ISO/IEC 18477-7:2017

INTERNATIONAL ISO/IEC
STANDARD 18477-7
Second edition
2017-05
Information technology — Scalable
compression and coding of
continuous-tone still images —
Part 7:
HDR Floating-Point Coding
Technologies de l’information — Compression échelonnable et codage
d’images plates en ton continu —
Partie 7: Codage de la virgule flottante en HDR
Reference number
©
ISO/IEC 2017
© ISO/IEC 2017, Published in Switzerland
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ii © ISO/IEC 2017 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and abbreviated terms . 1
4 Conventions . 9
4.1 Conformance language . 9
4.2 Operators . 9
4.2.1 Arithmetic operators .10
4.2.2 Logical operators .10
4.2.3 Relational operators .10
4.2.4 Precedence order of operators .10
4.2.5 Mathematical functions .11
5 General .11
5.1 Overview .11
5.2 High-level overview on JPEG XT ISO/IEC 18477-7 (informative) .11
5.3 Profiles .13
5.4 Encoder requirements .13
5.5 Decoder requirements.13
Annex A (normative) Encoding and decoding process .14
Annex B (normative) Boxes .19
Annex C (normative) Multi-component decorrelation .31
Annex D (normative) Half-exponential output transformation .32
Annex E (normative) Profiles .33
Annex F (informative) Implementation guidelines .43
Bibliography .49
© ISO/IEC 2017 – All rights reserved iii

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 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 the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/IEC JTC 1, Information technology, SC 29,
Coding of audio, picture, multimedia and hypermedia information.
This second edition cancels and replaces the first edition (ISO 18477-7:2016), of which it constitutes a
minor revision. The changes compared to the previous edition are as follows:
— a definition has been added for the term “horizontal subsampling factor” as 3.1.31;
— notes to entry have been added to terms throughout Clause 3;
— text in F.3 and F.4.2 has been modified;
— minor editorial changes.
A list of all the parts in the ISO/IEC 18477 series can be found on the ISO website.
iv © ISO/IEC 2017 – All rights reserved

Introduction
This document specifies a coded codestream format for storage of continuous-tone high and low
dynamic range photographic content. JPEG XT part 7 is a scalable image coding system supporting
multiple component images consisting of floating-point samples. It is by itself an extension of the coding
tools defined in ISO/IEC 18477-1 and the box-based format defined in ISO/IEC 18477-3; 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.
This standard low dynamic range image is typically constructed at the encoder side by tone mapping
from the high dynamic image; while the LDR image is always present, this document does not define a
process that generates this 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 eight bits, providing 256 representable values per channel.
If the dynamic range of the input scene is too large, however, an integer sample representation is no
longer applicable and sample values need to be specified in floating-point. These values typically are, or
are proportional to physical radiance values of three primaries. These primaries may be device specific
physical colours, or may be the basis of the CIE XYZ colourspace.
JPEG XT is primarily designed to provide coded data containing high dynamic range and wide colour
gamut content while simultaneously providing eight bits per pixel low dynamic range images using tools
defined in ISO/IEC 18477-1. The goal is to provide a backwards compatible coding specification that
allows legacy applications and existing tool chains 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 high dynamic range and wide colour gamut 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 other features defined in this document.
This document is itself based on ISO/IEC 18477-3, which defines a box-based file format similar to
other JPEG standards. The aim of this document is to provide a migration path for legacy applications
to support, potentially in a limited way, lossless coding and coding of high dynamic range images
consisting of samples represented in floating-point. 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 coded file format, referred to as JPEG
XT, which is designed primarily for storage and interchange of continuous-tone photographic content
© ISO/IEC 2017 – All rights reserved v

INTERNATIONAL STANDARD ISO/IEC 18477-7:2017(E)
Information technology — Scalable compression and
coding of continuous-tone still images —
Part 7:
HDR Floating-Point Coding
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 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 18477-2, Information technology — Scalable compression and coding of continuous-tone still
images — Part 2: Coding of high dynamic range 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/IEEE 60559, Information technology — Microprocessor Systems — Floating-Point arithmetic
Rec. ITU-T T.81 | ISO/IEC 10918–1:1994, Information technology — Digital compression and coding of
continuous-tone still images — Requirements and guidelines
Rec. 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
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