ISO/IEC 18477-2:2016
(Main)Information technology — Scalable compression and coding of continuous-tone still images — Part 2: Coding of high dynamic range images
Information technology — Scalable compression and coding of continuous-tone still images — Part 2: Coding of high dynamic range images
ISO/IEC 18477-2:2016 specifies a coding format, referred to as JPEG XT, which is designed primarily for continuous-tone photographic content.
Technologies de l'information — Compression échelonnable et codage d'images plates en ton continu — Partie 2: Codage d'images à gamme dynamique élevée
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INTERNATIONAL ISO/IEC
STANDARD 18477-2
First edition
2016-07-15
Information technology — Scalable
compression and coding of
continuous-tone still images —
Part 2:
Coding of high dynamic range images
Technologies de l’information — Compression échelonnable et codage
d’images plates en ton continu —
Partie 2: Codage d’images à gamme dynamique élevée
Reference number
ISO/IEC 18477-2:2016(E)
©
ISO/IEC 2016
---------------------- Page: 1 ----------------------
ISO/IEC 18477-2:2016(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, 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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO/IEC 2016 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 18477-2:2016(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms and symbols . 4
4.1 Abbreviated terms . 4
4.2 Symbols . 4
5 Conventions . 4
5.1 Conformance language . 4
5.2 Operators . 4
5.2.1 Arithmetic operators . 5
5.2.2 Logical operators . 5
5.2.3 Relational operators . 5
5.2.4 Precedence order of operators . 5
5.2.5 Mathematical functions . 6
6 General . 6
6.1 Elements specified . 6
6.2 High level overview of this document . 6
6.3 High level functional overview of decoding process . 7
6.4 Encoder requirements . 8
6.5 Decoder requirements. 8
7 Decoder definition . 8
7.1 Decoder functionality overview . 8
7.2 Legacy Inverse Decorrelation Block B3 . 9
7.3 Base Mapping and Colour space conversion Block B4 .10
7.4 Residual decode Blocks B5, B6 .11
7.5 Residual Mapping and Inverse Decorrelation Blocks B7, B8 .12
7.6 HDR Reconstruction Blocks B9, B10 .13
8 Codestream syntax for Main Profile .13
8.1 Main Profile Header Structure . .13
8.2 Parameter ASCII segment .14
8.3 Parameter binary segment .16
8.4 Residual codestream segment .16
Annex A (normative) Checksum computation .17
Bibliography .18
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ISO/IEC 18477-2:2016(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. In the field of information technology, ISO and IEC have established a joint technical committee,
ISO/IEC JTC 1.
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).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on 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 the following URL: www.iso.org/iso/foreword.html.
The committee responsible for this document is ISO/IEC JTC 1, Information technology, SC 29, Coding of
audio, picture, multimedia and hypermedia information.
A list of all parts in the ISO 18477 series, published under the general title Information technology --
Scalable compression and coding of continuous-tone still images, can be found on the ISO website.
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ISO/IEC 18477-2:2016(E)
Introduction
This document is an extension of ISO/IEC 18477-1, a compression system for continuous tone digital
still images which is backward 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, but will possibly not be able to reconstruct such
streams in full dynamic range, full quality or other features defined in this document.
The aim of this document is to provide a migration path for legacy applications to support coding of
high-dynamic range images. Existing tools depending on the existing standards will continue to work,
but will only be able to reconstruct a lossy and/or a low-dynamic range version of the image contained
in the codestream. This document specifies a codestream, referred to as JPEG XT, which is designed
primarily for storage and interchange of continuous-tone photographic content.
This document specifies a coded codestream format for storage of continuous-tone high and low
dynamic range photographic content. JPEG XT Part 2 is a scalable image coding system supporting
multiple component images in floating point. It is by itself an extension of the coding tools defined in
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 lower quality, low dynamic range, eight 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 or higher, providing greater
14
than 65 536 representable values to describe each channel within a pixel, resulting on over 2.8 × 10
representable colour values. In some less common scenarios, even greater bit depths are used.
The 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.
