved" and in addition indicates
that the provision will never be specified in the future.
4.2 Operators
NOTE Many of the operators used in this document are similar to those used in the C programming language.
4.2.1 Arithmetic operators
+ addition
− subtraction (as a binary operator) or negation (as a unary prefix operator)
× multiplication
/ division without truncation or rounding
smod x smod a is the unique value y between −⎾(a−1)/2⏋ and ⎿(a−1)/2⏌
for which y+N×a = x with a suitable integer N.
umod x umod a is the unique value y between 0 and a−1 for which
y+N×a = x with a suitable integer N
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ISO/IEC 18477-8:2020(E)
4.2.2 Logical operators
|| logical OR
&& logical AND
! logical NOT
∈ x ∈ {A, B} is defined as (x == A || x == B)
∉ x ∉ {A, B} is defined as (x != A && x != B)
4.2.3 Relational operators
> greater than
>= greater than or equal to
< less than
<= less than or equal to
== equal to
!= not equal to
4.2.4 Precedence order of operators
Operators are listed in descending order of precedence. If several operators appear in the same line,
they have equal precedence. When several operators of equal precedence appear at the same level in an
expression, evaluation proceeds according to the associativity of the operator either from right to left
or from left to right.
Operators Type of operation Associativity
(), [ ], . expression left to right
− unary negation
×, / multiplication left to right
umod, smod modulo (remainder) left to right
+, − addition and subtraction left to right
< , >, <=, >= relational left to right
4.2.5 Mathematical functions
⎾x⏋ ceil of x: returns the smallest integer that is greater than or equal to x
⎿x⏌ floor of x: returns the largest integer that is lesser than or equal to x
|x| absolute value, is –x for x < 0, otherwise x
sign(x) sign of x, 0 if x is zero, +1 if x is positive, −1 if x is negative
clamp(x,min,max) clamps x to the range [min,max]: returns min if x < min, max if x > max or oth-
erwise x
a
x raises the value of x to the power of a: x is a non-negative real number, a is a real
a
number; x is equal to exp(a×log(x)) where exp is the exponential function and
a
log() the natural logarithm; if x is 0 and a is positive, x is defined to be 0
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ISO/IEC 18477-8:2020(E)
5 General
5.1 General definitions
This clause gives an informative overview of the elements specified in this document. It also introduces
many of the terms which are defined in Clause 3. These terms are printed in italics upon first usage in
this clause.
There are three elements specified in this document:
a) An encoder is an embodiment of an encoding process. An encoder takes as input digital source image
data and encoder specifications and, by means of a specified set of procedures, generates as output
codestream.
b) A decoder is an embodiment of a decoding process. A decoder takes as input a codestream and, by
means of a specified set of procedures, generates as output digital reconstructed image data.
c) The codestream is a compressed image data representation, which includes all necessary data to
allow a (full or approximate) reconstruction of the sample values of a digital image. Additional data
might be required that define the interpretation of the sample data, such as colour space or the
spatial dimensions of the samples.
5.2 Overview of this document
This document allows near-lossless and lossless coding of high and intermediate dynamic range
of photographic images in a way that is backwards compatible to Rec. ITU-T T.81 | ISO/IEC 10918-1.
Decoders compliant to Rec. ITU-T T.81 | ISO/IEC
...
FINAL
INTERNATIONAL ISO/IEC
DRAFT
STANDARD FDIS
18477-8
ISO/IEC JTC 1/SC 29
Information technology — Scalable
Secretariat: JISC
compression and coding of
Voting begins on:
2020-03-13 continuous-tone still images —
Voting terminates on:
Part 8:
2020-05-08
Lossless and near-lossless coding
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 18477-8:2020(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 2020
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ISO/IEC FDIS 18477-8:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2020
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
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO/IEC 2020 – All rights reserved
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ISO/IEC FDIS 18477-8:2020(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Symbols . 5
3.3 Abbreviated terms . 6
4 Conventions . 6
4.1 Conformance language . 6
4.2 Operators . 6
4.2.1 Arithmetic operators . 6
4.2.2 Logical operators . 7
4.2.3 Relational operators . 7
4.2.4 Precedence order of operators . 7
4.2.5 Mathematical functions . 7
5 General . 8
5.1 General definitions . 8
5.2 Overview of this document . 8
5.3 Profiles .10
5.4 Encoder requirements .10
5.5 Decoder requirements.10
Annex A (normative) Encoding and decoding process .12
Annex B (normative) Boxes .17
Annex C (normative) Multi-component decorrelation transformation .25
Annex D (normative) Entropy coding of residual data in the DCT-bypass and large range mode .28
Annex E (normative) Discrete cosine transformation .39
Annex F (normative) Component upsampling .52
Annex G (normative) Quantization and noise shaping for the DCT-bypass process .54
Annex H (normative) Profiles .57
Bibliography .58
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ISO/IEC FDIS 18477-8:2020(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).
