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 18477-7:2017(E)
©
ISO/IEC 2017

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ISO/IEC 18477-7:2017(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO/IEC 2017, Published in Switzerland
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ISO/IEC 18477-7:2017(E)

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
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ISO/IEC 18477-7:2017(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 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.
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ISO/IEC 18477-7:2017(E)

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
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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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3.1.1
ASCII encoding
encoding of text characters and text strings according to ISO/IEC 10646
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.2
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
Note 1 to entry: See ISO/IEC 18477-6.
3.1.3
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
Note 1 to entry: See ISO/IEC 18477-6.
3.1.4
binary decision
choice between two alternatives
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.5
bitstream
partially encoded or decoded sequence of bits comprising an entropy-coded segment
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.6
block
8 × 8 array of samples or an 8 × 8 array of DCT coefficient values of one component
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.7
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.
3.1.8
byte
group of 8 bits
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.9
coder
embodiment of a coding process
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.10
coding
encoding or decoding
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
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3.1.11
coding model
procedure used to convert input data into symbols to be coded
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.12
(coding) process
general term for referring to an encoding process, a decoding process, or both
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.13
compression
reduction in the number of bits used to represent source image data
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.14
component
two-dimensional array of samples having the same designation in the output or display device
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
Note 2 to entry: An image typically consists of several components, e.g. red, green and blue.
3.1.15
continuous-tone image
image whose components have more than one bit per sample
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.16
data unit
8 × 8 block of samples of one component in DCT-based processes; a sample in lossless processes
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.17
decoder
embodiment of a decoding process
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.18
decoding process
process which takes as its input compressed image data and outputs a continuous-tone image
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.19
dequantization
inverse procedure to quantization by which the decoder recovers a representation of the DCT
coefficients
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.20
discrete cosine transform
DCT
either the forward discrete cosine transform or the inverse discrete cosine transform
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
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3.1.21
downsampling
procedure by which the spatial resolution of a component is reduced
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.22
encoder
embodiment of an encoding process
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.23
encoding process
process which takes as its input a continuous-tone image and outputs compressed image data
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.24
entropy-coded (data) segment
independently decodable sequence of entropy encoded bytes of compressed image data
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.25
entropy decoder
embodiment of an entropy decoding procedure
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.26
entropy decoding
lossless procedure which recovers the sequence of symbols from the sequence of bits produced by the
entropy encoder
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.27
entropy encoder
embodiment of an entropy encoding procedure
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.28
entropy encoding
lossless procedure which converts a sequence of input symbols into a sequence of bits such that the
average number of bits per symbol approaches the entropy of the input symbols
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.29
grayscale image
continuous-tone image that has only one component
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.30
high dynamic range
image or image data comprised of samples using a floating-point representation
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3.1.31
horizontal subsampling factor
relative number of vertical data units of a particular component with respect to the number of
horizontal data units in the other components in the frame
Note 1 to entry: See ISO/IEC 18477-1.
3.1.32
Huffman decoder
embodiment of a Huffman decoding procedure
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.33
Huffman decoding
entropy decoding procedure which recovers the symbol from each variable length code produced by
the Huffman encoder
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.34
Huffman encoder
embodiment of a Huffman encoding procedure
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.35
Huffman encoding
entropy encoding procedure which assigns a variable length code to each input symbol
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.36
joint photographic experts group
JPEG
informal name of the committee which created this document
Note 1 to entry: The “joint” comes from the ITU-T and ISO/IEC collaboration.
3.1.37
legacy codestream
collection of markers and syntax elements defined by Rec. ITU-T T.81 | ISO/IEC 10918-1 bare any syntax
elements defined by the family ISO/IEC 18477 standards
EXAMPLE The legacy codestream 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, Annex A.