JPEG XT is primarily designed to provide coded data containing high dynamic range and wide colour
gamut content while simultaneously providing 8 bits per pixel low dynamic range images using tools
defined in ISO/IEC 18477-1. The goal is to provide a backward compatible coding specification that
allows legacy applications and existing toolchains to continue to operate on codestreams conforming to
this this document.
JPEG XT has been designed to be backward 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 high dynamic range and wide colour gamut 32 bit float images while also
enabling low-complexity encoder and decoder implementations.
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INTERNATIONAL STANDARD ISO/IEC 18477-2:2016(E)
Information technology — Scalable compression and
coding of continuous-tone still images —
Part 2:
Coding of high dynamic range images
1 Scope
This document specifies a coding format, referred to as JPEG XT, which is designed primarily for
continuous-tone photographic content.
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 10918-1, Information technology — Digital compression and coding of continuous-tone still
images: Requirements and guidelines — Part 1
ISO/IEC 18477-1, Information technology — Scalable Compression and Coding of Continuous-Tone Still
Images, Core Coding System Specification
IEC 61966-2-1, sRGB Colour management — Default RGB colour space — sRGB
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
ASCII encoding
character encoding scheme defined by ANSI X3.4-1986
3.2
codestream
partially encoded or decoded sequence of bits comprising an entropy-coded segment
3.3
byte
group of 8 bits
3.4
coder
embodiment of a coding process
3.5
coding
encoding or decoding
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ISO/IEC 18477-2:2016(E)
3.6
(coding) process
general term for referring to an encoding process, a decoding process, or both
3.7
compression
reduction in the number of bits used to represent source image data
3.8
component
two-dimensional array of samples having the same designation in the output or display device
Note 1 to entry: An image typically consists of several components, e.g. red, green and blue.
3.9
continuous-tone image
image whose components have more than one bit per sample
3.10
discrete cosine transform
DCT
sum of cosine transforms at different frequencies
3.11
decoder
embodiment of a decoding process
3.12
decoding process
process which takes as its input compressed image data and outputs a continuous-tone image
3.13
downsampling
procedure by which the spatial resolution of a component is reduced
3.14
encoder
embodiment of an encoding process
3.15
encoding process
process which takes as its input a continuous-tone image and outputs compressed image data
3.16
grayscale image
continuous-tone image that has only one component
3.17
high dynamic range
image or image data comprised of more than eight bits per sample
3.18
Joint Photographic Experts Group
JPEG
informal name of the working group which created this part of ISO/IEC 18477
Note 1 to entry: The term “joint” comes from the ITU-T and ISO/IEC collaboration.
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ISO/IEC 18477-2:2016(E)
3.19
legacy decoder
embodiment of a decoding process conforming to Rec. ITU-T T.81 | ISO/IEC 10918-1, confined to the
lossy discrete cosine transformation (DCT) process and the baseline, sequential or progressive modes,
decoding at most four components to eight bits per component
3.20
lossless
descriptive term for encoding and decoding processes and procedures in which the output of the
decoding procedure(s) is identical to the input of the encoding procedure(s)
3.21
lossless coding
mode of operation which refers to any one of the coding processes defined in this part of ISO/IEC 18477
in which all of the procedures are lossless
3.22
lossy
descriptive term for encoding and decoding processes which are not lossless
3.23
low-dynamic range
image or image data comprised of data with no more than 8 bits per sample
3.24
marker
two-byte code in which the first byte is hexadecimal FF and the second byte is a value between 1 and
hexadecimal FE
3.25
marker segment
marker together with its associated set of parameters
3.26
minimum coded unit
MCU
smallest group of data units that is coded
3.27
pixel
collection of sample values in the spatial image domain having all the same sample coordinates
Note 1 to entry: A pixel may consist of three samples describing its red, green and blue value.