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 http:// 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.
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.
This second edition cancels and replaces the first edition (ISO/IEC 18477-8:2016), which has been
technically revised.
The main changes compared to the previous edition are as follows:
— Annex F.2 has been revised to adopt centred upsampling by default;
— minor editorial changes throughout.
A list of all parts in the ISO/IEC 18477 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.
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ISO/IEC FDIS 18477-8:2020(E)
Introduction
This document specifies a coded codestream format for storage of continuous-tone high and low
dynamic range photographic content. This is a scalable lossy to lossless image coding system supporting
multiple component images consisting of integer samples between 8- and 16-bit resolution, or floating
point samples of 16-bit resolution. It is by itself an extension of ISO/IEC 18477-6 and ISO/IEC 18477-7,
which specify intermediate range and high-dynamic range image decoding algorithms. Both of these
are based on the box-based file format specified in ISO/IEC 18477-3, which is again an extension of
ISO/IEC 18477-1; the codestream is composed in such a way that legacy applications conforming to
Rec. ITU-T T.81 | ISO/IEC 10918-1 are able to reconstruct a lossy, low dynamic range, 8 bits per sample
version of the image.
Today, the most widely used digital photography format, a minimal implementation of JPEG (specified
in Rec. ITU-T T.81 | ISO/IEC 10918-1), uses a bit depth of 8; each of the three channels that together
compose an image pixel is represented by 8 bits, providing 256 representable values per channel.
For more demanding applications, it is not uncommon to use a bit depth of 16, providing 65 536
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representable values to describe each channel within a pixel, resulting in over 2.8×10 representable
colour values. In some less common scenarios, even greater bit depths are used, requiring a floating-
point sample representation.
Most common photo and image formats use an 8-bit or 16-bit unsigned integer value to represent some
function of the intensity of each colour channel. While it might be theoretically possible to agree on
one method for assigning specific numerical values to real world colours, doing so is not practical.
Since any specific device has its own limited range for colour reproduction, the device’s range may be a
small portion of the agreed-upon universal colour range. As a result, such an approach is an extremely
inefficient use of the available numerical values, especially when using only 8 bits (or 256 unique
values) per channel. To represent pixel values as efficiently as possible, devices use a numeric encoding
optimized for their own range of possible colours or gamut.
This document is primarily designed to encode intermediate or high dynamic image sample values
without loss, or with a precisely controllable bounded loss using the tools defined in ISO/IEC 18477-1
and some minimal extensions of those tools. The goal is to provide a backwards-compatible coding
specification that allows legacy applications and existing toolchains to continue to operate on
codestreams conforming to this document.
JPEG XT has been designed to be backwards compatible to legacy applications while at the same time
having a small coding complexity; JPEG XT uses, whenever possible, functional blocks of Rec. ITU-T T.81
| ISO/IEC 10918-1 to extend the functionality of the legacy JPEG coding system. It is optimized for
storage and transmission of intermediate and high dynamic range and wide colour gamut 8- to 16-
bit integer or 16-bit floating point images while also enabling low-complexity encoder and decoder
implementations.
This document is an extension of ISO/IEC 18477-1, a compression system for continuous tone digital
still images which is backwards compatible with Rec. ITU-T T.81 | ISO/IEC 10918-1. That is, legacy
applications conforming to Rec. ITU-T T.81 | ISO/IEC 10918-1 will be able to reconstruct streams
generated by an encoder conforming to this document, though will possibly not be able to reconstruct
such streams in full dynamic range, full quality or without loss.