Note 1 to entry: See ISO/IEC 18477-6.
3.1.38
legacy decoder
embodiment of a decoding process conforming to Rec. ITU-T T.81 | ISO/IEC 10918-1, confined to the
lossy DCT process and the baseline, sequential or progressive modes, decoding at most four components
to eight bits per component
Note 1 to entry: See ISO/IEC 18477-6.
3.1.39
lossless
descriptive term for 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)
Note 1 to entry: See ISO/IEC 18477-8.
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3.1.40
lossless coding
mode of operation which refers to any one of the coding processes defined in this document in which all
of the procedures are lossless
Note 1 to entry: See ISO/IEC 18477-8.
3.1.41
lossy
descriptive term for encoding and decoding processes which are not lossless
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.42
low dynamic range
image or image data comprised of data with no more than eight bits per sample
Note 1 to entry: See ISO/IEC 18477-6.
3.1.43
marker
two-byte code in which the first byte is hexadecimal FF and the second byte is a value between 1 and
hexadecimal FE
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.44
marker segment
marker together with its associated set of parameters
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.45
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.
Note 2 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.46
precision
number of bits allocated to a particular sample or DCT coefficient
3.1.47
procedure
set of steps which accomplishes one of the tasks which comprise an encoding or decoding process
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.48
quantization value
integer value used in the quantization procedure
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.49
quantize
act of performing the quantization procedure for a DCT coefficient
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
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3.1.50
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 base image to form
the final output image
Note 1 to entry: See ISO/IEC 18477-6.
3.1.51
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
Note 1 to entry: See ISO/IEC 18477-6.
3.1.52
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
Note 1 to entry: See ISO/IEC 18477-6.
3.1.53
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: See ISO/IEC 18477-6.
3.1.54
sample
one element in the two-dimensional image array which comprises a component
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.55
sample grid
common coordinate system for all samples of an image
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
Note 2 to entry: The samples at the top left edge of the image have the coordinates (0,0), the first coordinate
increases towards the right, the second towards the bottom.
3.1.56
scan
single pass through the data for one or more of the components in an image
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.57
scan header
marker segment that contains a start-of-scan marker and associated scan parameters that are coded at
the beginning of a scan
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
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3.1.58
table specification data
coded representation from which the tables used in the encoder and decoder are generated and their
destinations specified
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.59
(uniform) quantization
procedure by which DCT coefficients are linearly scaled in order to achieve compression
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.60
upsampling
procedure by which the spatial resolution of a component is increased
Note 1 to entry: See ISO/IEC 18477-1.
3.1.61
vertical subsampling factor
relative number of vertical data units of a particular component with respect to the number of vertical
data units in the other components in the frame
Note 1 to entry: See ISO/IEC 18477-1.
3.1.62
zero byte
0×00 byte
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
3.1.63
zig-zag sequence
specific sequential ordering of the DCT coefficients from (approximately) lowest spatial frequency
to highest
Note 1 to entry: See Rec. ITU-T T.81 | ISO/IEC 10918-1.
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
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R additional number of DCT coefficient bits represented by refinement scans in the legacy decod-
h
ing path, 8 + R is the number of non-fractional bits (i.e. bits in front of the “binary dot”) of the
h
output of the inverse DCT process in the legacy decoding path
R additional number of DCT coefficient bits represented by refinement scans in the residual de-
r
coding path. p + R is the number of non-fractional bits of the output of the inverse DCT process
r
in the residual decoding path, where p is the frame-precision of the residual image as recorded
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
3.3 Abbreviated terms
ASCII American Standard Code for Information Interchange
LSB Least Significant Bit
MSB Most Significant Bit
HDR High Dynamic Range
LDR Low Dynamic Range
TMO Tone Mapping Operator
DCT Discrete Cosine Transformation
4 Conventions
4.1 Conformance language
This document consists of normative and informative text.
Normative text is that text which expresses mandatory requirements. The word “shall” is used to
express mandatory requirements strictly to be followed in order to conform to this document and
from which no deviation is permitted. A conforming implementation is one that fulfils all mandatory
requirements.
Informative text is text that is potentially helpful to the user, but not indispensable and can be removed,
changed or added editorially without affecting interoperability. All text in this document is normative,
with the following exceptions: the Introduction, any parts of the text that are explicitly labelled as
“informative”, and statements appearing with the preamble “NOTE” and behaviour described using the
word “should”. The word “should” is used to describe behaviour that is encouraged but is not required
for conformance to this document.
The keywords “may” and “need not” indicate a course of action that is permissible in a conforming
implementation.
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.
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