3.28
precision
number of bits allocated to a particular sample or discrete cosine transformation (DCT) coefficient
3.29
procedure
set of steps which accomplishes one of the tasks which comprise an encoding or decoding process
3.30
residual codestream
codestream that contains an encoded (according to Rec. ITU-T T.81 | ISO/IEC 10918-1) residual image
3.31
residual data
data that contains luminance ratio and red, green, and blue (RGB) differential data
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ISO/IEC 18477-2:2016(E)
3.32
residual image
pseudo image that contains encoded luminance ratio as luminance and encoded chrominance data that
is computed from red, green, and blue (RGB) differential data using Multiple Component Decorrelation
Transformation defined in ISO/IEC 18477-1
3.33
red, green, and blue
RGB
additive colour model
3.34
luminance ratio
array of per pixel ratio of HDR image luminance and LDR image luminance
3.35
quantization value
integer value used in the quantization procedure
3.36
quantize
act of performing the quantization procedure for a value
3.37
upsampling
procedure by which the spatial resolution of a component is increased
4 Abbreviated terms and symbols
4.1 Abbreviated terms
ASCII American Standard Code for Information Interchange
HDR High Dynamic Range
LDR Low Dynamic Range
4.2 Symbols
Nc number of components in an image
5 Conventions
5.1 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.
5.2 Operators
NOTE Many of the operators used in this document are similar to those used in the C programming language.
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ISO/IEC 18477-2:2016(E)
5.2.1 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 −−aa12//and1− 2 for which y+Na = x
() ()
with a suitable integer N
umod x mod a is the unique value y between 0 and a−1 for which y+Na = y with a suitable
integer N
5.2.2 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)
5.2.3 Relational operators
> greater than
>= greater than or equal to
< less than
<= less than or equal to
== equal to
!= not equal to
5.2.4 Precedence order of operators
Operators are listed below 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
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ISO/IEC 18477-2:2016(E)
umod, smod modulo (remainder) left to right
+,− addition and subtraction left to right
<, >, <=, >= relational left to right
5.2.5 Mathematical functions
⎾x⏋ Ceiling 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.
clamp(x,min,max) Clamps x to the range [min,max]: returns min if x < min, max if x > max or
otherwise x.
6 General
6.1 Elements specified
The purpose of this clause is to give an informative overview of the elements specified in this document.
Another purpose is to introduce many of the terms that 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 a
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.
6.2 High level overview of this document
This document allows backward compatible to Rec. ITU-T T.81 | ISO/IEC 10918-1 coding of high
dynamic range images. Rec. ITU-T T.81 | ISO/IEC 10918-1 compliant decoders will be able to parse
codestreams conforming to this document correctly, albeit in less precision, with a limited dynamic
range, and potential loss in sample bit precision.
This document includes multiple tools to reach the above functionality. A short overview on these
coding tools is given in this clause.
The high-level syntax of a 18477-2 codestream is identical to that defined in ISO/IEC 18477-1, which is
a subset of the syntax defined in Rec. ITU-T T.81 | ISO/IEC 10918-1. Marker definitions and the syntax
of the markers defined in the above recommendation remain in force and unchanged. However, this
document defines the APP marker, reserved in the legacy Recommendation | International Standard
11
for encoding additional syntax elements. Legacy decoders will skip and ignore such marker elements,
and hence will only be able to decode the image encoded by the legacy syntax elements. This document
codestream will be denoted the legacy codestream.
This document extends the legacy standard by introducing extended syntax defined in Clause 8. The
extended syntax uses the APP marker to hide additional data from legacy applications.
11
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ISO/IEC 18477-2:2016(E)
High dynamic range coding of floating point samples is supported through residual coding. As shown
in Figure 1 this coding method represents the high-dynamic range image by two codestreams, the
legacy codestream and the residual codestream. The former represents a represents a low-dynamic
range representation of the original image that is visible to legacy applications, the latter represents
a residual image that is merged with the low-dynamic range legacy image in the spatial domain to
obtain the high-dynamic range reconstructed output. The merging process first generates a precursor
image of the high-dynamic range image by transforming the legacy codestream to a linear colourspace.
The precision of the precursor image is then extended to full precision by the residual image which is
decoded from the residual codestream packaged in the residual codestream segments defined in Clause 8.
The residual codestream is the entropy-coded representation of the residual image. This document
defines multiple coding methods for encoding the residual image: The DCT baseline Huffman, extended
Huffman or progressive coding method defined in Rec. ITU-T T.81 | ISO/IEC 10918-1.
