ISO/IEC 18477-9:2016

INTERNATIONAL ISO/IEC
STANDARD 18477-9
First edition
2016-10-15
Information technology — Scalable
compression and coding of
continuous-tone still images —
Part 9:
Alpha channel coding
Technologies de l’information — Compression échelonnable et codage
d’images plates en ton continu
Reference number
ISO/IEC 18477-9:2016(E)
©
ISO/IEC 2016

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ISO/IEC 18477-9:2016(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
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ISO/IEC 18477-9:2016(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 . 6
3.3 Abbreviated terms . 6
4 Conventions . 6
4.1 Conformance language . 6
4.2 Operators . 6
4.2.1 Arithmetic operators . 7
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 . 8
5 General . 8
5.1 General definitions . 8
5.2 Overview of ISO/IEC 18477-9 . 8
5.2.1 Encoder requirements . 8
5.2.2 Decoder requirements . 8
Annex A (normative) Encoding and decoding process .10
Annex B (normative) Boxes .14
Annex C (normative) Profiles .21
Bibliography .23
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ISO/IEC 18477-9:2016(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, SC29, 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 18477-9:2016(E)

Introduction
This document specifies an extension for ISO/IEC 18477-3 compliant files that adds capabilities for
lossy or lossless storage of continuous or binary opacity information associated to the image; such
additional channels are commonly known as alpha channels. These channels are used for compositing
the image content with other content on the same physical media. An alpha value of 0 encodes maximal
transparency (and no opacity), while the maximal sample value represents maximal opacity (and
no transparency). Additionally, the image content itself may be premultiplied with the alpha value or
premultiplied and shaded with a background colour M, a process by which the original image A is replaced
by the image A’ defined as
A’ = α*A for pre-multiplication
A’ = α*A+(1−α)*M for pre-multiplication and shading
And A’ is encoded instead of A in the JPEG XT codestream. Reconstruction is then performed as follows:
If A denotes the sample value of the image contained in the ISO/IEC 18477-3 file at a specific spatial
location, B is the sample value of the background on which the image should be rendered, M is the matte
colour and α is the decoded value of the alpha channel, then the sample value of the image C composed
from A and B on the same position is given by:
C = α *A +(1−α)*B for non-premultiplied content;
C = A +(1−α)*B for premultiplied content;
C = A +(1−α)*(B-M) for premultiplied content with shade removal.
Encoding a premultiplied and shaded version of A’ with colour M enables legacy decoders that lack alpha
channel support to still decode and display the image with the appearance that it is composited on a
background with colour M. At the same time, new JPEG XT compliant decoders can composite the image
on any background by calculating image C from A, B and M.
This document provides facilities to encode the value of α for each spatial location, with or without
loss, either as a binary decision, i.e. α = 0 or α = 1, on a continuous scale of integers with a resolution
between 8 and 16 bits, or as floating point number between 0 and 1 with 16-bit precision. It uses coding
technology from other parts of the ISO/IEC 18477 family of standards for its encoding, and no new
technology besides that already defined in other parts is required for the reconstruction of the opacity
information.
This document can be freely combined with other parts of the ISO/IEC 18477 family, i.e. the sample
values A in the above formulae might be either 8-bit unsigned integers, i.e. represented by ISO/IEC 18477-
1, up to 16-bit integers using the encoding of ISO/IEC 18477-6 or floating point values encoded
by ISO/IEC 18477-7. The image content A may also be encoded without loss, using ISO/IEC 18477-8.
However, the compositing step itself to create the final output image C from the input images A and B is
not standardized.
The syntax of the codestream defined in this document is fully backward compatible to Rec. ITU-T T.81
| ISO/IEC 10918-1 and the ISO/IEC 18477 family of standards. Decoders unaware of the extensions
defined here will reconstruct a fully opaque version of the image and discard the alpha channel content.
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INTERNATIONAL STANDARD ISO/IEC 18477-9:2016(E)
Information technology — Scalable compression and
coding of continuous-tone still images —
Part 9:
Alpha channel 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, 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 18477-7:2016, Information technology — Scalable compression and coding of continuous-tone still
images — Part 7: HDR Floating-Point Coding
ISO/IEC 18477-8:2016, Information Technology: Scalable compression and coding of continuous-tone still
images — Lossless and near-lossless coding
ITU-T T.81 | ISO/IEC 10918-1, Information technology – Digital compression and coding of continuous-tone
still images: Requirements and guidelines
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following 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
ASCII encoding
encoding of text characters and text strings according to ISO/IEC 10646
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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
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
3.1.4
alpha channel
additional scalar image channel that encodes the opacity of each sample in the main image
3.1.5
alpha component
synonym for alpha channel
3.1.6
binary decision
choice between two alternatives
3.1.7
block
8×8 array of samples or an 8×8 array of DCT coefficient values of one component
3.1.8