This document is itself based on ISO/IEC 18477-3 that defines a box-based file format similar to
other JPEG standards. It also contains elements of ISO/IEC 18477-6 and ISO/IEC 18477-7. The aim
of this document is to provide a migration path for legacy applications to support lossless coding of
intermediate and high dynamic range images, that is images that are either represented by sample
values requiring 8- to 16-bit precision, or even using 16-bit floating point sample resolution. While Rec.
ITU-T T.81 | ISO/IEC 10918-1 already defines a lossless mode for integer samples, images encoded in
this mode cannot be decoded by applications only supporting the lossy 8-bit-mode; the coding engine
for lossless coding in Rec. ITU-T T.81 | ISO/IEC 10918-1 is completely different from the lossy coding
mode. Unlike the legacy standard, this document defines a lossless scalable coding engine supporting
all bit depths between 8 and 16 bits per sample, including 16-bit floating point samples, while also
staying compatible with legacy applications. Such applications will continue to work, but will only able
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ISO/IEC FDIS 18477-8:2020(E)
to reconstruct a lossy 8-bit standard low dynamic range (LDR) version of the full image contained in
the codestream. The ISO/IEC 18477 series specifies a coded file format, referred to as JPEG XT, which is
designed primarily for storage and interchange of continuous-tone photographic content.
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FINAL DRAFT INTERNATIONAL STANDARD ISO/IEC FDIS 18477-8:2020(E)
Information technology — Scalable compression and
coding of continuous-tone still images —
Part 8:
Lossless and near-lossless coding
1 Scope
This document specifies a coding format, referred to as JPEG XT, which is designed primarily for
continuous-tone photographic content. This document defines extensions that allow lossless coding of
such content while staying compatible with the core coding system specified in ISO/IEC 18477-1.
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 18477-1:2015, Information technology — Scalable compression and coding of continuous-tone still
images — Part 1: Scalable compression and coding of continuous-tone still images
ISO/IEC 18477-3:2015, Information technology — Scalable compression and coding of continuous-tone still
images — Part 3: Box file format
ISO/IEC 18477-6:2016, Information technology — Scalable compression and coding of continuous-tone
still images — Part 6: IDR Integer Coding
ISO/IEC 18477-7:2017, Information technology — Scalable compression and coding of continuous-tone still
images — Part 7: HDR Floating-Point Coding
ITU-T T.81 | ISO/IEC 10918-1, Information technology — Digital compression and coding of continuous
tone still images — Requirements and guidelines
ITU-T BT.601, Studio encoding parameters of digital television for standard 4:3 and wide screen 16:9
aspect ratios
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1.1
AC coefficient
any DCT coefficient for which the frequency is not zero in at least one dimension
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3.1.2
ASCII encoding
encoding of text characters and text strings
Note 1 to entry: In accordance with ISO/IEC 10646.
3.1.3
base decoding path
process of decoding legacy codestream and refinement data to the base image, jointly with all further
steps until residual data is added to the values obtained from the residual codestream
3.1.4
base image
collection of sample values obtained by entropy decoding the DCT coefficients of the legacy codestream
and the refinement codestream, and inversely DCT transforming them jointly
3.1.5
box
structured collection of data describing the image or the image decoding process embedded into one or
multiple APP marker segments
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Note 1 to entry: See ISO/IEC 18477-3:2015, Annex B for the definition of boxes.
3.1.6
coding process
encoding process, a decoding process, or both
3.1.7
DC coefficient
DCT coefficient for which the frequency is zero in both dimensions
3.1.8
DCT coefficient
amplitude of a specific cosine basis function
Note 1 to entry: May refer to an original DCT coefficient, to a quantized DCT coefficient, or to a dequantized DCT
coefficient.
3.1.9
decoder
embodiment of a decoding process
3.1.10
decoding process
process which takes as its input compressed image data and outputs a continuous-tone image
3.1.11
dequantization
inverse procedure to quantization by which the decoder recovers a representation of the DCT
coefficients
3.1.12
encoder
embodiment of an encoding process
3.1.13
encoding process
process which takes as its input a continuous-tone image and outputs compressed image data
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3.1.14
extension image
residual image
sample values as reconstructed by inverse quantization and inverse DCT transformation applied to the
entropy-decoded coefficients described by the residual scan and residual refinement scans
[SOURCE: ISO/IEC 18477-6:2016, 3.1.54]
3.1.15
fixed point discrete cosine transformation
implementation of the discrete cosine transformation based on fixed point arithmetic
Note 1 to entry: As specified in Annex E.