Figure 1 — Coding process
6.3 High level functional overview of decoding process
Figure 2 — Overview of the decoding process
Figure 2 depicts the decoding process. The process begins with the legacy decoder block which
reconstructs the base image. This image is then optionally chroma upsampled followed by the Inverse
Decorrelation block. The output of this transformation is a low-dynamic range image with 8 bits per
sample in a RGB-type colour space. This upper path is the backward compatible part.
The low-dynamic range components are further mapped by the Base Mapping and Colour Space
Conversion block to a floating point image which is called the precursor image. The precursor image is
optionally converted to HDR colour space if the colour space of the precursor image is different and the
luminance of the precursor image is calculated. The noise level may be used to avoid division by zero
and to reduce the coding artefacts.
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ISO/IEC 18477-2:2016(E)
The residual decoder path uses the residual image stored in the residual codestream in the APP
11
markers. The residual image is decoded and optionally upsampled. The Residual Mapping and Inverse
Decorrelation block maps the residual image to the residual data. The residual data consists of at least
luminance ratio data and optional RGB differential data. This mapping uses the luminance computed by
the Base Mapping and Colour Space Conversion block.
The HDR Reconstruction block uses the resulting residual data and the precursor image to calculate
the reconstructed HDR Image.
6.4 Encoder requirements
An encoder is required to meet the compliance tests and to generate the codestream according to the
syntax and to limit the coding parameters to those valid within this document.
6.5 Decoder requirements
A decoding process converts compressed image data to reconstructed image data. The process shall
follow the decoding operation specified in this document and ISO/IEC 18477-1 for the residual and base
image codestreams. The decoder shall parse the codestream syntax to extract the parameters, the
residual data and the precursor image. The parameters shall be used to merge the residual data and
precursor image into the reconstructed HDR Image.
In order to comply with this document, a decoder:
a) shall convert a codestream conforming to this document without considering the residual codestream
into a low dynamic range image.
b) may additionally, convert a conforming codestream including the residual codestream according to
the codestream syntax, into a high dynamic range continuous tone image.
c) shall implement at least all the functional blocks of the JPEG XT decoding process defined in this
document.
7 Decoder definition
7.1 Decoder functionality overview
The decoder relies on a layered approach to extend capabilities of Rec. ITU-T T.81 | ISO/IEC 10918-1
compliant codestream. An encoder compliant to this document decomposes an HDR image into a base
layer and an HDR residual layer. The base layer is a tone mapped image obtained from the original
floating point HDR with either a local or global tonemapping or/and gamut mapping operator. The base
layer codestream constitutes the backward compatible part and is access
...
INTERNATIONAL ISO/IEC
STANDARD 18477-2
First edition
Information technology — Scalable
compression and coding of
continuous-tone still images —
Part 2:
Coding of high dynamic range images
Technologies de l’information — Compression échelonnable et codage
d’images plates en ton continu —
Partie 2: Codage d’images à gamme dynamique élevée
PROOF/ÉPREUVE
Reference number
ISO/IEC 18477-2:2016(E)
©
ISO/IEC 2016
---------------------- Page: 1 ----------------------
ISO/IEC 18477-2:2016(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, 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
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO/IEC 2016 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/IEC 18477-2:2016(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms and symbols . 4
4.1 Abbreviated terms . 4
4.2 Symbols . 4
5 Conventions . 4
5.1 Conformance language . 4
5.2 Operators . 4
5.2.1 Arithmetic operators . 5
5.2.2 Logical operators . 5
5.2.3 Relational operators . 5
5.2.4 Precedence order of operators . 5
5.2.5 Mathematical functions . 5
6 General . 6
6.1 Elements specified . 6
6.2 High level overview of this document . 6
6.3 High level functional overview of decoding process . 7
6.4 Encoder requirements . 7
6.5 Decoder requirements. 8
7 Decoder definition . 8
7.1 Decoder functionality overview . 8
7.2 Legacy Inverse Decorrelation Block B3 . 9
7.3 Base Mapping and Colour space conversion Block B4 . 9
7.4 Residual decode Blocks B5, B6 .11
7.5 Residual Mapping and Inverse Decorrelation Blocks B7, B8 .11
7.6 HDR Reconstruction Blocks B9, B10 .12
8 Codestream syntax for Main Profile .13
8.1 Main Profile Header Structure . .13
8.2 Parameter ASCII segment .13
8.3 Parameter binary segment .15
8.4 Residual codestream segment .15
Annex A (normative) Checksum computation .16
Bibliography .17
© ISO/IEC 2016 – All rights reserved PROOF/ÉPREUVE iii
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ISO/IEC 18477-2:2016(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. In the field of information technology, ISO and IEC have established a joint technical committee,
ISO/IEC JTC 1.