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.
3.1.9
byte
group of 8 bits
3.1.10
coder
embodiment of a coding process
3.1.11
coding
encoding or decoding
3.1.12
coding process
general reference to an encoding process, a decoding process, or both
3.1.13
compression
reduction in the number of bits used to represent source image data
3.1.14
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.
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3.1.15
composition
process of merging the decoded image data with background image data using opacity information and
generating one single final output image
3.1.16
continuous-tone image
image whose components have more than one bit per sample
3.1.17
decoder
embodiment of a decoding process
3.1.18
decoding process
process which takes as its input compressed image data and outputs a continuous-tone image
3.1.19
encoder
embodiment of an encoding process
3.1.20
encoding process
process which takes as its input a continuous-tone image and outputs compressed image data
3.1.21
entropy decoder
embodiment of an entropy decoding procedure
3.1.22
entropy decoding
lossless procedure which recovers the sequence of symbols from the sequence of bits produced by the
entropy encoder
3.1.23
entropy encoder
embodiment of an entropy encoding procedure
3.1.24
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
3.1.25
fixed point discrete cosine transformation
fixed point DCT
implementation of the discrete cosine transformation based on fixed point arithmetic following the
specifications in ISO/IEC 18477-8:2016, Annex E
3.1.26
high dynamic range
HDR
image or image data comprised of more than eight bits per sample
3.1.27
integer based discrete cosine transformation
integer point 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 as specified in ISO/IEC 18477-8:2016,
Annex E
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3.1.28
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.
3.1.29
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 ISO/IEC 18477 family of standards. That is, the legacy codestream consists of
the collection of all markers except those APP markers that describe JPEG XT boxes by the syntax
11
defined in ISO/IEC 18477-3:2015, Annex A
3.1.30
legacy decoding path
collection of operations to be performed on the entropy coded data as described by Rec. ITU-T T.81 |
ISO/IEC 10918-1 jointly with the Legacy Refinement scans before this data is merged with the residual
data to form the final output image
3.1.31
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
3.1.32
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)
3.1.33
lossless coding
mode of operation which refers to any one of the coding processes defined in ISO/IEC 18477-8:2016 in
which all of the procedures are lossless
3.1.34
lossy
encoding and decoding processes which are not lossless
3.1.35
low-dynamic range
LDR
image or image data comprised of data with no more than eight bits per sample
3.1.36
pixel
collection of sample values in the spatial image domain having all the same sample coordinates, e.g. a
pixel may consist of three samples describing its red, green and blue value
3.1.37
point transformation
application of a location independent global function to reconstructed sample values in the spatial domain
3.1.38
precision
number of bits allocated to a particular sample or DCT coefficient
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3.1.39
premultiplied content
image component that has already multiplied by the scaled value of the alpha channel on a pixel-by-
pixel basis to ease the composition of the image with the background
3.1.40
procedure
set of steps which accomplishes one of the tasks which comprise an encoding or decoding process
3.1.41
quantize
act of performing the quantization procedure for a DCT coefficient
3.1.42
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
3.1.43
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
3.1.44
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
3.1.45
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.
3.1.46
sample
one element in the two-dimensional image array which comprises a component
3.1.47
sample grid
common coordinate system for all samples of an image
Note 1 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.
3.1.48
superbox
box that carries other boxes as payload data
3.1.49
sub box
box that is contained as payload data within a superbox
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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, 8+h is the number of
h
non-fractional bits (i.e. bits in front of the “binary dot”) of the output of the inverse DCT process.
R Additional number of DCT coefficients bits represented by refinement scans in the residual, P+R
r h
is the number of non-fractional bits (i.e. bits in front of the “binary dot”) of the output of the in-
verse 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
3.3 Abbreviated terms
ASCII American Standard Code for Information Interchange
DCT discrete cosine transformation
LSB least significant bit
MSB most significant bit
TMO tone mapping operator
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.
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4.2.1 Arithmetic operators
+ addition
− subtraction (as a binary operator) or negation (as a unary prefix operator)
* multiplication
/ division without truncation or rounding.
umod x umod a is the unique value y bet ween 0 and a−1for which
y+N*a = x with a suitable integer N
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 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 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 modulo (remainder) left to right
+, − addition and subtraction left to right
< , > , < = , > = relational left to right
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4.2.5 Mathematical functions
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
 