3.1.16
forward DCT bypass
transformation that takes an 8×8 sample block and prepares it for entropy coding without applying a
discrete cosine transformation
3.1.17
forward fixed point DCT
transformation of an 8×8 sample block from the spatial domain to the frequency domain using the fixed
point arithmetic
Note 1 to entry: As specified in Annex E.
3.1.18
forward integer DCT
transformation of an 8×8 sample block from the spatial domain to the frequency domain using the
integer approximation of the discrete cosine transformation
Note 1 to entry: As specified in Annex E.
3.1.19
inverse DCT bypass
transformation that takes an 8×8 sample block as generated by entropy decoding and level-shifts it
without applying a discrete cosine transformation
3.1.20
inverse fixed point DCT
transformation of an 8×8 sample block from the frequency domain to the spatial domain using the fixed
point arithmetic
Note 1 to entry: As specified in Annex E.
3.1.21
inverse integer DCT
the transformation of an 8×8 sample block from the frequency domain to the spatial domain using the
integer approximation of the discrete cosine transformation
Note 1 to entry: As specified in Annex E.
3.1.22
frequency
two-dimensional index into the two-dimensional array of DCT coefficients
[SOURCE: ISO/IEC 10918-1:1994, 3.1.61]
3.1.23
high dynamic range
HDR
image or image data comprised of more than eight bits per sample
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3.1.24
Huffman encoding
entropy encoding procedure which assigns a variable length code to each input symbol
3.1.25
intermediate dynamic range
image or image data comprised of more than eight bits per sample
3.1.26
legacy codestream
collection of markers and syntax elements
Note 1 to entry: The legacy codestream, as defined by Rec. ITU-T T.81 | ISO/IEC 10918-1 and any syntax elements
defined by the ISO/IEC 18477 series, consists of the collection of all markers except those APP markers that
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describe JPEG XT boxes by the syntax defined in ISO/IEC 18477-3:2015, Annex A.
3.1.27
lossless
encoding and decoding processes and procedures in which the output of the decoding procedure(s) is
identical to the input to the encoding procedure(s)
3.1.28
lossless coding
mode of operation which refers to any one of the coding processes in which all of the procedures are
lossless
Note 1 to entry: Coding processes defined in ISO/IEC 18477-8.
3.1.29
lossy
encoding and decoding processes which are not lossless
3.1.30
low-dynamic range
LDR
image or image data comprised of data with no more than eight bits per sample
3.1.31
noise shaping
signal processing technique that removes quantization noise from the low frequency components and
injects it into the high frequency domain where it can be removed by filtering
3.1.32
point transformation
application of a location independent global function to reconstructed sample values in the spatial domain
3.1.33
residual decoding path
collection of operations applied to the entropy coded data contained in the residual data box and
residual refinement scan boxes up to the point where this data is merged with the legacy data to form
the final output image
3.1.34
residual image
sample values as reconstructed by inverse quantization and inverse DCT transformation applied to the
entropy-decoded coefficients described by the residual scan and residual refinement scans
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3.1.35
residual scan
additional pass over the image data invisible to legacy decoders which provides additive and/or
multiplicative correction data of the legacy scans to allow reproduction of high-dynamic range or wide
colour gamut data
3.1.36
refinement scan
additional pass over the image data invisible to legacy decoders which provides additional least
significant bits to extend the precision of the DCT transformed coefficients
Note 1 to entry: Refinement scans can be either applied in the legacy or residual decoding path.