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).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/IEC JTC 1, Information technology, SC 29, Coding of
audio, picture, multimedia and hypermedia information.
iv PROOF/ÉPREUVE © ISO/IEC 2016 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/IEC 18477-2:2016(E)
Introduction
This document is an extension of ISO/IEC 18477-1, a compression system for continuous tone digital
still images which is backward 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, but will possibly not be able to reconstruct such
streams in full dynamic range, full quality or other features defined in this document.
The aim of this document is to provide a migration path for legacy applications to support coding of
high-dynamic range images. Existing tools depending on the existing standards will continue to work,
but will only be able to reconstruct a lossy and/or a low-dynamic range version of the image contained
in the codestream. This document specifies a codestream, referred to as JPEG XT, which is designed
primarily for storage and interchange of continuous-tone photographic content.
This document specifies a coded codestream format for storage of continuous-tone high and low
dynamic range photographic content. JPEG XT Part 2 is a scalable image coding system supporting
multiple component images in floating point. It is by itself an extension of the coding tools defined in
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 lower quality, low dynamic range, eight 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 or higher, providing greater
14
than 65 536 representable values to describe each channel within a pixel, resulting on over 2.8 × 10
representable colour values. In some less common scenarios, even greater bit depths are used.
The 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.
JPEG XT is primarily designed to provide coded data containing high dynamic range and wide colour
gamut content while simultaneously providing 8 bits per pixel low dynamic range images using tools
defined in ISO/IEC 18477-1. The goal is to provide a backward compatible coding specification that
allows legacy applications and existing toolchains to continue to operate on codestreams conforming to
this this document.
JPEG XT has been designed to be backward 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 high dynamic range and wide colour gamut 32 bit float images while also
enabling low-complexity encoder and decoder implementations.
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INTERNATIONAL STANDARD ISO/IEC 18477-2:2016(E)
Information technology — Scalable compression and
coding of continuous-tone still images —
Part 2:
Coding of high dynamic range images
1 Scope
This document specifies a coding format, referred to as JPEG XT, which is designed primarily for
continuous-tone photographic content.
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 10918-1, Information technology — Digital compression and coding of continuous-tone still
images: Requirements and guidelines — Part 1
ISO/IEC 18477-1, Information technology — Scalable Compression and Coding of Continuous-Tone Still
Images, Core Coding System Specification
IEC 61966-2-1, sRGB Colour management — Default RGB colour space — sRGB
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
ASCII encoding
character encoding scheme defined by ANSI X3.4-1986
3.2
codestream
partially encoded or decoded sequence of bits comprising an entropy-coded segment
3.3
byte
group of 8 bits
3.4
coder
embodiment of a coding process
3.5
coding
encoding or decoding
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ISO/IEC 18477-2:2016(E)
3.6
(coding) process
general term for referring to an encoding process, a decoding process, or both
3.7
compression
reduction in the number of bits used to represent source image data
3.8
component
two-dimensional array of samples having the same designation in the output or display device
Note 1 to entry: An image typically consists of several components, e.g. red, green and blue.
3.9
continuous-tone image
image whose components have more than one bit per sample
3.10
discrete cosine transform
DCT
sum of cosine transforms at different frequencies
3.11
decoder
embodiment of a decoding process
3.12
decoding process
process which takes as its input compressed image data and outputs a continuous-tone image
3.13
downsampling
procedure by which the spatial resolution of a component is reduced
3.14
encoder
embodiment of an encoding process
3.15
encoding process
process which takes as its input a continuous-tone image and outputs compressed image data
3.16
grayscale image
continuous-tone image that has only one component
3.17
high dynamic range
image or image data comprised of more than eight bits per sample
3.18
Joint Photographic Experts Group
JPEG
informal name of the working group which created this part of ISO/IEC 18477
Note 1 to entry: The term “joint” comes from the ITU-T and ISO/IEC collaboration.