 
|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 otherwise x
5 General
5.1 General definitions
Clause 5 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
Clause 5.
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.
5.2 Overview of ISO/IEC 18477-9
5.2.1 Encoder requirements
An encoder is only required to meet the compliance tests and to generate the codestream according
to the syntax defined in this document. How the codestream is algorithmically constructed and how
the boxes are laid out is implementation specific and not within scope of this document. Subsequent
parts of the ISO/IEC 18477 series may, however, define additional restrictions and requirements, either
within the document itself, or within profiles that restrict the freedom of the encoder further.
An encoder claiming to be compliant to one of these profiles then shall conform to the syntax constraints
defined in the corresponding profile of the corresponding part of ISO/IEC 18477.
5.2.2 Decoder requirements
A decoding process converts compressed image data to reconstructed image data. It shall follow the
decoding operation specified in this document and ISO/IEC 18477-1 to reconstruct a legacy 8 bits per
channel standard low dynamic range image. It is not required that a conforming decoder is capable of
decoding and interpreting all box types defined in this or other members of the ISO/IEC 18477 family
of standards. A decoder implementation is always free to skip over box types it is unable or not willing
to support.
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In order to comply with this document, a decoder:
a) shall convert a codestream conforming to this document without considering any boxes into to
a low dynamic range image;
b) shall additionally convert a conforming codestream including the information in a subset of the
boxes into an image (of potentially higher precision, higher quality or higher bit depths) and into an
alpha-channel;
c) shall implement at least all the functional blocks of the JPEG XT decoding process defined in the
profile it claims to be conforming to, where profiles are defined in this and other parts of the
ISO/IEC 18477 family of standards. For that, a conforming decoder shall correctly interpret all box
types required in the definition of the profile.
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Annex A
(normative)

Encoding and decoding process
A.1 Decoding process (normative)
Annex A defines the functionality of a subset of the boxes of the file format, as specified in
ISO/IEC 18477-3, which are required for decoding an alpha channel. The alpha channel defines opacity
data accompanying the image represented by the respective legacy codestream and boxes defined in
other parts of the ISO/IEC 18477 family of standards.
The decoding process of the alpha channel data is depicted in Figure A.1. The decoder first decodes the
foreground image contained in the codestream and boxes of the ISO/IEC 18477-3 conforming file, giving
one or three components per sample process. The decoding process of the foreground image from the
codestream is specified in other parts of the ISO/IEC 18477 family of standards and not repeated here.
The decoder then proceeds to decode the codestream in the Alpha Codestream box B1 and the Alpha
refinement box B1a if they are present, giving a precursor alpha channel plane denoted by H . The
i
frame dimensions indicated in the frame header of the Alpha Codestream box (see Annex B) and the
frame header of the Residual Alpha Data box shall be identical to the frame dimensions indicated in the
frame header of the legacy codestream. Decoding then proceeds to the Residual Alpha box B5, and the
Residual Alpha Refinement box B5a. If decoded, they provide either a lossless error residual, denoted
by Q to enable lossless coding of the
...