3.1.37
superbox
box that carries other boxes as payload data
3.1.38
sub box
box that is contained as payload data within a superbox
3.1.39
uniform quantization
procedure by which DCT coefficients are linearly scaled in order to achieve compression
3.1.40
upsampling
procedure by which the spatial resolution of a component is increased
3.2 Symbols
X width of the sample grid in positions
Y height of the sample grid in positions
Nf number of components in an image
s subsampling factor of component i in horizontal direction
i,x
s subsampling factor of component i in vertical direction
i,y
H subsampling indicator of component i in the frame header
i
V subsampling indicator of component i in the frame header
i
v sample value at the sample grid position x,y
x,y
R additional number of DCT coefficients bits represented by refinement scans in the base image,
h
8+R is the number of non-fractional bits (i.e. bits in front of the "binary dot") of the output of
h
the inverse DCT process in the base image
R additional number of DCT coefficients bits represented by refinement scans in the residual,
r
P+R is the number of non-fractional bits (i.e. bits in front of the "binary dot") of the output of
h
the inverse DCT process in the residual image where P is the bit depth indicated in the frame
header of the residual codestream
R additional bits in the HDR image. 8+Rb is the sample precision of the reconstructed HDR image
b
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ISO/IEC FDIS 18477-8:2020(E)
3.3 Abbreviated terms
ASCII American Standard Code for Information Interchange
LSB least significant bit
MSB most significant bit
TMO tone mapping operator
DCT discrete cosine transformation
FCT fixed point multi-component transformation
ICT irreversible multi-component transformation
RCT reversible multi-component transformation
JPEG joint photographic experts group
4 Conventions
4.1 Conformance language
The keyword "reserved" indicates a provision that is not specified at this time, shall not be used, and
may be specified in the future. The keyword "forbidden" indicates "reserved" and in addition indicates
that the provision will never be specified in the future.
4.2 Operators
NOTE Many of the operators used in this document are similar to those used in the C programming language.
4.2.1 Arithmetic operators
+ addition
− subtraction (as a binary operator) or negation (as a unary prefix operator)
× multiplication
/ division without truncation or rounding
smod x smod a is the unique value y between −⎾(a−1)/2⏋ and ⎿(a−1)/2⏌
for which y+N×a = x with a suitable integer N.
umod x umod a is the unique value y between 0 and a−1 for which
y+N×a = x with a suitable integer N
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ISO/IEC FDIS 18477-8:2020(E)
4.2.2 Logical operators
|| logical OR
&& logical AND
! logical NOT
∈ x ∈ {A, B} is defined as (x == A || x == B)
∉ x ∉ {A, B} is defined as (x != A && x != B)
4.2.3 Relational operators
> greater than
>= greater than or equal to
< less than
<= less than or equal to
== equal to
!= not equal to
4.2.4 Precedence order of operators
Operators are listed in descending order of precedence. If several operators appear in the same line,
they have equal precedence. When several operators of equal precedence appear at the same level in an
expression, evaluation proceeds according to the associativity of the operator either from right to left
or from left to right.
Operators Type of operation Associativity
(), [ ], . expression left to right
− unary negation
×, / multiplication left to right
umod, smod modulo (remainder) left to right
+, − addition and subtraction left to right
< , >, <=, >= relational left to right
4.2.5 Mathematical functions
⎾x⏋ ceil of x: returns the smallest integer that is greater than or equal to x
⎿x⏌ floor of x: returns the largest integer that is lesser than or equal to x
|x| absolute value, is –x for x < 0, otherwise x
sign(x) sign of x, 0 if x is zero, +1 if x is positive, −1 if x is negative
clamp(x,min,max) clamps x to the range [min,max]: returns min if x < min, max if x > max or oth-
erwise x
a
x raises the value of x to the power of a: x is a non-negative real number, a is a real
a
number; x is equal to exp(a×log(x)) where exp is the exponential function and
a
log() the natural logarithm; if x is 0 and a is positive, x is defined to be 0
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ISO/IEC FDIS 18477-8:2020(E)
5 General
5.1 General definitions
This clause gives an informative overview of the elements specified in this document. It also introduces
many of the terms which are defined in Clause 3. These terms are printed in italics upon first usage in
this clause.
There are three elements specified in this document:
a) An encoder is an embodiment of an encoding process. An encoder takes as input digital source image
data and encoder specifications and, by means of a specified set of procedures, generates as output
codestream.
b) A decoder is an embodiment of a decoding process. A decoder takes as input a codestream and, by
means of a specified set of procedures, generates as output digital reconstructed image data.
c) The codestream is a compressed image data representation, which includes all necessary data to
a
...
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