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ISO/IEC 18477-2:2016(E)
3.19
legacy decoder
embodiment of a decoding process conforming to Rec. ITU-T T.81 | ISO/IEC 10918-1, confined to the
lossy discrete cosine transformation (DCT) process and the baseline, sequential or progressive modes,
decoding at most four components to eight bits per component
3.20
lossless
descriptive term for encoding and decoding processes and procedures in which the output of the
decoding procedure(s) is identical to the input of the encoding procedure(s)
3.21
lossless coding
mode of operation which refers to any one of the coding processes defined in this part of ISO/IEC 18477
in which all of the procedures are lossless
3.22
lossy
descriptive term for encoding and decoding processes which are not lossless
3.23
low-dynamic range
image or image data comprised of data with no more than 8 bits per sample
3.24
marker
two-byte code in which the first byte is hexadecimal FF and the second byte is a value between 1 and
hexadecimal FE
3.25
marker segment
marker together with its associated set of parameters
3.26
minimum coded unit
MCU
smallest group of data units that is coded
3.27
pixel
collection of sample values in the spatial image domain having all the same sample coordinates
Note 1 to entry: A pixel may consist of three samples describing its red, green and blue value.
3.28
precision
number of bits allocated to a particular sample or discrete cosine transformation (DCT) coefficient
3.29
procedure
set of steps which accomplishes one of the tasks which comprise an encoding or decoding process
3.30
residual codestream
codestream that contains an encoded (according to Rec. ITU-T T.81 | ISO/IEC 10918-1) residual image
3.31
residual data
data that contains luminance ratio and red, green, and blue (RGB) differential data
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ISO/IEC 18477-2:2016(E)
3.32
residual image
pseudo image that contains encoded luminance ratio as luminance and encoded chrominance data that
is computed from red, green, and blue (RGB) differential data using Multiple Component Decorrelation
Transformation defined in ISO/IEC 18477-1
3.33
red, green, and blue
RGB
additive colour model
3.34
luminance ratio
array of per pixel ratio of HDR image luminance and LDR image luminance
3.35
quantization value
integer value used in the quantization procedure
3.36
quantize
act of performing the quantization procedure for a value
3.37
upsampling
procedure by which the spatial resolution of a component is increased
4 Abbreviated terms and symbols
4.1 Abbreviated terms
ASCII American Standard Code for Information Interchange
HDR High Dynamic Range
LDR Low Dynamic Range
4.2 Symbols
Nc Number of components in an image
5 Conventions
5.1 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.
5.2 Operators
NOTE Many of the operators used in this document are similar to those used in the C programming language.
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ISO/IEC 18477-2:2016(E)
5.2.1 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 −−aa12//and1− 2 for which y+Na = x with a
() ()
suitable integer N
umod x mod a is the unique value y between 0 and a−1 for which y+Na = y with a suitable integer N
5.2.2 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)
5.2.3 Relational operators
> Greater than
>= Greater than or equal to
< Less than
<= Less than or equal to
== Equal to
!= Not equal to
5.2.4 Precedence order of operators
Operators are listed below 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
5.2.5 Mathematical functions
⎾x⏋ Ceiling 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.
clamp(x,min,max) Clamps x to the range [min,max]: returns min if x < min, max if x > max or otherwise x.
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ISO/IEC 18477-2:2016(E)
6 General
6.1 Elements specified
The purpose of this clause is to give an informative overview of the elements specified in this document.
Another purpose is to introduce many of the terms that 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 a
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.
6.2 High level overview of this document
This document allows backward compatible to Rec. ITU-T T.81 | ISO/IEC 10918-1 coding of high
dynamic range images. Rec. ITU-T T.81 | ISO/IEC 10918-1 compliant decoders will be able to parse
codestreams conforming to this document correctly, albeit in less precision, with a limited dynamic
range, and potential loss in sample bit precision.
This document includes multiple tools to reach the above functionality. A short overview on these
coding tools is given in this clause.
The high-level syntax of a 18477-2 codestream is identical to that defined in ISO/IEC 18477-1, which is
a subset of the syntax defined in Rec. ITU-T T.81 | ISO/IEC 10918-1. Marker definitions and the syntax
of the markers defined in the above recommendation remain in force and unchanged. However, this
document defines the APP marker, reserved in the legacy Recommendation | International Standard
11
for encoding additional syntax elements. Legacy decoders will skip and ignore such marker elements,
and hence will only be able to decode the image encoded by the legacy syntax elements. This document
codestream will be denoted the legacy codestream.
This document extends the legacy standard by introducing extended syntax defined in Clause 8. The
extended syntax uses the APP marker to hide additional data from legacy applications.
11
High dynamic range coding of floating point samples is supported through residual coding. As shown
in Figure 1 this coding method represents the high-dynamic range image by two codestreams, the
legacy codestream and the residual codestream. The former represents a represents a low-dynamic
range representation of the original image that is visible to legacy applications, the latter represents
a residual image that is merged with the low-dynamic range legacy image in the spatial domain to
obtain the high-dynamic range reconstructed output. The merging process first generates a precursor
image of the high-dynamic range image by transforming the legacy codestream to a linear colourspace.
The precision of the precursor image is then extended to full precision by the residual image which is
decoded from the residual codestream packaged in the residual codestream segments defined in Clause 8.
The residual codestream is the entropy-coded representation of the residual image. This document
defines multiple coding methods for encoding the residual image: The DCT baseline Huffman, extended
Huffman or progressive coding method defined in Rec. ITU-T T.81 | ISO/IEC 10918-1.
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ISO/IEC 18477-2:2016(E)
Figure 1 — Coding process
6.3 High level functional overview of decoding process
Figure 2 — Overview of the decoding process
Figure 2 depicts the decoding process. The process begins with the legacy decoder block which
reconstructs the base image. This image is then optionally chroma upsampled followed by the Inverse
Decorrelation block. The output of this transformation is a low-dynamic range image with 8 bits per
sample in a RGB-type colour space. This upper path is the backward compatible part.
The low-dynamic range components are further mapped by the Base Mapping and Colour Space
Conversion block to a floating point image which is called the precursor image. The precursor image is
optionally converted to HDR colour space if the colour space of the precursor image is different and the
luminance of the precursor image is calculated. The noise level may be used to avoid division by zero
and to reduce the coding artefacts.
The residual decoder path uses the residual image stored in the residual codestream in the APP
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markers. The residual image is decoded and optionally upsampled. The Residual Mapping and Inverse
Decorrelation block maps the residual image to the residual data. The residual data consists of at least
luminance ratio data and optional RGB differential data. This mapping uses the luminance computed by
the Base Mapping and Colour Space Conversion block.
The HDR Reconstruction block uses the resulting residual data and the precursor image to calculate
the reconstructed HDR Image.
6.4 Encoder requirements
An encoder is required to meet the compliance tests and to generate the codestream according to the
syntax and to limit the coding parameters to those valid within this document.
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6.5 Decoder requirements
A decoding process converts compressed image data to reconstructed image data. The process shall
follow the decoding operation specified in this document and ISO/IEC 18477-1 for the residual and base
image codestreams. The decoder shall parse the codestream syntax to extract the parameters, the
residual data and the precursor image. The parameters shall be used to merge the residual data and
precursor image into the reconstructed HDR Image.
In order to comply with this document, a decoder:
a) shall convert a codestream conforming to this document without considering the residual codestream
into a low dynamic range image.
b) may additionally, convert a conforming codestream including the residual codestream according to
the codestream syntax, into a high dynamic range continuous tone image.
c) shall implement at least all the functional blocks of the JPEG XT decoding process defined in this
document.
7 Decoder definition
7.1 Decoder functionality overview
The decoder relies on a layered approach to extend capabilities of Rec. ITU-T T.81 | ISO/IEC 10918-1
compliant codestream. An encoder compliant to this document decomposes an HDR image into a base
layer and an HDR residual layer. The base layer is a tone mapped image obtained from the original
floating point HDR with either a local or global tonemapping or/and gamut mapping operator. The base
layer codestream constitutes the backward compatible part and is accessible by all legacy decoders.
The base layer codestream is accessible in exactly the same way
...
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